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  • richardmitnick 12:10 pm on April 23, 2019 Permalink | Reply
    Tags: "Falsifiability and physics", , , , , Dark Matter, , , Karl Popper (1902-1994) "The Logic of Scientific Discovery", , ,   

    From Symmetry: “Falsifiability and physics” 

    Symmetry Mag
    From Symmetry

    Matthew R. Francis

    Illustration by Sandbox Studio, Chicago with Corinne Mucha

    Can a theory that isn’t completely testable still be useful to physics?

    What determines if an idea is legitimately scientific or not? This question has been debated by philosophers and historians of science, working scientists, and lawyers in courts of law. That’s because it’s not merely an abstract notion: What makes something scientific or not determines if it should be taught in classrooms or supported by government grant money.

    The answer is relatively straightforward in many cases: Despite conspiracy theories to the contrary, the Earth is not flat. Literally all evidence is in favor of a round and rotating Earth, so statements based on a flat-Earth hypothesis are not scientific.

    In other cases, though, people actively debate where and how the demarcation line should be drawn. One such criterion was proposed by philosopher of science Karl Popper (1902-1994), who argued that scientific ideas must be subject to “falsification.”

    Popper wrote in his classic book The Logic of Scientific Discovery that a theory that cannot be proven false—that is, a theory flexible enough to encompass every possible experimental outcome—is scientifically useless. He wrote that a scientific idea must contain the key to its own downfall: It must make predictions that can be tested and, if those predictions are proven false, the theory must be jettisoned.

    When writing this, Popper was less concerned with physics than he was with theories like Freudian psychology and Stalinist history. These, he argued, were not falsifiable because they were vague or flexible enough to incorporate all the available evidence and therefore immune to testing.

    But where does this falsifiability requirement leave certain areas of theoretical physics? String theory, for example, involves physics on extremely small length scales unreachable by any foreseeable experiment.

    String Theory depiction. Cross section of the quintic Calabi–Yau manifold Calabi yau.jpg. Jbourjai (using Mathematica output)

    Cosmic inflation, a theory that explains much about the properties of the observable universe, may itself be untestable through direct observations.

    Some critics believe these theories are unfalsifiable and, for that reason, are of dubious scientific value.

    At the same time, many physicists align with philosophers of science who identified flaws in Popper’s model, saying falsification is most useful in identifying blatant pseudoscience (the flat-Earth hypothesis, again) but relatively unimportant for judging theories growing out of established paradigms in science.

    “I think we should be worried about being arrogant,” says Chanda Prescod-Weinstein of the University of New Hampshire. “Falsifiability is important, but so is remembering that nature does what it wants.”

    Prescod-Weinstein is both a particle cosmologist and researcher in science, technology, and society studies, interested in analyzing the priorities scientists have as a group. “Any particular generation deciding that they’ve worked out all that can be worked out seems like the height of arrogance to me,” she says.

    Tracy Slatyer of MIT agrees, and argues that stringently worrying about falsification can prevent new ideas from germinating, stifling creativity. “In theoretical physics, the vast majority of all the ideas you ever work on are going to be wrong,” she says. “They may be interesting ideas, they may be beautiful ideas, they may be gorgeous structures that are simply not realized in our universe.”

    Particles and practical philosophy

    Take, for example, supersymmetry. SUSY is an extension of the Standard Model in which each known particle is paired with a supersymmetric partner.

    Standard Model of Supersymmetry via DESY

    The theory is a natural outgrowth of a mathematical symmetry of spacetime, in ways similar to the Standard Model itself. It’s well established within particle physics, even though supersymmetric particles, if they exist, may be out of scientists’ experimental reach.

    SUSY could potentially resolve some major mysteries in modern physics. For one, all of those supersymmetric particles could be the reason the mass of the Higgs boson is smaller than quantum mechanics says it should be.

    CERN CMS Higgs Event

    CERN ATLAS Higgs Event

    “Quantum mechanics says that [the Higgs boson] mass should blow up to the largest mass scale possible,” says Howard Baer of the University of Oklahoma. That’s because masses in quantum theory are the result of contributions from many different particles involved in interactions—and the Higgs field, which gives other particles mass, racks up a lot of these interactions. But the Higgs mass isn’t huge, which requires an explanation.

    “Something else would have to be tuned to a huge negative [value] in order to cancel [the huge positive value of those interactions] and give you the observed value,” Baer says. That level of coincidence, known as a “fine-tuning problem,” makes physicists itchy. “It’s like trying to play the lottery. It’s possible you might win, but really you’re almost certain to lose.”

    If SUSY particles turn up in a certain mass range, their contributions to the Higgs mass “naturally” solve this problem, which has been an argument in favor of the theory of supersymmetry. So far, the Large Hadron Collider has not turned up any SUSY particles in the range of “naturalness.”


    CERN map

    CERN LHC Tunnel

    CERN LHC particles

    However, the broad framework of supersymmetry can accommodate even more massive SUSY particles, which may or may not be detectable using the LHC. In fact, if naturalness is abandoned, SUSY doesn’t provide an obvious mass scale at all, meaning SUSY particles might be out of range for discovery with any earthly particle collider. That point has made some critics queasy: If there’s no obvious mass scale at which colliders can hunt for SUSY, is the theory falsifiable?

    A related problem confronts dark matter researchers: Despite strong indirect evidence for a large amount of mass invisible to all forms of light, particle experiments have yet to find any dark matter particles. It could be that dark matter particles are just impossible to directly detect. A small but vocal group of researchers has argued that we need to consider alternative theories of gravity instead.

    Fritz Zwicky, the Father of Dark Matter research.No image credit after long search

    Astronomer Vera Rubin at the Lowell Observatory in 1965, worked on Dark Matter (The Carnegie Institution for Science)

    U Washington ADMX Axion Dark Matter Experiment

    DEAP Dark Matter detector, The DEAP-3600, suspended in the SNOLAB deep in Sudbury’s Creighton Mine

    Dark Side-50 Dark Matter Experiment at Gran Sasso

    Slatyer, whose research involves looking for dark matter, considers the criticism partly as a problem of language. “When you say ‘dark matter,’ [you need] to distinguish dark matter from specific scenarios for what dark matter could be,” she says. “The community has not always done that well.”

    In other words, specific models for dark matter can stand or fall, but the dark matter paradigm as a whole has withstood all tests so far. But as Slatyer points out, no alternative theory of gravity can explain all the phenomena that a simple dark matter model can, from the behavior of galaxies to the structure of the cosmic microwave background.

    Prescod-Weinstein argues that we’re a long way from ruling out all dark matter possibilities. “How will we prove that the dark matter, if it exists, definitively doesn’t interact with the Standard Model?” she says. “Astrophysics is always a bit of a detective game. Without laboratory [detection of] dark matter, it’s hard to make definitive statements about its properties. But we can construct likely narratives based on what we know about its behavior.”

    Similarly, Baer thinks that we haven’t exhausted all the SUSY possibilities yet. “People say, ‘you’ve been promising supersymmetry for 20 or 30 years,’ but it was based on overly optimistic naturalness calculations,” he says. “I think if one evaluates the naturalness properly, then you find that supersymmetry is still even now very natural. But you’re going to need either an energy upgrade of LHC or an ILC [International Linear Collider] in order to discover it.”

    ILC schematic, being planned for the Kitakami highland, in the Iwate prefecture of northern Japan

    Beyond falsifiability of dark matter or SUSY, physicists are motivated by more mundane concerns. “Even if these individual scenarios are in principle falsifiable, how much money would [it] take and how much time would it take?” Slatyer says. In other words, rather than try to demonstrate or rule out SUSY as a whole, physicists focus on particle experiments that can be performed within a certain number of budgetary cycles. It’s not romantic, but it’s true nevertheless.

    Illustration by Sandbox Studio, Chicago with Corinne Mucha

    Is it science? Who decides?

    Historically, sometimes theories that seem untestable turn out to just need more time. For example, 19th century physicist Ludwig Boltzmann and colleagues showed they could explain many results in thermal physics and chemistry if everything were made up of “atoms”—what we call particles, atoms, and molecules today—governed by Newtonian physics.

    Since atoms were out of reach of experiments of the day, prominent philosophers of science argued that the atomic hypothesis was untestable in principle, and therefore unscientific.

    However, the atomists eventually won the day: J. J. Thompson demonstrated the existence of electrons, while Albert Einstein showed that water molecules could make grains of pollen dance on a pond’s surface.

    Atoms provide a case study for how falsifiability proved to be the wrong criterion. Many other cases are trickier.

    For instance, Einstein’s theory of general relativity is one of the best-tested theories in all of science. At the same time, it allows for physically unrealistic “universes,” such as a “rotating” cosmos where movement back and forth in time is possible, which are contradicted by all observations of the reality we inhabit.

    General relativity also makes predictions about things that are untestable by definition, like how particles move inside the event horizon of a black hole: No information about these trajectories can be determined by experiment.

    The first image of a black hole, Messier 87 Credit Event Horizon Telescope Collaboration, via NSF 4.10.19

    Yet no knowledgeable physicist or philosopher of science would argue that general relativity is unscientific. The success of the theory is due to enough of its predictions being testable.

    Eddington/Einstein exibition of gravitational lensing solar eclipse of 29 May 1919

    Another type of theory may be mostly untestable, but have important consequences. One such theory is cosmic inflation, which (among other things) explains why we don’t see isolated magnetic monopoles and why the universe is a nearly uniform temperature everywhere we look.

    The key property of inflation—the extremely rapid expansion of spacetime during a tiny split second after the Big Bang—cannot be tested directly. Cosmologists look for indirect evidence for inflation, but in the end it may be difficult or impossible to distinguish between different inflationary models, simply because scientists can’t get the data. Does that mean it isn’t scientific?


    Alan Guth, from Highland Park High School and M.I.T., who first proposed cosmic inflation

    HPHS Owls

    Lambda-Cold Dark Matter, Accelerated Expansion of the Universe, Big Bang-Inflation (timeline of the universe) Date 2010 Credit: Alex MittelmannColdcreation

    Alan Guth’s notes:

    “A lot of people have personal feelings about inflation and the aesthetics of physical theories,” Prescod-Weinstein says. She’s willing to entertain alternative ideas which have testable consequences, but inflation works well enough for now to keep it around. “It’s also the case that the majority of the cosmology community continues to take inflation seriously as a model, so I have to shrug a little when someone says it’s not science.”

    On that note, Caltech cosmologist Sean M. Carroll argues that many very useful theories have both falsifiable and unfalsifiable predictions. Some aspects may be testable in principle, but not by any experiment or observation we can perform with existing technology. Many particle physics models fall into that category, but that doesn’t stop physicists from finding them useful. SUSY as a concept may not be falsifiable, but many specific models within the broad framework certainly are. All the evidence we have for the existence of dark matter is indirect, which won’t go away even if laboratory experiments never find dark matter particles. Physicists accept the concept of dark matter because it works.

    Slatyer is a practical dark matter hunter. “The questions I’m most interested asking are not even just questions that are in principle falsifiable, but questions that in principle can be tested by data on the timescale of less than my lifetime,” she says. “But it’s not only problems that can be tested by data on a timescale of ‘less than Tracy’s lifetime’ are good scientific questions!”

    Prescod-Weinstein agrees, and argues for keeping an open mind. “There’s a lot we don’t know about the universe, including what’s knowable about it. We are a curious species, and I think we should remain curious.”

    See the full article here .


    Please help promote STEM in your local schools.

    Stem Education Coalition

    Symmetry is a joint Fermilab/SLAC publication.

  • richardmitnick 11:41 am on April 18, 2019 Permalink | Reply
    Tags: "What gravitational waves can say about dark matter", , , , , , , Dark Matter, , ,   

    From Symmetry: “What gravitational waves can say about dark matter” 

    Symmetry Mag
    From Symmetry

    Caitlyn Buongiorno

    Scientists think that, under some circumstances, dark matter could generate powerful enough gravitational waves for equipment like LIGO to detect.

    Artwork by Sandbox Studio, Chicago

    In 1916, Albert Einstein published his theory of general relativity, which established the modern view of gravity as a warping of the fabric of spacetime. The theory predicted that objects that interact with gravity could disturb that fabric, sending ripples across it.

    Any object that interacts with gravity can create gravitational waves. But only the most catastrophic cosmic events make gravitational waves powerful enough for us to detect. Now that observatories have begun to record gravitational waves on a regular basis, scientists are discussing how dark matter—only known so far to interact with other matter only through gravity—might create gravitational waves strong enough to be found.

    Fritz Zwicky discovered Dark Matter when observing the movement of the Coma Cluster. But , Vera Rubin a Woman in STEM denied the Nobel, did most of the work on Dark Matter.

    Fritz Zwicky from http:// palomarskies.blogspot.com

    Coma cluster via NASA/ESA Hubble

    Astronomer Vera Rubin at the Lowell Observatory in 1965, worked on Dark Matter (The Carnegie Institution for Science)

    Vera Rubin measuring spectra, worked on Dark Matter (Emilio Segre Visual Archives AIP SPL)

    Vera Rubin, with Department of Terrestrial Magnetism (DTM) image tube spectrograph attached to the Kitt Peak 84-inch telescope, 1970. https://home.dtm.ciw.edu

    The spacetime blanket

    In the universe, space and time are invariably linked as four-dimensional spacetime. For simplicity, you can think of spacetime as a blanket suspended above the ground.

    Spacetime with Gravity Probe B. NASA

    Jupiter might be a single Cheerio on top of that blanket. The sun could be a tennis ball. R136a1—the most massive known star—might be a 40-pound medicine ball.

    Each of these objects weighs down the blanket where it sits: the heavier the object, the bigger the dip in the blanket. Like objects of different weights on a blanket, objects of different masses have different effects on the fabric of spacetime. A dip in spacetime is gravitational field.

    The gravitational field of one object can affect another object. The other object might fall into the first object’s gravitational field and orbit around it, like the moon around Earth, or Earth around the sun.

    Alternatively, two bodies with gravitational fields might spiral toward each other, getting closer and closer until they collide. As this happens, they create ripples in spacetime—gravitational waves.

    On September 14, 2015, scientists used the Laser Interferometer Gravitational-Wave Observatory, or LIGO, to make the first direct observation of gravitational waves, part of the buildup to the crash between two massive black holes.

    Since that first detection, the LIGO collaboration—together with the collaboration that runs a partner gravitational-wave observatory called Virgo—has detected gravitational waves from at least 10 more mergers of black holes and, in 2017, the first merger between two neutron stars.

    VIRGO Gravitational Wave interferometer, near Pisa, Italy

    Caltech/MIT Advanced aLigo Hanford, WA, USA installation

    Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA

    Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project

    Gravitational waves. Credit: MPI for Gravitational Physics/W.Benger

    Gravity is talking. Lisa will listen. Dialogos of Eide

    ESA/eLISA the future of gravitational wave research

    Localizations of gravitational-wave signals detected by LIGO in 2015 (GW150914, LVT151012, GW151226, GW170104), more recently, by the LIGO-Virgo network (GW170814, GW170817). After Virgo came online in August 2018

    Skymap showing how adding Virgo to LIGO helps in reducing the size of the source-likely region in the sky. (Credit: Giuseppe Greco (Virgo Urbino group)

    Dark matter is believed to be five times as prevalent as visible matter. Its gravitational effects are seen throughout the universe. Scientists think they have yet to definitively see gravitational waves caused by dark matter, but they can think of numerous ways this might happen.

    Primordial black holes

    Scientists have seen the gravitational effects of dark matter, so they know it must be there—or at least, something must be going on to cause those effects. But so far, they’ve never directly detected a dark matter particle, so they’re not sure exactly what dark matter is like.

    One idea is that some of the dark matter could actually be primordial black holes.

    Imagine the universe as an infinitely large petri dish. In this scenario, the Big Bang is the point where matter-bacteria begins to grow. That point quickly expands, moving outward to encompass more and more of the petri dish. If that growth is slightly uneven, certain areas will become more densely inhabited by matter than others.

    These pockets of dense matter—mostly photons at this point in the universe—might have collapsed under their own gravity and formed early black holes.

    “I think it’s an interesting theory, as interesting as a new kind of particle,” says Yacine Ali-Haimoud, an assistant professor of physics at New York University. “If primordial black holes do exist, it would have profound implications on the conditions in the very early universe.”

    By using gravitational waves to learn about the properties of black holes, LIGO might be able to prove or constrain this dark matter theory.

    Unlike normal black holes, primordial black holes don’t have a minimum mass threshold they need to reach in order to form. If LIGO were to see a black hole less massive than the sun, for example, it might be a primordial black hole.

    Even if primordial black holes do exist, it’s doubtful that they account for all of the dark matter in the universe. Still, finding proof of primordial black holes would expand our fundamental understanding of dark matter and how the universe began.

    Neutron star rattles

    Dark matter seems to interact with normal matter only through gravity, but, based on the way known particles interact, theorists think it’s possible that dark matter might also interact with itself.

    If that is the case, dark matter particles might bind together to form dark objects that are as compact as a neutron star.

    We know that stars drastically “weigh down” the fabric of spacetime around them. If the universe were populated with compact dark objects, there would be a chance that at least some of them would end up trapped inside of ordinary matter stars.

    A normal star and a dark object would interact only through gravity, allowing the two to co-exist without much of a fuss. But any disruption to the star—for example, a supernova explosion—could create a rattle-like disturbance between the resulting neutron star and the trapped dark object. If such an event occurred in our galaxy, it would create detectable gravitational waves

    “We understand neutron stars quite well,” says Sanjay Reddy, University of Washington physics professor and senior fellow with the Institute for Nuclear Theory. “If something ‘odd’ happens with gravitational waves, we would know there was potentially something new going on that might involve dark matter.”

    The likelihood that any exist in our solar system is limited. Chuck Horowitz, Maria Alessandra Papa and Reddy recently analyzed LIGO’s data and found no indication of compact dark objects of a specific mass range within Earth, Jupiter or the sun.

    Further gravitational-wave studies could place further constraints on compact dark objects. “Constraints are important,” says Ann Nelson, a physics professor at the University of Washington. “They allow us to improve existing theories and even formulate new ones.”

    Axion stars

    One light dark matter candidate is the axion, named by physicist Frank Wilczek after a brand of detergent, in reference to its ability to tidy up a problem in the theory of quantum chromodynamics.

    Scientists think it could be possible for axions to bind together into axion stars, similar to neutron stars but made up of extremely compact axion matter.

    “If axions exist, there are scenarios where they can cluster together and form stellar objects, like ordinary matter,” says Tim Dietrich, a LIGO-Virgo member and physicist. “We don’t know if axion stars exist, and we won’t know for sure until we find constraints for our models.”

    If an axion star merged with a neutron star, scientists might not be able to tell the difference between the two with their current instruments. Instead, scientists would need to rely on electromagnetic signals accompanying the gravitational wave to identify the anomaly.

    It’s also possible that axions could bunch around a binary black hole or neutron star system. If those stars then merged, the changes in the axion “cloud” would be visible in the gravitational wave signal. A third possibility is that axions could be created by the merger, an action that would be reflected in the signal.

    This month, the LIGO-Virgo collaborations began their third observing run and, with new upgrades, expect to detect a merger event every week.

    Gravitational-wave detectors have already proven their worth in confirming Einstein’s century-old prediction. But there is still plenty that studying gravitational waves can teach us. “Gravitational waves are like a completely new sense for science,” Ali-Haimoud says. “A new sense means new ways to look at all the big questions in physics.”

    See the full article here .


    Please help promote STEM in your local schools.

    Stem Education Coalition

    Symmetry is a joint Fermilab/SLAC publication.

  • richardmitnick 3:21 pm on April 9, 2019 Permalink | Reply
    Tags: , , Dark Matter, , , , , ,   

    From Symmetry: “A tiny new experiment at the LHC” 

    Symmetry Mag
    From Symmetry

    03/05/19 [Sorry, missed this one.]
    Caitlyn Buongiorno

    Illustration by Sandbox Studio, Chicago with Ana Kova

    The story of the latest experiment approved for installation at the Large Hadron Collider starts with a theorist and a question about dark matter.

    Jonathan Feng originally described himself as a high-energy-collider guy, specifically a high-energy-collider theorist. Then a well-placed question at a talk started him on a winding path from colliders to cosmology, from theory to experiment, and finally right back to high-energy physics where he began.

    That path led to today, when the CERN Research Board approved the experiment Feng recently co-founded, called FASER, for installation at the Large Hadron Collider.

    A 3D picture of the planned FASER detector as seen in the TI12 tunnel. The detector is precisely aligned with the collision axis in ATLAS, 480 m away from the collision point. (Image: FASER/CERN)

    Let’s start from the beginning. As a graduate student at SLAC National Accelerator Laboratory, Feng was studying supersymmetry, also called SUSY. SUSY predicts the existence of a whole host of massive new particles, which scientists continue to look for with experiments at accelerators like the LHC.

    Standard Model of Supersymmetry via DESY

    On a fateful visit to Fermi National Accelerator Laboratory, Feng gave a talk about his latest ideas for a model of supersymmetric particles. When he finished and transitioned into questions, someone in the audience pointed out a seemingly significant flaw: The existence of dark matter might have already negated his entire presentation.

    “As it turned out the model was okay, but that was really a wake-up call for me,” says Feng, now a professor at UC Irvine. “I realized I better start learning about dark matter and connecting it to supersymmetry.”

    Unlike evidence for supersymmetric particles, evidence for dark matter particles has already shown up in scientific observations. We know that dark matter is there because of the gravitational effects it has on galaxies, including our own. In fact, dark matter is five times as prevalent as visible matter and thought to make up the foundations upon which most galaxies are built.

    Caterpillar Project A Milky-Way-size dark-matter halo and its subhalos circled, an enormous suite of simulations . Griffen et al. 2016

    Fritz Zwicky discovered Dark Matter when observing the movement of the Coma Cluster. But Vera Rubin, Vera Rubin a Woman in STEM denied the Nobel, did most of the work on Dark Matter.

    Fritz Zwicky from http:// palomarskies.blogspot.com

    Coma cluster via NASA/ESA Hubble

    Astronomer Vera Rubin at the Lowell Observatory in 1965, worked on Dark Matter (The Carnegie Institution for Science)

    Vera Rubin measuring spectra, worked on Dark Matter (Emilio Segre Visual Archives AIP SPL)

    Vera Rubin, with Department of Terrestrial Magnetism (DTM) image tube spectrograph attached to the Kitt Peak 84-inch telescope, 1970. https://home.dtm.ciw.edu

    Despite the abundance of dark matter in the universe, scientists have not yet been able to directly observe it. They think that’s because, other than through the force of gravity, dark matter rarely interacts with normal matter.

    For decades, scientists have searched for dark matter particles like this, ones that interact only weakly with known particles. One type of weakly interacting massive particles, called WIMPs, has a possible connection with supersymmetry.

    Like dark matter particles, SUSY particles could also be weakly interacting and massive, a fact that makes theorists wonder if dark matter particles are SUSY particles. Feng in particular was intrigued by the fact that the predicted number of SUSY particles left over from the Big Bang and the number required to account for all the dark matter in the universe were essentially the same.

    Suddenly, instead of negating models of supersymmetry, dark matter was bolstering them.

    “It’s just an amazing coincidence,” Feng says. “I still think that it’s almost too good to not be relevant to nature.”

    Feng spent the next 10 years focused on popularizing this coincidence, which supported theories behind the search for WIMPs. But as time went on and WIMPs continued to prove elusive, Feng grew restless. In 2008, he began also focusing on other possibilities.

    One such possibility was a different kind of weakly interacting particle—this one light, not massive. Such a dark matter particle would be even more difficult to detect than a WIMP. But it could be that there are other particles, called portal particles, that could be the bridge between normal matter and this light dark matter. Portal particles would be capable of communicating with both, and they would be easier to detect than dark matter particles.

    These portal particles could be produced in the decays of light particles like pions or kaons. As Feng and three postdocs thought about where to look for portal particles in 2017, they realized there’s a place where the pions and kaons they might come from are produced in droves: the LHC.

    Every second, millions of protons are collided in the LHC. The energy from those collisions transforms the protons into a multitude of other particles. Those particles then speed off in all directions.

    Beams of particles are brought into collision at four different points along the Large Hadron Collider. The four large detectors—ATLAS, ALICE, CMS and LHCb—are built around those collision points. Artwork by Sandbox Studio, Chicago with Ana Kova.

    The LHC’s huge experiments are built to surround the places where the particle beams collide to give them the best chance of catching these particles. But Feng’s team realized that the LHC detectors had an important blind spot: straight down the beam pipes.

    During collisions, it could be that portal particles (labeled A’) are escaping detection by traveling down the beam pipe.
    Artwork by Sandbox Studio, Chicago with Ana Kova

    The LHC beam pipes travel in a circle, not a straight line. Magnets turn the beams of particles inside them at a very, very slight angle so that they can travel around in a ring over and over and potentially collide at four locations.

    The FASER collaboration discovered a disused tunnel, called TI12, in just the right location to intercept portal particles that could be escaping from collisions in the ATLAS detector. Artwork by Sandbox Studio, Chicago with Ana Kova.

    A portal particle coming from a kaon or pion created in a particle collision at the LHC would be neutral and therefore unaffected by the magnetic field, so it would continue in a straight line as the rest of the beam curved away. At a distance of 500 meters, the escaping particles would have spread out only 7 centimeters from one another, making it possible for a detector as small as a sheet of paper to catch almost all of them.

    The portal particles would continue traveling straight, unaffected by the magnets that bend beams of particles around the ring of the LHC. They would travel through the earth and interact within the FASER detector. Artwork by Sandbox Studio, Chicago with Ana Kova.

    Feng and postdocs Iftah Galon, Felix Kling and Sebastian Trojanowski pulled out a map of CERN and traced a straight line from the collision point inside the ATLAS detector. Just in the spot where the portal particles would appear, they found a tunnel, TI12, left over from the LEP collider that had previously inhabited the LHC’s underground home.

    “That was really exciting,” says Galon, postdoctoral associate at the New High Energy Theory Center at Rutgers University. Suddenly, the idea to detect particles that had potentially been escaping the LHC for years unnoticed by the gigantic detectors around them wasn’t just a fantasy, “it was actually feasible.”

    In August of 2017, Feng and his postdocs excitedly published a paper proposing a new experiment, pointing out this unused tunnel as the perfect location. They called it FASER, a slightly forced acronym for ForwArd Search ExpeRiment at the LHC. They expected an experimentalist to jump on the idea within a few weeks, a month at the most.

    By the time two months had passed, Feng says he realized that wasn’t going to happen.

    Instead of being deterred, Feng, Galon, Kling and Trojanowski continued reaching out to their contacts and giving talks about FASER. If they couldn’t inspire the experimentalists to take on their idea, they were going to have to get involved themselves.

    In February of 2018, Feng met a CERN research physicist named Jamie Boyd.

    “That was the big break, when Jamie got wind of this and got on board,” Feng says. “He’s been extraordinarily effective at putting together the experimental side of FASER.”

    Before the LHC even started producing collisions, Boyd was working on ATLAS, one of the two largest experiments at LHC. For over 10 years, he cultivated relationships and experience, making him the perfect person to campaign for FASER.

    He also realized that FASER would need help from other experiments.

    “With any experiment, you create a number of back-up parts,” Boyd says. These back-ups are kept around in case something happens to the main equipment. Instead of halting the entire experiment, scientists can simply replace faulty parts and continue taking data. Boyd realized that a few of the many copies of back-up parts created early on for ATLAS and LHCb could safely be donated to FASER instead.

    “Other experiments’ generous contributions is partially why FASER could get off the ground so quickly,” he says.

    Somewhat unusually for a group of theorists, Feng, Galon, Kling and Trojanowski became founding members of the FASER experiment, with Feng and Boyd serving as co-spokespersons.

    From there, things came together at a whirlwind pace. In July they had a conceptual design and a collaboration of 14 people. In October, the ATLAS and LHCb collaboartaions donated essential parts. In November, their team had jumped up to 25 people and had produced a technical design to propose to CERN. In Febraury, they secured full funding for construction from the Heising-Simons and Simons Foundations. And on March 5, the group received the final go ahead from the CERN Research Board to integrate FASER into the LHC schedule.

    The LHC is down for upgrades until late 2020, so FASER will need to be built, tested, installed and ready for operation by then.

    “We have a very clear and very hard deadline,” Boyd says. “Because FASER is small, the LHC won’t stop its beam to wait for us. We have to match the beam shutdown schedule, not the other way around.”

    FASER’s main purpose is to detect portal particles produced by the LHC, but it also stands to provide other important insights. It could find heavy versions of hypothetical particles called axions. Less massive versions of axions are dark matter candidates, but the axions FASER could detect would be too heavy and unstable to be dark matter.

    “We look at the world, we look at physics, and we ask ourselves where new physics could be hiding,” Galon says. “If FASER finds any new particles, then we’ve done our job correctly.”

    FASER could also catch neutrinos, other weakly interacting particles that scientists already know about but have yet to directly observe in detectors at the LHC. This would provide scientists with an opportunity to study a previously unexplored energy range of neutrinos and test our current understanding of how neutrinos interact.

    FASER scientists expect to have data to analyze starting in 2021 and hope to make significant contributions to physics by the end of their first three-year run.

    See the full article here .


    Please help promote STEM in your local schools.

    Stem Education Coalition

    Symmetry is a joint Fermilab/SLAC publication.

  • richardmitnick 1:57 pm on April 9, 2019 Permalink | Reply
    Tags: APEX at JLab, , Dark Matter, , ,   

    From Symmetry: “All hands on deck” 

    Symmetry Mag
    From Symmetry

    Ali Sundermier

    Illustration by Sandbox Studio, Chicago with Ana Kova

    Some theorists have taken to designing their own experiments to broaden the search for dark matter.

    From a young age, Philip Schuster knew he wanted to go into particle physics. As an undergraduate, he became involved in a number of research projects with experimentalists. But, like many other students pursuing a career in physics, he reached a point when he had to narrow his path.

    “When you’re going into graduate school, you have to make a very stark choice between going in the direction of theory or experiment,” says Schuster, now a theorist at SLAC National Accelerator Laboratory. “One of the reasons for that is that either one takes a tremendous investment and commitment of time. It’s just not practical to do both simultaneously.”

    As much as he enjoyed the hands-on feeling of experiments, Schuster felt a stronger pull down the theory route. But he continued to keep an eye on what was happening in the world of experiment.

    Falling through the cracks

    Toward the end of his graduate education around 2007, Schuster wound up embedded with an experimental group working with data from the Large Hadron Collider. Although his work was still theoretical, this experience rekindled an interest in experimental physics that carried through into his postdoctoral fellowship.

    Together with Natalia Toro, also a theorist SLAC, and Rouven Essig, a theorist at Stony Brook University, Schuster began developing a series of ideas for an experiment that could leverage existing equipment to look for new forces that might be related to dark matter. The three teamed up with Bogdan Wojtsekhowski, an experimentalist at Thomas Jefferson National Accelerator Facility, to co-lead the experiment, called A Prime Experiment, or APEX.


    At the time, spearheading experiments was considered a dangerous move for theorists. Many feared that physicists could end up falling into the cracks between theory and experiment, landing in a place where their work would be unappreciated by both sides. But the seemingly impossible hunt for dark matter called for new approaches.

    “We knew we were taking a risk,” Schuster says. “And because so few people were doing it at the time, the risk felt even more vivid.

    “I remember being a little worried about it from time to time. But whenever I stood back, I could see that we had this physics problem that was going to require both theory and new experiments to answer.”

    A deepening divide

    There wasn’t always so much distinction between experiment and theory in physics. From Galileo Galilei to Isaac Newton, many of the great physicists had to use both theory and experiment. But as the field expanded, so did the scale of the experiments and the complexity of the theory. The larger and more challenging the experiments grew, and the more elaborate the theories became, the higher the level of specialization and expertise scientists required to work on them.

    “At first it wasn’t so much a split as it was just a sharpening of roles,” Schuster says. “People who tended to be a little bit more inclined to mathematical modeling versus actually tinkering. But with the discovery and development of quantum mechanics, you really had to specialize in something to make any sort of progress. The divide deepened out of a necessity, and it just became much more entrenched with time.”

    New perspectives

    But in the past decade, a new trend has emerged. In the scramble to detect dark matter particles, more and more theorists have been dreaming up experiments that can tackle the problem from new perspectives.

    “Over the last few years, we’ve been going back to the drawing board,” says Mariangela Lisanti, a theorist at Princeton University. “There has been a renaissance in dark matter science that calls for a much closer collaboration between the two communities, so people have been moving closer to that boundary as a result.”

    A large part of this, Essig says, is that physicists have been expanding the type of dark matter candidates they’re interested in, requiring new ideas on how to find them.

    “Most of us go into science because we want to understand the world,” says David Spergel, a theoretical astrophysicist at Princeton University. “We want to be able to compare theoretical ideas with experiments, and there’s no better way to do that than to be directly involved in the experiment. I think it’s very valuable for us to ask the question, ‘What types of new experiments should be done to advance our knowledge of fundamental physics?’”

    Back to the basics

    To broaden the search for dark matter, physicists have gone back to the basics, in a way—designing smaller-scale experiments that can often fit on tabletops. These smaller and less expensive experimental setups and collaborations provide a perfect avenue for theorists to explore new ideas.

    “These little experiments are kind of moving into the mainstream, and that’s been a really good thing,” says Jonathan Feng, a theorist at University California, Irvine. “There are some really interesting ideas out there, and any one of them can actually discover dark matter or some new particle and just change our whole view of what’s going on.”

    Many of these small experiments are fueled by collaboration between theorists and experimentalists. Recently, Feng worked with experimentalists to design FASER, a small dark matter experiment sitting in the LHC tunnel that looks for exotic weakly interacting particles produced in collisions.

    A 3D picture of the planned FASER detector as seen in the TI12 tunnel. The detector is precisely aligned with the collision axis in ATLAS, 480 m away from the collision point. (Image: FASER/CERN)

    David Casper, an experimentalist involved in the project, says that Feng and the other theorists have been instrumental in the process.

    “This experiment was really their idea,” Casper says. “This wasn’t theorists becoming involved in experiment. It was experimentalists joining theorists to make their idea a reality.”

    Flooding the field

    This synergy between theorists and experimentalists in the hunt has been a driving force for why many physicists do what they do. Lisanti says she’s always been interested in flying close to the interface between the two disciplines.

    “Collaborating closely with experimentalists and thinking of new ways to shed light on patterns in the data is what I love spending my days doing,” she says. “I can’t imagine any other thing that would be more fun.”

    Now, the trend of theorists proposing experiments has become so common that it’s almost expected of new students entering the field. The hope is that flooding the field with new ideas could finally lead to the discovery of dark matter.

    “I was at a conference a few months ago and I heard a few people joking that you’re not a real theorist until you’ve done an experiment,” Feng says. “For a while people had this idea that theorists were only meant to devise high-minded, beautiful thoughts and theories of everything. But sometimes we need to get our hands dirty and make sure we’re covering as many bases as we can.”

    See the full article here .


    Please help promote STEM in your local schools.

    Stem Education Coalition

    Symmetry is a joint Fermilab/SLAC publication.

  • richardmitnick 9:21 am on March 30, 2019 Permalink | Reply
    Tags: "Physicists constrain dark matter", , , , , Dark Matter, , Moscow Institute of Physics and Technology,   

    From Moscow Institute of Physics and Technology via EurekAlert: “Physicists constrain dark matter” 

    MIPT bloc


    Varvara Bogomolova

    From Moscow Institute of Physics and Technology




    This image of Centaurus A, one of the closest active galaxies to Earth, combines the data from observations in multiple frequency ranges. Credit: ESO/WFI (optical), MPIfR/ESO/APEX/A. Weiss et al. (submillimeter), NASA/CXC/CfA/R. Kraft et al. (X-ray)

    Wide Field Imager on the 2.2 meter MPG/ESO telescope at Cerro LaSilla

    MPG/ESO 2.2 meter telescope at Cerro La Silla, Chile, 600 km north of Santiago de Chile at an altitude of 2400 metres

    ESO/MPIfR APEX high on the Chajnantor plateau in Chile’s Atacama region, at an altitude of over 4,800 m (15,700 ft)

    NASA/Chandra X-ray Telescope

    Researchers from Russia, Finland, and the U.S. have put a constraint on the theoretical model of dark matter particles by analyzing data from astronomical observations of active galactic nuclei. The new findings provide an added incentive for research groups around the world trying to crack the mystery of dark matter: No one is quite sure what it is made of. The paper was published in the Journal of Cosmology and Astroparticle Physics.

    The question of what particles make up dark matter is a crucial one for modern particle physics. Despite the expectations that dark matter particles would be discovered at the Large Hadron Collider, this did not happen. A number of then-mainstream hypotheses about the nature of dark matter had to be rejected. Diverse observations indicate that dark matter exists, but apparently something other than the particles in the Standard Model constitutes it. Physicists thus have to consider further options that are more complex. The Standard Model needs to be extended.

    Standard Model of Particle Physics (LATHAM BOYLE AND MARDUS OF WIKIMEDIA COMMONS)

    Among the candidates for inclusion are hypothetical particles that may have masses in the range from 10?²? to 10?¹? times the mass of the electron. That is, the heaviest speculated particle has a mass 40 orders of magnitude greater than that of the lightest.

    One theoretical model treats dark matter as being made up of ultralight particles. This offers an explanation for numerous astronomical observations. However, such particles would be so light that they would interact very weakly with other matter and light, making them exceedingly hard to study. It is almost impossible to spot a particle of this kind in a lab, so researchers turn to astronomical observations.

    “We are talking about dark matter particles that are 28 orders of magnitude lighter than the electron. This notion is critically important for the model that we decided to test. The gravitational interaction is what betrays the presence of dark matter. If we explain all the observed dark matter mass in terms of ultralight particles, that would mean there is a tremendous number of them. But with particles as light as these, the question arises: How do we protect them from acquiring effective mass due to quantum corrections? Calculations show that one possible answer would be that these particles interact weakly with photons — that is, with electromagnetic radiation. This offers a much easier way to study them: by observing electromagnetic radiation in space,” said Sergey Troitsky, a co-author of the paper and chief researcher at the Institute for Nuclear Research of the Russian Academy of Sciences.

    When the number of particles is very high, instead of individual particles, you can treat them as a field of certain density permeating the universe. This field coherently oscillates over domains that are on the order of 100 parsecs in size, or about 325 light years. What determines the oscillation period is the mass of the particles. If the model considered by the authors is correct, this period should be about one year. When polarized radiation passes through such a field, the plane of radiation polarization oscillates with the same period. If periodic changes like this do in fact occur, astronomical observations can reveal them. And the length of the period — one terrestrial year — is very convenient, because many astronomical objects are observed over several years, which is enough for the changes in polarization to manifest themselves.

    The authors of the paper decided to use the data from Earth-based radio telescopes, because they return to the same astronomical objects many times during a cycle of observations. Such telescopes can observe remote active galactic nuclei — regions of superheated plasma close to the centers of galaxies. These regions emit highly polarized radiation. By observing them, one can track the change in polarization angle over several years.

    “At first it seemed that the signals of individual astronomical objects were exhibiting sinusoidal oscillations. But the problem was that the sine period has to be determined by the dark matter particle mass, which means it must be the same for every object. There were 30 objects in our sample. And it may be that some of them oscillated due to their own internal physics, but anyway, the periods were never the same,” Troitsky goes on. “This means that the interaction of our ultralight particles with radiation may well be constrained. We are not saying such particles do not exist, but we have demonstrated that they don’t interact with photons, putting a constraint on the available models describing the composition of dark matter.”

    “Just imagine how exciting that was! You spend years studying quasars, when one day theoretical physicists turn up, and the results of our high-precision and high angular resolution polarization measurements are suddenly useful for understanding the nature of dark matter,” enthusiastically adds Yuri Kovalev, a co-author of the study and laboratory director at the Moscow Institute of Physics and Technology and Lebedev Physical Institute of the Russian Academy of Sciences.

    In the future, the team plans to search for manifestations of hypothesized heavier dark matter particles proposed by other theoretical models. This will require working in different spectral ranges and using other observation techniques. According to Troitsky, the constraints on alternative models are more stringent.

    “Right now, the whole world is engaged in the search for dark matter particles. This is one of the great mysteries of particle physics. As of today, no model is accepted as favored, better-developed, or more plausible with regard to the available experimental data. We have to test them all. Inconveniently, dark matter is “dark” in the sense that it hardly interacts with anything, particularly with light. Apparently, in some scenarios it could have a slight effect on light waves passing through. But other scenarios predict no interactions at all between our world and dark matter, other than those mediated by gravity. This would make its particles very hard to find,” concludes Troitsky.

    See the full article here .


    Please help promote STEM in your local schools.

    Stem Education Coalition

    MIPT Bldg

    Moscow Institute of Physics and Technology is a leading Russian university that trains students in various fields of modern science and technology.
    MIPT was set up on September 17, 1951 by Resolution#3517-1635 of the Soviet Cabinet of Ministers on the basis of the Department of Physics and Technology at the Lomonosov Moscow State University. The department started working on November 25, 1946. On October 1, 1951, the resolution was approvedby executive order of the Soviet Education Ministry. On November 2, 2009, MIPT was granted the status of National Research University by the Russian government.
    MIPT has a very rich history. Its founders included academicians Pyotr Kapitsa, Nikolay Semenov and Sergey Khristianovich. Its first professors were Nobel Prize winners Kapitsa, Semenov and Lev Landau, and its first rector was Ivan Petrov. There are Nobel Prize winners among MIPT’s graduates as well. Many MIPT professors are leading Russian scientists, including over 80 members of the Russian Academy of Sciences.
    From the outset, MIPT has used a unique system for training specialists, known as the Phystech System, which combines fundamental science, engineering disciplines and student research.
    With a history rich in major events and longstanding traditions, MIPT pays well-deserved attention to its symbols. MIPT has an original emblem, which embodies its devotion to science.
    MIPT Emblem
    Every 5 years MIPT marks two anniversaries, celebrating the creation of the Department of Physics and Technology at Moscow State University on November 25, 1946 and the creation of Moscow Institute of Physics and Technology, which took place five years later.

  • richardmitnick 2:35 pm on March 21, 2019 Permalink | Reply
    Tags: "The Milky Way Contains the Mass of 1.5 Trillion Suns", , , , , , Dark Matter, Milkdromeda   

    From Sky & Telescope: “The Milky Way Contains the Mass of 1.5 Trillion Suns” 

    SKY&Telescope bloc

    From Sky & Telescope

    March 18, 2019
    Monica Young

    Astronomers are using Gaia and the Hubble Space Telescope to make the most precise measure of the Milky Way’s mass to date. The new result puts our galaxy on par with — if not more massive than — Andromeda Galaxy.

    ESA/GAIA satellite

    NASA/ESA Hubble Telescope

    The mass of the Milky Way has long been debated, to the point that we don’t even know where it stands in the Local Group of galaxies.

    Local Group. Andrew Z. Colvin 3 March 2011

    Is it the heavyweight champion, or does our sister galaxy, Andromeda, outweigh us?

    Andromeda Galaxy Adam Evans

    Laura Watkins (Space Telescope Science Institute) and colleagues have used data recently released by the European Space Agency’s Gaia satellite, as well as roughly ten years of Hubble Space Telescope observations, to peg the motions of 46 tightly packed bunches of stars. Known as globular clusters, their orbits help pin down the Milky Way’s mass.

    This artist’s impression shows a computer generated model of the Milky Way and the accurate positions of the globular clusters used in this study surrounding it.
    ESA / Hubble, NASA / L. Calçada

    Milky Way NASA/JPL-Caltech /ESO R. Hurt

    Our galaxy’s gravitational pull determines the clusters’ movements, explains coauthor N. Wyn Evans (University of Cambridge, UK). If our galaxy is more massive, the clusters will move faster under the stronger pull of its gravity. The key is to understand exactly how fast the clusters are moving.

    Many previous measurements have measured the speed at which a cluster is approaching or receding from Earth. “However,” Evans says, “we were able to also measure the sideways motion of the clusters, from which the total velocity, and consequently the galactic mass, can be calculated.”

    The team finds a mass equivalent to 1.5 trillion Suns. The results appear in The Astrophysical Journal.

    The Milky Way’s disk of stars (labeled here as “thin disk”) are relatively insignificant to the galaxy’s massive dark matter halo.
    NASA / ESA / A. Feild

    Milky Way Dark Matter Halo. Jürg Diemand, UCSC/UCO/ Lick

    A Tricky Scale

    Astronomers have been fussing over the mass of the Milky Way the way parents fuss over their newborns. Understandably so: Just as a baby’s weight serves as an indicator of more important things, like their growth and well-being, the heft of our galaxy affects everything from our understanding of its formation to the nature of dark matter.

    But while the pediatrician will usually tell you your baby’s weight to within a percent (equivalent to a tenth of an ounce if you’re in the U.S.), the Milky Way’s mass is known only to within a factor of two. Imagine putting your newborn on the scale, only to have the needle waver between 5 and 10 — is baby failing to thrive? Or doing just fine? The uncertainty would render the result meaningless.

    On the galactic scale, of course, there are a few more zeroes involved: Over the years, astronomers have found that the Milky Way’s mass is somewhere between 0.5 trillion and 3 trillion Suns. There are plenty of reasons for the large range. First, studying our galaxy is difficult because we’re inside of it; things like dust or the galactic plane of stars can block our view. Second, even when astronomers trace the orbits of objects — such as globular clusters — measuring their motion across the sky is trickier. It takes many years of observations to nail down their so-called proper motions. That’s what Watkins and her colleagues have done, using dedicated Hubble programs that have monitored stellar motions over roughly 10 years, as well as the second data release from the Gaia mission that has been monitoring stars since 2014.

    By far the trickiest part of the problem, though, is that much of the mass astronomers are trying to measure can’t be seen. The bulk of the Milky Way is in dark matter, not stars. Moreover, the Milky Way’s dark matter halo may extend 1 million light-years out from the galaxy’s center. Even if astronomers follow the orbit of a globular cluster around the galaxy, it will only reveal the mass inside its orbit. The farthest globular cluster in Watkins’s study is out at 130,000 light-years. To measure the mass beyond that distance, the astronomers must make some assumptions about the nature and shape of the dark matter halo.

    A More Exact Mass

    The globular cluster NGC 4147 is about 60,000 light-years from Earth.
    ESA / Hubble / NASA / T. Sohn et al.

    Nevertheless, the new measurement is so precise that it has helped narrow things down. “Together with another analysis of similar data by Posti & Helmi, this [Astronomy and Astrophysics] has tipped the scale towards a heavier Milky Way,” says Ana Bonaca (Harvard-Smithsonian Center for Astrophysics), who was not involved in the study. “Thanks to these studies, we now know that a very low value for the mass of the Milky Way is unlikely.”

    For astronomers, this new mass estimate will be most relevant for understanding the Milky Way’s swarm of satellite galaxies. For the rest of us: Phew — we’re not smaller than Andromeda after all!

    There’s still work to be done, though. The ideal tracer would be in the outer halo, Bonaca notes, out beyond 300,000 light-years. The trick is finding something that far out that we can still see, such as globular clusters, dwarf galaxies, or even streams of stars that the Milky Way’s gravity has torn from an infalling cluster or dwarf. Watkins and colleagues for their part think it’s likely that Gaia will continue to estimate the motions of many more globular clusters. No doubt, researchers will continue to narrow down the Milky Way’s mass using this and other methods for some time to come.

    Milkdromeda -Andromeda on the left-Earth’s night sky in 3.75 billion years-NASA

    See the full article here .


    Please help promote STEM in your local schools.

    Stem Education Coalition

    Sky & Telescope magazine, founded in 1941 by Charles A. Federer Jr. and Helen Spence Federer, has the largest, most experienced staff of any astronomy magazine in the world. Its editors are virtually all amateur or professional astronomers, and every one has built a telescope, written a book, done original research, developed a new product, or otherwise distinguished him or herself.

    Sky & Telescope magazine, now in its eighth decade, came about because of some happy accidents. Its earliest known ancestor was a four-page bulletin called The Amateur Astronomer, which was begun in 1929 by the Amateur Astronomers Association in New York City. Then, in 1935, the American Museum of Natural History opened its Hayden Planetarium and began to issue a monthly bulletin that became a full-size magazine called The Sky within a year. Under the editorship of Hans Christian Adamson, The Sky featured large illustrations and articles from astronomers all over the globe. It immediately absorbed The Amateur Astronomer.

    Despite initial success, by 1939 the planetarium found itself unable to continue financial support of The Sky. Charles A. Federer, who would become the dominant force behind Sky & Telescope, was then working as a lecturer at the planetarium. He was asked to take over publishing The Sky. Federer agreed and started an independent publishing corporation in New York.

    “Our first issue came out in January 1940,” he noted. “We dropped from 32 to 24 pages, used cheaper quality paper…but editorially we further defined the departments and tried to squeeze as much information as possible between the covers.” Federer was The Sky’s editor, and his wife, Helen, served as managing editor. In that January 1940 issue, they stated their goal: “We shall try to make the magazine meet the needs of amateur astronomy, so that amateur astronomers will come to regard it as essential to their pursuit, and professionals to consider it a worthwhile medium in which to bring their work before the public.”

  • richardmitnick 8:35 am on March 15, 2019 Permalink | Reply
    Tags: "How Much Of The Dark Matter Could Neutrinos Be?", , , , , Dark Matter, , , Neutrinos are the only Standard Model particles that behave like dark matter should. But they can’t be the full story   

    From Ethan Siegel: “How Much Of The Dark Matter Could Neutrinos Be?” 

    From Ethan Siegel
    Mar 14, 2019

    They’re the only Standard Model particles that behave like dark matter should. But they can’t be the full story.

    While the web of dark matter (purple) might seem to determine cosmic structure formation on its own, the feedback from normal matter (red) can severely impact galactic scales. Both dark matter and normal matter, in the right ratios, are required to explain the Universe as we observe it. Neutrinos are ubiquitous, but standard, light neutrinos cannot account for most (or even a significant fraction) of the dark matter. (ILLUSTRIS COLLABORATION / ILLUSTRIS SIMULATION)

    All throughout the Universe, there’s more than what we’re capable of seeing. When we look out at the stars moving around within galaxies, the galaxies moving withing groups and clusters, or the largest structures of all that make up the cosmic web, everything tells the same disconcerting story: we don’t see enough matter to explain the gravitational effects that occur. In addition to the stars, gas, plasma, dust, black holes and more, there must be something else in there causing an additional gravitational effect.

    Traditionally, we’ve called this dark matter, and we absolutely require it to explain the full suite of observations throughout the Universe. While it cannot be made up of normal matter — things made of protons, neutrons, and electrons — we do have a known particle that could have the right behavior: neutrinos. Let’s find out how much of the dark matter neutrinos could possibly be.

    The neutrino was first proposed in 1930, but was not detected until 1956, from nuclear reactors. In the years and decades since, we’ve detected neutrinos from the Sun, from cosmic rays, and even from supernovae. Here, we see the construction of the tank used in the solar neutrino experiment in the Homestake gold mine from the 1960s.(BROOKHAVEN NATIONAL LABORATORY)

    At first glance, neutrinos are the perfect dark matter candidate. They barely interact at all with normal matter, and neither absorb nor emit light, meaning that they won’t generate an observable signal capable of being picked up by telescopes. At the same time, because they interact through the weak force, it’s inevitable that the Universe created enormous numbers of them in the extremely early, hot stages of the Big Bang.

    We know that there are leftover photons from the Big Bang, and very recently we’ve also detected indirect evidence that there are leftover neutrinos as well. Unlike the photons, which are massless, it’s possible that neutrinos have a non-zero mass. If they have the right value for their mass based on the total number of neutrinos (and antineutrinos) that exist, they could conceivably account for 100% of the dark matter.

    The largest-scale observations in the Universe, from the cosmic microwave background [CMB]to the cosmic web to galaxy clusters to individual galaxies, all require dark matter to explain what we observe. The large-scale structure requires it, but the seeds of that structure, from the Cosmic Microwave Background, require it too. (CHRIS BLAKE AND SAM MOORFIELD)

    CMB per ESA/Planck

    ESA/Planck 2009 to 2013

    So how many neutrinos are there? That depends on the number of types (or species) of neutrino.

    Although we can detect neutrinos directly using enormous tanks of material designed to capture their rare interactions with matter, this is both incredibly inefficient and is only going to capture a tiny fraction of them. We can see neutrinos that are the result of particle accelerators, nuclear reactors, fusion reactions in the Sun, and cosmic rays interacting with our planet and atmosphere. We can measure their properties, including how they transform into one another, but not the total number of types of neutrino.

    In this illustration, a neutrino has interacted with a molecule of ice, producing a secondary particle — a muon — that moves at relativistic speed in the ice, leaving a trace of blue light behind it. Directly detecting neutrinos has been a herculean but successful effort, and we are still trying to puzzle out the full suite of their nature. (NICOLLE R. FULLER/NSF/ICECUBE)

    U Wisconsin ICECUBE neutrino detector at the South Pole

    But there is a way to make the critical measurement from particle physics, and it comes from a rather unexpected place: the decay of the Z-boson. The Z-boson is the neutral boson that mediates the weak interaction, enabling certain types of weak decays. The Z couples to both quarks and leptons, and whenever you produce one in a collider experiment, there’s a chance that it will simply decay into two neutrinos.

    Those neutrinos are going to be invisible! We cannot typically detect the neutrinos we create from particle decays in colliders, as it would take a detector with the density of a neutron star to capture them. But by measuring what percentage of the decays produce “invisible” signals, we can infer how many types of light neutrino (whose mass is less than half the Z-boson mass) there are. It’s a spectacular and unambiguous result known for decades now: there are three.

    Standard Model of Particle Physics (LATHAM BOYLE AND MARDUS OF WIKIMEDIA COMMONS)

    This diagram displays the structure of the Standard Model, illustrating the key relationships and patterns. In particular, this diagram depicts all of the particles in the Standard Model, the role of the Higgs boson, and the structure of electroweak symmetry breaking, indicating how the Higgs vacuum expectation value breaks electroweak symmetry, and how the properties of the remaining particles change as a consequence. Note that the Z-boson couples to both quarks and leptons, and can decay through neutrino channels. (LATHAM BOYLE AND MARDUS OF WIKIMEDIA COMMONS)

    Coming back to dark matter, we can calculate, based on all the different signals we see, how much extra dark matter is necessary to give us the right amount of gravitation. In every way we know how to look, including:

    from colliding galaxy clusters,
    from galaxies moving within X-ray emitting clusters,
    from the fluctuations in the cosmic microwave background,
    from the patterns found in the large-scale structure of the Universe,
    and from the internal motions of stars and gas within individual galaxies,

    we find that we require about five times the abundance of normal matter to exist in the form of dark matter. It’s a great success of dark matter for modern cosmology that just by adding one ingredient to solve one puzzle, a whole slew of other observational puzzles are also solved.

    Four colliding galaxy clusters, showing the separation between X-rays (pink) and gravitation (blue), indicative of dark matter. On large scales, cold dark matter is necessary, and no alternative or substitute will do.(X-RAY: NASA/CXC/UVIC./A.MAHDAVI ET AL. OPTICAL/LENSING: CFHT/UVIC./A. MAHDAVI ET AL. (TOP LEFT); X-RAY: NASA/CXC/UCDAVIS/W.DAWSON ET AL.; OPTICAL: NASA/ STSCI/UCDAVIS/ W.DAWSON ET AL. (TOP RIGHT); ESA/XMM-NEWTON/F. GASTALDELLO (INAF/ IASF, MILANO, ITALY)/CFHTLS (BOTTOM LEFT); X-RAY: NASA, ESA, CXC, M. BRADAC (UNIVERSITY OF CALIFORNIA, SANTA BARBARA), AND S. ALLEN (STANFORD UNIVERSITY) (BOTTOM RIGHT))

    NASA/Chandra X-ray Telescope

    CFHT Telescope, Maunakea, Hawaii, USA, at Maunakea, Hawaii, USA,4,207 m (13,802 ft) above sea level

    NASA/ESA Hubble Telescope

    ESA/XMM Newton

    If you have three species of light neutrino, it would only take a relatively small amount of mass to account for all the dark matter: a few electron-Volts (about 3 or 4 eV) per neutrino would do it. The lightest particle found in the Standard Model besides the neutrino is the electron, and that has a mass of about 511 keV, or hundreds of thousands of times the neutrino mass we want.

    Unfortunately, there are two big problems with having light neutrinos that are that massive. When we look in detail, the idea of massive neutrinos is insufficient to make up 100% of the dark matter.

    A distant quasar will have a big bump (at right) coming from the Lyman-series transition in its hydrogen atoms. To the left, a series of lines known as a forest appears. These dips are due to the absorption of intervening gas clouds, and the fact that the dips have the strengths they do place constraints on the temperature of dark matter. It cannot be hot. (M. RAUCH, ARAA V. 36, 1, 267 (1998))

    The first problem is that neutrinos, if they are the dark matter, would be a form of hot dark matter. You might have heard the phrase “cold dark matter” before, and what it means is that the dark matter must be moving slowly compared to the speed of light at early times.


    If dark matter were hot, and moving quickly, it would prevent the gravitational growth of small-scale structure by easily streaming out of it. The fact that we form stars, galaxies, and clusters of galaxies so early rules this out. The fact that we see the weak lensing signals we do rules this out. The fact that we see the pattern of fluctuations in the cosmic microwave background rules this out. And direct measurements of clouds of gas in the early Universe, through a technique known as the Lyman-α forest, definitively rule this out. Dark matter cannot be hot.

    The dark matter structures which form in the Universe (left) and the visible galactic structures that result (right) are shown from top-down in a cold, warm, and hot dark matter Universe. From the observations we have, at least 98%+ of the dark matter must be cold. (ITP, UNIVERSITY OF ZURICH)

    A number of collaborations have measured the oscillations of one species of neutrinos to another, and this enables us to infer the mass differences between the different types. Since the 1990s, we’ve been able to infer that the mass difference between two of the species are on the order of about 0.05 eV, and the mass difference between a different two species is approximately 0.009 eV. Direct constraints on the mass of the electron neutrino come from tritium decay experiments, and show that the electron neutrino must be less massive than about 2 eV.

    A neutrino event, identifiable by the rings of Cerenkov radiation that show up along the photomultiplier tubes lining the detector walls, showcase the successful methodology of neutrino astronomy. This image shows multiple events, and is part of the suite of experiments paving our way to a greater understanding of neutrinos. (SUPER KAMIOKANDE COLLABORATION)

    Super-Kamiokande experiment. located under Mount Ikeno near the city of Hida, Gifu Prefecture, Japan

    Beyond that, the cosmic microwave background [CMB [above] (from Planck [above]) and the large-scale structure data (from the Sloan Digital Sky Survey) tells us that the sum of all the neutrino masses is at most approximately 0.1 eV, as too much hot dark matter would definitively affect these signals. From the best data we have, it appears that the mass values that the known neutrinos have are very close to the lowest values that the neutrino oscillation data implies.

    In other words, only a tiny fraction of the total amount of dark matter is allowed to be in the form of light neutrinos. Given the constraints we have today, we can conclude that approximately 0.5% to 1.5% of the dark matter is made up of neutrinos. This isn’t insignificant; the light neutrinos in the Universe have about the same mass as all the stars in the Universe. But their gravitational effects are minimal, and they cannot make up the needed dark matter.

    THE The Sudbury neutrino observatory, which was instrumental in demonstrating neutrino oscillations and the massiveness of neutrinos. With additional results from atmospheric, solar, and terrestrial observatories and experiments, we may not be able to explain the full suite of what we’ve observed with only 3 Standard Model neutrinos, and a sterile neutrino could still be very interesting as a cold dark matter candidate. (A. B. MCDONALD (QUEEN’S UNIVERSITY) ET AL.,SUDBURY NEUTRINO OBSERVATORY INSTITUTE

    There is an exotic possibility, however, that means we might still have a chance for neutrinos to make a big splash in the world of dark matter: it’s possible that there’s a new, extra type of neutrino. Sure, we have to fit in with all the constraints from particle physics and cosmology that we have already, but there’s a way to make that happen: to demand that if there’s a new, extra neutrino, it’s sterile.

    A sterile neutrino has nothing to do with its gender or fertility; it merely means that it doesn’t interact through the conventional weak interactions today, and that a Z-boson won’t couple to it. But if neutrinos can oscillate between the conventional, active types and a heavier, sterile type, it could not only behave as though it were cold, but could make up 100% of the dark matter. There are experiments that are completed, like LSND and MiniBooNe, as well as experiments planned or in process, like MicroBooNe, PROSPECT, ICARUS and SBND, that are highly suggestive of sterile neutrinos being a real, important part of our Universe.

    LSND experiment at Los Alamos National Laboratory and Virginia Tech>



    Yale PROSPECT Neutrino experiment

    Yale PROSPECT—A Precision Oscillation and Spectrum Experiment

    INFN Gran Sasso ICARUS, since moved to FNAL


    FNAL Short Baseline Neutrino Detector [SBND]

    Scheme of the MiniBooNE experiment at FNAL

    A high-intensity beam of accelerated protons is focused onto a target, producing pions that decay predominantly into muons and muon neutrinos. The resulting neutrino beam is characterized by the MiniBooNE detector. (APS / ALAN STONEBRAKER)

    If we restrict ourselves to the Standard Model alone, we simply cannot account for the dark matter that must be present in our Universe. None of the particles we know of have the right behavior to explain all of the observations. We can imagine a Universe where neutrinos have relatively large amounts of mass, and that would result in a Universe with significant quantities of dark matter. The only problem is that dark matter would be hot, and lead to an observably different Universe than the one we see today.

    Still, the neutrinos we know of do behave like dark matter, although it only makes up about 1% of the total dark matter out there. That’s not totally insignificant; it equals the mass of all the stars in our Universe! And most excitingly, if there truly is a sterile neutrino species out there, a series of upcoming experiments ought to reveal it over the next few years. Dark matter might be one of the greatest mysteries out there, but thanks to neutrinos, we have a chance at understanding it at least a little bit.

    See the full article here .


    Please help promote STEM in your local schools.

    Stem Education Coalition

    “Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

  • richardmitnick 10:52 am on March 7, 2019 Permalink | Reply
    Tags: "The hypothetical effect we are investigating is not the result of increased gravity" Budker said., "What if It's Not Dark Matter Making The Universe's Extra 'Gravity' But Light?", As we move out from the galactic centre the orbital motion of the stars and gas in the disc should theoretically slow down with the decrease in velocity proportional to the distance from the centre., , , , But that something might not be dark matter according to a team of researchers specifically plasma physicist Dmitri Ryutov retired from the Lawrence Livermore National Laboratory in California, But unless all our current understanding about the physical Universe (and all the data we've collected on the phenomenon is wrong) something out there is definitely making extra gravity., , Dark Matter, For now dark matter is still king. But there's no harm and potentially a lot of good in looking for other explanations too., , So astrophysicists hypothesised dark matter. We don't know what it is and we can't detect it directly., So the theory would need a bit of work to be compatible with our actual observations of the Universe., , What if it's the mass of light?, When placed in the context of a mathematical system called Maxwell-Proca electrodynamics these electromagnetic stresses can generate additional centripetal forces   

    From Science Alert: “What if It’s Not Dark Matter Making The Universe’s Extra ‘Gravity’, But Light?” 


    From Science Alert

    7 MAR 2019

    (NASA/ESA/ Hubble)

    NASA/ESA Hubble Telescope

    We’ve been looking for decades for dark matter, yet the mysterious stuff remains undetectable to our instruments. Now, astrophysicists have explored an intriguing possibility: what if it’s not dark matter that’s affecting galactic rotation after all. What if it’s the mass of light instead?

    In a 1980 paper [The Astrophysical Journal], the American astronomer Vera Rubin pretty conclusively proved something really weird about galaxies: their rims are rotating far faster than they should be.

    Fritz Zwicky discovered Dark Matter when observing the movement of the Coma Cluster. But Vera Rubin, Vera Rubin a Woman in STEM denied the Nobel, did most of the work on Dark Matter.

    Fritz Zwicky from http:// palomarskies.blogspot.com

    Coma cluster via NASA/ESA Hubble

    Astronomer Vera Rubin at the Lowell Observatory in 1965, worked on Dark Matter (The Carnegie Institution for Science)

    Vera Rubin measuring spectra, worked on Dark Matter (Emilio Segre Visual Archives AIP SPL)

    Vera Rubin, with Department of Terrestrial Magnetism (DTM) image tube spectrograph attached to the Kitt Peak 84-inch telescope, 1970. https://home.dtm.ciw.edu

    As we move out from the galactic centre, the orbital motion of the stars and gas in the disc should theoretically slow down, with the decrease in velocity proportional to the distance from the centre.

    This is called Keplerian decline, or decreasing rotation curve, and it can be observed quite neatly in planetary systems like our own Solar System. But most galaxies don’t actually do this.

    Instead, their rotation curves either remain flat, or actually increase. Those outer stars are orbiting much more quickly than they should be, based on the gravitational effect of the matter we can observe.

    So astrophysicists hypothesised dark matter. We don’t know what it is, and we can’t detect it directly. But unless all our current understanding about the physical Universe (and all the data we’ve collected on the phenomenon is wrong), something out there is definitely making extra gravity.

    But that something might not be dark matter, according to a team of researchers – specifically, plasma physicist Dmitri Ryutov, who recently retired from the Lawrence Livermore National Laboratory in California, and Dmitry Budker and Victor Flambaum of the Johannes Gutenberg University of Mainz in Germany.

    In a new paper [The Astrophysical Journal], they lay out an argument that light particles (photons) are at least partially the source of the phenomenon – causing an effect that isn’t gravity, but behaves a heck of a lot like it.

    “The hypothetical effect we are investigating is not the result of increased gravity,” Budker said.

    “By assuming a certain photon mass, much smaller than the current upper limit, we can show that this mass would be sufficient to generate additional forces in a galaxy and that these forces would be roughly large enough to explain the rotation curves. This conclusion is extremely exciting.”

    The effect they describe is a sort of “negative pressure” caused by electromagnetic stresses related to the photon mass.

    When placed in the context of a mathematical system called Maxwell-Proca electrodynamics, these electromagnetic stresses can generate additional centripetal forces, acting predominantly on interstellar gas. The team calls this Proca stress, and it acts a lot like gravity.

    So, yes, it’s all purely hypothetical at this point. And it’s not perfect.

    On the one hand, short-lived stars that are born from gas (and rapidly return to gas before completing one orbit) would be strongly coupled with the gas; the Proca stresses acting on the gas would be indirectly also acting on these stars.

    But longer-lived stars create a problem. The Sun, for example, is around 4.6 billion years old, and orbits the galactic centre once every 230 million years, so it’s had a few turns on the roundabout. According to the team’s calculations, it should have a highly elliptical orbit under Proca stresses.

    And yet it does not. So the theory would need a bit of work to be compatible with our actual observations of the Universe. For now, dark matter is still king. But there’s no harm, and potentially a lot of good, in looking for other explanations too.

    “We don’t currently consider photon mass to be the solution to the rotation-curve problem. But it could be part of the solution,” Budker said.

    “However, we need to keep an open mind as long as we do not actually know what dark matter is.”

    See the full article here .


    Please help promote STEM in your local schools.

    Stem Education Coalition

  • richardmitnick 5:12 pm on March 5, 2019 Permalink | Reply
    Tags: As one of the main outstanding questions in fundamental physics the identification of the nature of dark matter is a key scientific driver for the future of particle physics., Because of gravitational lensing an effect related to Einstein’s general theory of relativity matter that stands between a light source and its observer can bend the light from the source so that th, , Dark Matter, From comparing the known position of the source (e.g. obtained through direct emission of visible particles from the source) to its distorted image one can reconstruct the distribution of the matter c, Invisible particles can be detected in ATLAS as they recoil against the visible ones (in this case the jet of particles), It is through the gravitational effect of dark matter on other matter in space that astronomers inferred its existence, Many astronomers had been observing the motion of galaxies and found a discrepancy with respect to their expectation that only accounted for matter that was emitting light. This was corroborated in th, More recently supercomputer simulations of the structure of our universe show that only including visible matter will not reproduce the structures that are observed in the universe while if dark matte, The first evidence for the existence of dark matter came as early as the 1930s, The most popular example of a more complete theory that includes a dark matter candidate is supersymmetry (SUSY), The presence of dark matter and its amount in the universe can also be inferred from the variations of temperature in the early universe. This leftover amount of dark matter is called its “relic den, Using WIMP models as our starting point for LHC searches doesn’t mean that we are bound to the idea that dark matter should be described with a single particle and a single interaction!   

    From CERN ATLAS: “Searching for Dark Matter with the ATLAS detector” 

    CERN/ATLAS detector

    CERN ATLAS Higgs Event


    5th March 2019
    Caterina Doglioni
    Dan Tovey

    Figure 1: An event with a highly energetic jet of particles and no other significant visible energy (monojet) recorded in 2016 by the ATLAS detector. This is how invisible particles can be detected in ATLAS, as they recoil against the visible ones (in this case, the jet of particles). The direction of the invisible particle is indicated by the dashed line. (Image: ATLAS Collaboration/CERN)

    When we look around us, at all the things we can touch and see – all of this is visible matter. And yet, this makes up less than 5% of the universe.

    We now know that the vast majority of matter is dark. This dark matter does not emit or reflect light, nor have we yet observed any known particle interacting with it. It is through the gravitational effect of dark matter on other matter in space that astronomers inferred its existence.

    The first evidence for the existence of dark matter came as early as the 1930s[1].

    Fritz Zwicky discovered Dark Matter when observing the movement of the Coma Cluster.

    Fritz Zwicky from http:// palomarskies.blogspot.com

    Coma cluster via NASA/ESA Hubble

    But most of the real work was done by Vera Rubin a Woman in STEM

    Astronomer Vera Rubin at the Lowell Observatory in 1965, worked on Dark Matter (The Carnegie Institution for Science)

    Vera Rubin measuring spectra, worked on Dark Matter (Emilio Segre Visual Archives AIP SPL)

    Vera Rubin, with Department of Terrestrial Magnetism (DTM) image tube spectrograph attached to the Kitt Peak 84-inch telescope, 1970. https://home.dtm.ciw.edu

    Many astronomers had been observing the motion of galaxies, and found a discrepancy with respect to their expectation that only accounted for matter that was emitting light. This was corroborated in the 70s through observations of the rotational velocity of galaxies made by Vera Rubin and collaborators.

    Figure 2: Percentage of ordinary matter, dark matter and dark energy in the universe, as measured by the Planck satellite. (Image: E. Ward/ATLAS Collaboration, Credit: ESA and the Planck Collaboration)

    Because of gravitational lensing, an effect related to Einstein’s general theory of relativity, matter that stands between a light source and its observer can bend the light from the source so that the observed image is distorted.

    Gravitational Lensing NASA/ESA

    From comparing the known position of the source (e.g. obtained through direct emission of visible particles from the source) to its distorted image, one can reconstruct the distribution of the matter causing the distortion. Observations of gravitational lensing also pointed to additional matter with respect to what was visible.

    More recently, supercomputer simulations of the structure of our universe show that only including visible matter will not reproduce the structures that are observed in the universe, while if dark matter is included then a closer agreement is obtained between observations and simulations.

    The presence of dark matter and its amount in the universe can also be inferred from the variations of temperature in the early universe. This leftover amount of dark matter is called its “relic density”, and it amounts to about 27% of the matter-energy content of the universe.

    However, none of the observations or simulations involving dark matter give a clear indication of what dark matter is made of. We only know that if dark matter is a particle[2], then it must have mass, since it interacts with other matter through the force of gravity. We can hope to understand its nature by observing rare dark matter particles and their interactions from space (where we have already seen its effects), and by trying to produce them in controlled laboratory conditions.

    How particle collisions can create dark matter in a lab

    Experiments at particle accelerators have revealed much about the nature of visible (ordinary) matter, starting from the first prototypes that aided the discovery of the proton and the antiproton to the recent discovery of the Higgs boson. All of the particles observed so far are part of the Standard Model of Particle Physics, describing the fundamental components of matter and their non-gravitational interactions.

    Standard Model of Particle Physics

    Standard Model of Particle Physics from Symmetry Magazine

    The most powerful accelerator ever built is the Large Hadron Collider (LHC) at CERN in Geneva, accelerating protons and colliding them with a total energy of 13 TeV. According to Einstein’s most famous equation, E=mc2, the more energy (E) the more massive particles (with a mass m) one can create (13 TeV corresponds to roughly 14 thousand times the rest mass of a proton). The hope is that at the LHC we can create massive dark matter particles by colliding known particles, in the same way we create the Higgs boson in proton-proton collisions.

    Particles are regularly accelerated to very high energies in the universe in “natural” particle accelerators, such as supernovae explosions, and then collide with other particles in our atmosphere. Cosmic rays, for example, are particles that are generated in outer space and make it to Earth. However, the advantage of laboratory particle accelerators such as the LHC is that there we know the initial conditions of the collisions – namely the type and energy of the particles being collided. We can also create a large (and known) number of collisions and observe them in a controlled environment. These are essential features for detecting dark matter particles at experiments like ATLAS.

    Characteristics of dark matter and consequences for detector signatures

    Since dark matter is dark, it will not interact significantly with instruments made of ordinary matter. For this reason, the underlying signature of dark matter production at the LHC, used by all ATLAS searches, is the presence of invisible particles in proton-proton collisions.

    One might reasonably ask how invisible particles can be observed, since they are by definition undetectable! We solve this problem with a little ingenuity. Before each collision, the protons travel along the direction of the LHC beams, and not in directions perpendicular to the beams. This means that their momenta in these perpendicular directions – their “transverse momentum” – is zero. A fundamental principle of physics is that momentum is conserved and so, after the collision, the sum of the transverse momenta of the products of the collision should still be zero. Therefore, if we add up the transverse momenta of all the visible particles produced in the collision and find it not to be zero, then this could be because we have missed the momentum carried away by invisible particles.

    Figure 3: Diagram showing how missing transverse momentum (ETmiss) is determined in the transverse cross-section of a LHC detector. The LHC beams are entering/exiting through the plane. (Image: C. Doglioni, L.T. Wang & E. Ward/ATLAS Collaboration)

    This happens routinely in ATLAS, in the case of physics processes involving neutrinos. We refer to this missed transverse momentum as “ETmiss”. LHC searches for dark matter look for collisions with large values of ETmiss, where the dark matter is produced in association with other, visible particles from the Standard Model, such as photons, quarks or gluons (forming “jets” of particles), or electrons, muons or tau leptons. While ETmiss can be difficult to measure because it relies on accurate measurements of all the other particles in the collision, it is a powerful tool for observing dark matter.

    A further requirement for the identification of dark matter particles in collisions is that the invisible particles should not decay as they travel through the ATLAS detector. In order for an invisible particle to be a candidate for the “relic” dark matter produced in the Big Bang, it should have a lifetime of at least the age of the universe – of the order of 14 billion years. Particles created in LHC collisions take about 40 nanoseconds to cross the ATLAS detector, so requiring that their lifetime be longer than this is not enough, on its own, to prove they constitute the dark matter. Complementary information from astroparticle experiments searching for relic dark matter would be required. However, it is a very good start!

    It is worth noting that other particles that are connected to dark matter might also be detected at the LHC, for example new short-lived particles that can decay both into dark matter and into known matter. Observing those would be an important complement to an observation of dark matter particles from space, as it would allow us to better understand the landscape of dark matter interactions.

    What could dark matter be? Theoretical hypotheses

    Experimentally, there are very few indications of what dark matter might be. We can, however, make theoretical hypotheses on the nature of dark matter, which are useful to experimentalists. The theorist and experimentalist communities often collaborate, for example within the LHC Dark Matter Working Group[3]. Theoretical models of dark matter can tell us more about how the interaction of dark matter with ordinary matter may take place. From that, we can predict what to expect in our detectors if that model were realised in nature. This is relevant for designing detectors sensitive to dark matter, and for deciding how to analyse the products of the collisions once they have been recorded. It is also useful to know what to look for, as we have to decide in real-time which collisions to save data from (this is done using the ATLAS trigger system). A solid theoretical framework for dark matter is also necessary to put LHC searches into context and to compare them with dark matter searches from other instruments.

    Searches for dark matter at the LHC are commonly guided by theoretical models that would allow us to explain the relic density of dark matter with one or a few kinds of particles. A class of models that satisfies these requirements includes a dark matter particle that only interacts weakly with ordinary particles and has a mass within the energy range that can be probed at the LHC – a Weakly Interacting Massive Particle (WIMP).

    Using WIMP models as our starting point for LHC searches doesn’t mean that we are bound to the idea that dark matter should be described with a single particle and a single interaction! This is especially important when you consider that the content of dark matter in the universe is five times the content of ordinary matter, and ordinary matter is described by a variety of different particles and interactions. At the LHC, we have begun our tour into possible theoretical models of dark matter[4] hoping that the few most prominent components and interactions of dark matter will be detected first, just as the electron, proton and electromagnetic interaction were discovered before all other particles of the Standard Model.

    Figure 4: Key particle discoveries from 1898 to today! (Image: E. Ward/ATLAS Collaboration)

    The simplest models one can build in terms of particle content are those where the dark matter particle is added to the Standard Model. In these models, the interaction between visible and dark matter must proceed through existing particles, such as the Z or Higgs boson. This means that the Z or Higgs boson could decay into two dark matter particles[5], in addition to their ordinary decay modes involving Standard Model particles.

    These models are called “portal” models of dark matter, as known particles act as the portal between what we know (ordinary matter) and what we don’t know (dark matter). While models with a Z boson portal are fairly constrained by precision measurements, including those done at the LEP collider at CERN during the 1990s, now is the first time in the history of particles that we can study the properties of the Higgs boson in detail. We could discover whether one or more of those properties lead to a connection to dark matter.

    In addition to dark matter, one can also conceive of another particle not included in the Standard Model that acts as a portal particle. These are called “mediator” particles, since they mediate a new interaction between ordinary matter and dark matter. In the simplest versions of these models, the mediator is an unstable heavy particle that is produced directly from the interaction of Standard Model particles, such as quarks at the LHC. Therefore, it must also be able to decay into those same particles, or into a pair of dark matter particles. If a model of this kind occurs in nature, we have a chance to directly discover this mediator particle at the LHC, as we would be able to detect its Standard Model decay products. Other simple models don’t have a mediator that can also decay to Standard Model particles, but instead foresee the production of dark matter particles in association with Standard Model particles that can aid the detection of the process over known backgrounds.

    While these models are commonly used to interpret the results of many LHC searches in terms of dark matter, they are too simple to represent the full complexity of a dark matter theory. However, they are still useful as building blocks for more complete theories with more ingredients.

    The most popular example of a more complete theory that includes a dark matter candidate is supersymmetry (SUSY). SUSY was one of the first dark matter models to be studied extensively at the LHC. An appealing feature of supersymmetry is that it also solves a stability problem of the relatively low mass of the Higgs boson and other electroweak particles of the Standard Model (around 100 GeV) compared to the Planck scale (10^19 GeV), at which gravity is expected to become strong and the Standard Model must break down. Quantum field theories like the Standard Model naturally prevent such large differences in energy scale from developing, so a physical mechanism is required to generate them. SUSY models provide such a mechanism and, in many cases, predict the existence of a new stable, invisible particle – the lightest supersymmetric particle (LSP) – which has exactly the right properties to be a WIMP dark matter particle. The search for particles predicted by SUSY is a major focus of the ATLAS physics programme. If produced in LHC collisions, these particles could decay to produce a variety of Standard Model particles that can be observed in the ATLAS detector, together with two escaping LSP dark matter particles that generate the characteristic ETmiss signature discussed above.

    Many other theories, of various degrees of completeness and complexity, contain dark matter particle candidates. Some of them predict new particles similar to the Higgs boson that can decay into dark matter, while others go beyond the WIMP paradigm and include mediators with extremely feeble interactions with known particles that only decay after traveling significant distances inside (or outside!) the detector, or more complex sectors of particles mirroring the Standard Model[6]. It is important for LHC searches to cover all this ground, while also preparing for unexpected, not-yet-theorised discoveries. No stone must be left unturned!

    Experimental techniques and results

    ATLAS already measures many processes involving invisible particles, namely neutrinos from the Standard Model. Fig. 5 shows the results of the measurement of the number of Z bosons decaying into a pair of neutrinos (about one fifth of all Z boson decays). As shown in the diagram in Fig. 6, we use a visible object (in this case a photon) to detect the presence of invisible particles and measure their missing transverse energy, as explained in the previous section.

    Figure 6: Diagram of a Z boson decaying into a neutrino-antineutrino pair where the Z boson is produced in association with a photon. (Image: ATLAS Collaboration/CERN)

    Figure 7: Diagram of a new mediator particle decaying into a pair of dark matter particles, produced in association with a photon. (Image: ATLAS Collaboration/CERN)

    A very similar technique can be used for detecting the presence of dark matter particles. If we take the process in Fig. 6, replace the neutrinos with dark matter particles, replace the Z boson with a generic mediator between ordinary matter and dark matter, then we have the diagram in Fig. 7.

    The detector signature of the processes shown in Fig. 6 and Fig. 7 is identical (and is shown in the event display in Fig. 8). Since we cannot distinguish the processes on a collision-by-collision basis, we have to take a different approach. We start by collecting a large number of events that have a large amount of missing transverse momentum and a highly energetic object. Then, we estimate precisely the number of expected events from Standard Model processes (called “backgrounds”), and look for an excess of additional events that could be due to dark matter processes. This kind of search is called “ETmiss+X”, where X stands for what the dark matter recoils against[7].

    So far, we have not found any excess with respect to backgrounds in this kind of search,as shown in Fig. 12 where the data agrees with the Standard Model-only prediction. Still, the journey of ETmiss+X searches at the LHC is far from over. Adding data and improving the experimental precision of future searches will enable us to search for even weaker dark matter interactions yielding processes that are still rarer than those to which we are already sensitive.

    Figure 8: A visualisation of a photon and ETmiss event recorded in 2016, is shown in the ATLAS detector. A photon with transverse momentum of 265 GeV (yellow bar) is balanced by a ETmiss of 268 GeV (red dashed line in the opposite side of the detector). (Image: ATLAS Collaboration/CERN)

    The advantage of this kind of search is that it makes no specific assumption about the nature of the invisible particles, other than that they are produced in association with a Standard Model particle. It is therefore well-suited to cast a wide net on a variety of dark matter models, as long as the model’s signature includes invisible particles and includes dark matter–Standard Model interactions. Conversely, the very large Standard Model backgrounds in ETmiss+X searches can be reduced by giving up some of their generality, for example by requiring distinctive particles (e.g. top quarks, the Higgs boson or related particles) to be produced in association with the dark matter.

    The mediator particle can also decay to visible particles, leading to a peak or “resonance” in the total mass of those particles. Searches for new particles using resonances in the total mass of visible particles have led to numerous discoveries at colliders, including, most recently, the Higgs boson at the LHC. Given that the LHC is the highest-energy laboratory particle collider, the most obvious goal is to search for extremely massive particles that could not have been produced before.

    Still, dark matter mediators could also appear at lower masses, escaping detection because of very low couplings to protons. This is a region where it has been increasingly difficult to perform searches due to the overwhelming Standard Model backgrounds that exceed the experiment’s data capacity if recorded in their entirety. Since background events are indistinguishable from events coming from decays of dark matter mediators, there is a risk of discarding both. Being able to detect this kind of process has provided motivation for overcoming technical limitations. All the main LHC experiments now employ data-taking techniques that allow them to retain a smaller amount of information for some events, so that more events can be recorded[8]. These searches have not yet yielded any new particles, but improvements to the data selection and data acquisition system may bring surprises for the next LHC run.

    The results of searches for invisible and visible dark matter-mediator decays bring complementary information on different parameters of dark matter models. Together, they could help to characterise the nature of a discovery. We must keep in mind, though, that these searches are interpreted in terms of the processes shown in Fig. 7, which stem from a very simple theoretical model. In this model, the only two new particles are the dark matter and the mediator of the interaction, and that may not describe the full complexity of the unknown matter in the universe.

    This is why ATLAS searches target many other experimental signatures in addition to MET-X and resonance searches. For example, models including putative new Higgs bosons yield an assortment of detector signals that can be targeted by different searches. These results can be compared to see whether there are regions in the model parameter space where we haven’t yet looked and, in some cases, they can be combined to strengthen the discovery potential or constraints on dark matter models. A comprehensive summary of these kinds of searches for dark matter, as well as their connection to astrophysical searches (described in the next section), can be found in a new ATLAS paper published today (arXiv: 1903.01400).

    Compared with ETmiss+X searches, detector signatures from SUSY scenarios offer the possibility to make use of some additional tricks to identify a dark matter signal from the Standard Model background.

    In many models, SUSY particles are produced in pairs due to a requirement to conserve a quantity called “R-parity”[9] (sometimes also denoted “matter-parity”). Whenever a SUSY particle decays, the resulting decay products must include exactly one lighter SUSY particle. The decay chain ends when the lightest SUSY particle, which is a candidate dark matter particle, is produced.

    In contrast to many non-SUSY dark matter models, SUSY particle decays can generate many visible Standard Model particles of high energy. Hence, events containing SUSY particles can be identified by requiring these particles as well as missing transverse momentum. A further trick is to make use of constraints on the momenta of the visible particles produced in the SUSY decays coming from the high masses of their SUSY particle parents. In particular, when two visible particles are produced from two identical decay chains in a SUSY event, we can measure properties of the event which can take on much larger values than those expected in Standard Model background events. An example is shown in Fig. 10.

    Figure 9: Missing transverse momentum distribution in data after selecting events with an energetic photon and ETmiss, compared to the Standard Model predictions. The different background processes are shown in different colours. The expected spectra of an example WIMP dark matter scenario is illustrated with red dashed lines. (Image: ATLAS Collaboration/CERN)

    Figure 10: Distribution in data of a quantity sensitive to the production of pairs of SUSY particles whose decays include dark matter particles, after selecting events with two electrons or muons and ETmiss, compared to the Standard Model predictions. The different background processes are shown in different colours. The expected spectra of example SUSY dark matter scenarios are illustrated with blue and green dashed lines. (Image: ATLAS Collaboration/CERN)

    With the help of these tools, SUSY searches are able to set tight requirements for events with a given set of characteristics, targeting specific models. This makes them less general than ETmiss+X searches, but also less impacted by large numbers of background events.

    ATLAS has not yet found evidence of SUSY LSPs, and has strongly constrained many of the models that would simultaneously solve the dark matter puzzle and provide an explanation for the low mass of the Higgs boson. Nevertheless, many SUSY variants remain interesting and the search isn’t over, as described in the dedicated feature article.

    Many other searches for particles from more complex dark matter theories, e.g. those in footnote 7, are also performed in ATLAS even though we don’t cover them in detail in this article. Some of the characteristics of these particles make them behave very differently compared with the particles the LHC was built to observe. Therefore, searching for these (still well-motivated) variants of dark matter is generally more challenging and requires dedicated techniques to identify and reconstruct candidate particles that would hint at the presence of dark matter. These searches are now at the forefront of the ATLAS and LHC quest for dark matter, and have gathered at least as much interest as searches for WIMPs and their associated particles.

    Connecting collider searches to astrophysical searches

    Searches for dark matter at the LHC are typically searches for the production, rather than the interaction or annihilation, of potential dark matter particles. As such, data from ATLAS would not provide proof that a new particle constitutes the dark matter – the sensitivity to dark matter lifetimes is just too short (see above). Nevertheless, ATLAS data could establish consistency with the predictions of dark matter models, and within those models ATLAS can provide complementary information to the broad range of astroparticle searches for the interaction of relic dark matter particles being carried out around the world. This complementarity can be illustrated taking, for example, the simple dark matter-mediator model.

    Figure 11. Diagram showing dark matter (DM) interactions and their corresponding experimental detection techniques, with time going from left to right. (a) shows DM annihilation to Standard Model (SM) particles, as sought by Indirect Detection (ID) experiments. (b) shows DM -> SM particle scattering, targeted by Direct Detection (DD) experiments. (c) shows the production of DM particles from the annihilation of SM particles at colliders. (d) again shows the pair production of DM at colliders, but in this case the interaction occurs through a mediator particle between DM and SM particles. (Image: C. Doglioni & A. Boveia/ATLAS Collaboration)

    Within this model, in order for dark matter particles to be produced in pairs at the LHC, two strongly interacting quarks or gluons from the colliding protons must interact to produce the two dark matter particles (Fig. 11(b)). These same interactions could enable relic dark matter particles trapped in the Milky Way galaxy to scatter off atomic nuclei on Earth, generating the nuclear recoil signature exploited by “direct” astroparticle searches for dark matter such as XENON in Europe, LUX in North America and PANDA-X in China. Constraints from ATLAS searches can therefore be translated, albeit with assumptions on the mediator–proton and mediator–dark matter interaction, into constraints on the possible signals in those experiments (Fig. 12).

    Figure 12: A comparison of the inferred limits from ATLAS data, including those from both ETmiss+X and mediator resonance searches, to the constraints from direct detection experiments on the WIMP-proton scattering cross section in the context of a model with a new vector particle mediating the Standard Model-dark matter interaction, fixing the given mediator / quarks (gq) and mediator / dark matter (gDM) couplings to the value in the plot. (Image: ATLAS Collaboration/CERN)

    Furthermore, the same interactions also enable relic dark matter particles produced in the early universe to annihilate and create Standard Model particles (Fig. 11(a)). This leads to the signatures for dark matter sought by “indirect” dark matter search experiments – typically high-energy photons (observed by telescopes such as HESS, MAGIC and VERITAS), neutrinos (observed by neutrino telescopes such as IceCube) or anti-particles (detected by space experiments such as AMS on the International Space Station). Results from collider searches can therefore also be compared with results from those experiments.

    The complementarity between recent ATLAS searches and astroparticle searches for dark matter is illustrated by Fig. 12, for the case of the simple dark matter-mediator model.

    When interpreting and combining ATLAS results and those from astroparticle dark matter searches, we need to consider whether the dark matter model being tested is consistent with the observed density of relic dark matter particles. This has been measured with a precision better than 1% through observations of the cosmic microwave background [CMB] by satellites like Planck. When considering a particular dark matter model, this only sets an upper limit on the amount of dark matter the model should produce. This is because, in principle, the dark matter could consist of multiple types of particles, with any one type only contributing a fraction of the amount measured by Planck.

    The relic dark matter density constraint is particularly important for SUSY dark matter models, where the models can often predict more dark matter than the Planck satellite observed. Special characteristics of the model, such as closely-spaced SUSY particle masses or increased dark matter interactions, can reduce this density to values consistent with Planck observations, and searches for models with these characteristics are a high priority for ATLAS.

    The relic dark matter density constraint is particularly important for SUSY dark matter models, where the models can often predict more dark matter than the Planck satellite observed. Special characteristics of the model, such as closely-spaced SUSY particle masses or increased dark matter interactions, can reduce this density to values consistent with Planck observations, and searches for models with these characteristics are a high priority for ATLAS.

    Outlook: where do we go from here?

    ATLAS is searching for dark matter at the LHC in synergy with other experimental collaborations, such as CMS and LHCb. LHC experiments have not yet discovered dark matter candidates from Run 1/2 data, but there is a large number of proton-proton collisions ahead. The upcoming LHC data-taking period (2021-2023, known as Run 3) is expected to more than double the current dataset, and the high-luminosity period beginning 2026 will deliver at least another factor of 10 more data. The experiments will be able to probe dark matter processes that are rarer and more challenging to reconstruct than the ones studied today. In view of the upcoming data-taking, experiments are also making use of more advanced data-collection and data-analysis techniques, such as machine learning[10].
    Direct and indirect searches for signals of the existing dark matter in our galactic neighbourhood are important complementary strategies to LHC searches, since astrophysical experiments are able to detect relic dark matter and they are necessary to confirm that a new invisible particle discovered at the LHC could make up dark matter. We will continue the dialogue with these experiments, exchanging scientific results and perspectives, share theoretical models, and extend the discussion to the broader astrophysics community.
    Other experiments can probe dark matter models to which the LHC experiments are not sensitive, for example models where the interactions between dark matter and ordinary matter are too feeble for dark matter to be produced in collisions of known particles. These experiments are being discussed in the Physics Beyond Colliders effort that recently started at CERN.

    As one of the main outstanding questions in fundamental physics, the identification of the nature of dark matter is a key scientific driver for the future of particle physics. For this reason dark matter searches are a main focus of the discussions, including both experimentalists and theorists, which have taken place in recent initiatives to draw up roadmaps for the future of the field. While the nature of dark matter is currently still unknown, it is clear that the quest to better understand it will be a highlight of humanity’s study of the fundamental constituents of the universe for many years to come.

    [1] For an exhaustive overview of the history of dark matter, with ideas on dark matter that date even further back in time, see Bertone and Hooper’s “A History of Dark Matter” (arXiv: 1605.04909), or Bertone, de Swart and van Dongen’s “How dark matter came to matter” (arXiv: 1703.00013).

    [2] This piece will not discuss the possibility that scientists haven’t understood all of the details of the structure of space-time, including how gravity acts. That hypothesis is discussed in more detail in this article and its references: “Shaking the dark matter paradigm” (Symmetry magazine, 2017).

    [3] For this reason, the community of theorists and experimentalists looking for dark matter at the LHC has joined forces, forming first the Dark Matter Forum and then the Dark Matter Working Group. The goal and results of those group are described here.

    [4] This article does not contain an exhaustive list of models. For a graduate-level lecture series on models of dark matter see, for example, the TASI “Lectures on Dark Matter Physics” by M. Lisanti (arXiv: 1603.03797).

    [5] If the dark matter mass is less than half of that of the Z or the Higgs boson.

    [6] For an introduction to these kind of models see, for example, “If You Can’t Find Dark Matter, Look First for a Dark Force” (Nautilus article, 2017), “Hunting for Dark Matter’s ‘Hidden Valley’” (BNL feature story, 2016), “Voyage into the dark sector” (Symmetry magazine, 2018) and “Long-lived physics” (CERN article, 2018).

    [7] For more information on the missing transverse momentum+jet search, see the 2017 ATLAS Physics Briefing “Chasing the Invisible”.

    [8] For more information on this kind of searches, see the 2018 ATLAS Physics Briefing “A new data-collection method for ATLAS aids in the hunt for new physics”.

    [9] R-parity ensures that in SUSY models protons, and hence all of the atoms in the universe, are unable to decay to other particles quickly by exchanging SUSY particles. In models without R-parity conservation, this can also be prevented. However introducing R-Parity is the simplest possibility.

    [10] For more information on ongoing efforts on Machine Learning, see the DarkMachines research collective. For general perspectives on data acquisition and collection see the HEP Software Foundation.

    See the full article here .

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