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  • richardmitnick 12:04 pm on May 14, 2019 Permalink | Reply
    Tags: >Model-dependent vs model-independent research, , , , , , , , , , , Symmetry Magazine   

    From Symmetry: “Casting a wide net” 

    Symmetry Mag
    From Symmetry

    05/14/19
    Jim Daley

    1
    Illustration by Sandbox Studio, Chicago

    In their quest to discover physics beyond the Standard Model, physicists weigh the pros and cons of different search strategies.

    On October 30, 1975, theorists John Ellis, Mary K. Gaillard and D.V. Nanopoulos published a paper [Science Direct] titled “A Phenomenological Profile of the Higgs Boson.” They ended their paper with a note to their fellow scientists.

    “We should perhaps finish with an apology and a caution,” it said. “We apologize to experimentalists for having no idea what is the mass of the Higgs boson… and for not being sure of its couplings to other particles, except that they are probably all very small.

    “For these reasons, we do not want to encourage big experimental searches for the Higgs boson, but we do feel that people performing experiments vulnerable to the Higgs boson should know how it may turn up.”

    What the theorists were cautioning against was a model-dependent search, a search for a particle predicted by a certain model—in this case, the Standard Model of particle physics.

    Standard Model of Particle Physics

    It shouldn’t have been too much of a worry. Around then, most particle physicists’ experiments were general searches, not based on predictions from a particular model, says Jonathan Feng, a theoretical particle physicist at the University of California, Irvine.

    Using early particle colliders, physicists smashed electrons and protons together at high energies and looked to see what came out. Samuel Ting and Burton Richter, who shared the 1976 Nobel Prize in physics for the discovery of the charm quark, for example, were not looking for the particle with any theoretical prejudice, Feng says.

    That began to change in the 1980s and ’90s. That’s when physicists began exploring elegant new theories such as supersymmetry, which could tie up many of the Standard Model’s theoretical loose ends—and which predict the existence of a whole slew of new particles for scientists to try to find.

    Of course, there was also the Higgs boson. Even though scientists didn’t have a good prediction of its mass, they had good motivations for thinking it was out there waiting to be discovered.

    And it was. Almost 40 years after the theorists’ tongue-in-cheek warning about searching for the Higgs, Ellis found himself sitting in the main auditorium at CERN next to experimentalist Fabiola Gianotti, the spokesperson of the ATLAS experiment at the Large Hadron Collider who, along with CMS spokesperson Joseph Incandela, had just co-announced the discovery of the particle he had once so pessimistically described.

    CERN CMS Higgs Event


    CERN ATLAS Higgs Event

    Model-dependent vs model-independent

    Scientists’ searches for particles predicted by certain models continue, but in recent years, searches for new physics independent of those models have begun to enjoy a resurgence as well.

    “A model-independent search is supposed to distill the essence from a whole bunch of specific models and look for something that’s independent of the details,” Feng says. The goal is to find an interesting common feature of those models, he explains. “And then I’m going to just look for that phenomenon, irrespective of the details.”

    Particle physicist Sara Alderweireldt uses model-independent searches in her work on the ATLAS experiment at the Large Hadron Collider.

    CERN ATLAS Image Claudia Marcelloni CERN/ATLAS

    Alderweireldt says that while many high-energy particle physics experiments are designed to make very precise measurements of a specific aspect of the Standard Model, a model-independent search allows physicists to take a wider view and search more generally for new particles or interactions. “Instead of zooming in, we try to look in as many places as possible in a consistent way.”

    Such a search makes room for the unexpected, she says. “You’re not dependent on the prior interpretation of something you would be looking for.”

    Theorist Patrick Fox and experimentalist Anadi Canepa, both at Fermilab, collaborate on searches for new physics.


    In Canepa’s work on the CMS experiment, the other general-purpose particle detector at the LHC, many of the searches are model-independent.

    While the nature of these searches allows them to “cast a wider net,” Fox says, “they are in some sense shallower, because they don’t manage to strongly constrain any one particular model.”

    At the same time, “by combining the results from many independent searches, we are getting closer to one dedicated search,” Canepa says. “Developing both model-dependent and model-independent searches is the approach adopted by the CMS and ATLAS experiments to fully exploit the unprecedented potential of the LHC.”

    Driven by data and powered by machine learning

    Model-dependent searches focus on a single assumption or look for evidence of a specific final state following an experimental particle collision. Model-independent searches are far broader—and how broad is largely driven by the speed at which data can be processed.

    “We have better particle detectors, and more advanced algorithms and statistical tools that are enabling us to understand searches in broader terms,” Canepa says.

    One reason model-independent searches are gaining prominence is because now there is enough data to support them. Particle detectors are recording vast quantities of information, and modern computers can run simulations faster than ever before, she says. “We are able to do model-independent searches because we are able to better understand much larger amounts of data and extreme regions of parameter and phase space.”

    Machine-learning is a key part of this processing power, Canepa says. “That’s really a change of paradigm, because it really made us make a major leap forward in terms of sensitivity [to new signals]. It really allows us to benefit from understanding the correlations that we didn’t capture in a more classical approach.”

    These broader searches are an important part of modern particle physics research, Fox says.

    “At a very basic level, our job is to bequeath to our descendants a better understanding of nature than we got from our ancestors,” he says. “One way to do that is to produce lots of information that will stand the test of time, and one way of doing that is with model-independent searches.”

    Models go in and out of fashion, he adds. “But model-independent searches don’t feel like they will.”

    See the full article here .


    five-ways-keep-your-child-safe-school-shootings

    Please help promote STEM in your local schools.


    Stem Education Coalition

    Symmetry is a joint Fermilab/SLAC publication.


     
  • richardmitnick 11:18 am on May 7, 2019 Permalink | Reply
    Tags: "A universe is born", , , , , , , , , , , , Symmetry Magazine, The Planck epoch   

    From Symmetry: “A universe is born” 

    Symmetry Mag
    From Symmetry

    05/07/19
    Diana Kwon

    Take a (brief) journey through the early history of our cosmos.

    Timeline of the Inflationary Universe WMAP

    The universe was a busy place during the first three minutes. The cosmos we see today expanded from a tiny speck to much closer to its current massive size; the elementary particles appeared; and protons and neutrons combined into the first nuclei, filling the universe with the precursors of elements.

    By developing clever theories and conducting experiments with particle colliders, telescopes and satellites, physicists have been able to wind the film of the universe back billions of years—and glimpse the details of the very first moments in the history of our cosmic home.

    Take an abridged tour through this history:

    The Planck epoch
    Time: < 10^-43 seconds

    The Planck Epoch https:// http://www.slideshare.net ericgolob the-big-bang-10535251

    Welcome to the Planck epoch, named after the smallest scale of measurements possible in particle physics today. This is currently the closet scientists can get to the beginning of time.

    Theoretical physicists don’t know much about the earliest moments of the universe. After the Big Bang theory gained popularity, scientists thought that in the first moments, the cosmos was at its hottest and densest and that all four fundamental forces—electromagnetic, weak, strong and gravitational—were combined into a single, unified force. But the current leading theoretical framework for our universe’s beginning doesn’t necessarily require these conditions.

    The universe expands
    Time: From 10^-43 seconds to about 10^-36 seconds

    In this stage, which began either at Planck time or shortly after it, scientists think the universe underwent superfast, exponential expansion in a process known as inflation.

    Inflation

    4
    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:
    5

    Physicists first proposed the theory of inflation in the 1980s to address the shortcomings of the Big Bang theory, which, despite its popularity, could not explain why the universe was so flat and uniform, and why its different parts began expanding simultaneously.

    During inflation, quantum fluctuations could have stretched out to produce a pattern that later determined the locations of galaxies. It might have been only after this period of inflation the universe became a hot, dense fireball as described in the Big Bang theory.

    The elementary particles are born
    Time: ~10^-36 seconds

    When the universe was still very hot, the cosmos was like a gigantic accelerator, much more powerful than the Large Hadron Collider, running at extremely high energies. In it, the elementary particles we know today were born.

    Scientists think that first came exotic particles, followed by more familiar ones, such as electrons, neutrinos and quarks. It could be that dark matter particles came about during this time.

    3
    Quarks APS/Alan Stonebraker

    The quarks soon combined, forming the familiar protons and neutrons, which are collectively known as baryons. Neutrinos were able to escape this plasma of charged particles and began traveling freely through space, while photons continued to be trapped by the plasma.

    Standard Model of Particle Physics

    The first nuclei emerge
    Time: ~1 second to 3 minutes

    Scientists think that when the universe cooled enough for violent collisions to subside, protons and neutrons clumped together into nuclei of the light elements—hydrogen, helium and lithium—in a process known as Big Bang nucleosynthesis.

    Protons are more stable than neutrons, due to their lower mass. In fact, a free neutron decays with a 15-minute half-life, while protons may not decay at all, as far as we know.

    So as the particles combined, many protons remained unpaired. As a result, hydrogen—protons that never found a partner—make up around 74% of the mass of “normal” matter in our cosmos. The second most abundant element is helium, which makes up approximately 24%, followed by trace amounts of deuterium, lithium, and helium-3 (helium with a three-baryon core).

    Periodic table Sept 2017. Wikipedia

    Scientists have been able to accurately measure the density of baryons in our universe. Most of those measurements line up with theorists’ estimations of what the quantities ought to be, but there is one lingering issue: Lithium calculations are off by a factor of three. It could be that the measurements are off, but it could also be that something we don’t yet know about happened during this time period to change the abundance of lithium.

    The cosmic microwave background becomes visible
    Time: 380,000 years

    Hundreds of thousands of years after inflation, the particle soup had cooled enough for electrons to bind to nuclei to form electrically neutral atoms. Through this process, which is also known as recombination, photons became free to traverse the universe, creating the cosmic microwave background.

    CMB per ESA/Planck


    ESA/Planck 2009 to 2013

    Today, the CMB is one of the most valuable tools for cosmologists, who probe its depths in search of answers for many of the universe’s lingering secrets, including the nature of inflation and the cause of matter-antimatter asymmetry.

    Shortly after the CMB became detectable, neutral hydrogen particles formed into a gas that filled the universe. Without any objects emitting high-energy photons, the cosmos was plunged into the dark ages for millions of years.

    Dark Energy Camera Enables Astronomers a Glimpse at the Cosmic Dawn. CREDIT National Astronomical Observatory of Japan

    The earliest stars shine
    Time: ~100 million years

    The dark ages ended with the formation of the first stars and the occurrence of reionization, a process through which highly energetic photons stripped electrons off neutral hydrogen atoms.

    Reionization era and first stars, Caltech

    Scientists think that the vast majority of the ionizing photons emerged from the earliest stars. But other processes, such as collisions between dark matter particles, may have also played a role.

    At this time, matter began to form the first galaxies. Our own galaxy, the Milky Way, contains stars that were born when the universe was only several hundred million years old.

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

    Our sun is born
    Time: 9.2 billion years

    3
    NASA

    The sun is one of a few hundred billion stars in the Milky Way. Scientists think it formed from a giant cloud of gas that consisted mostly hydrogen and helium.

    Today
    Time: 13.8 billion years

    Today, our cosmos sits at a cool 2.7 Kelvin (minus 270.42 degrees Celsius). The universe is expanding at an increasing rate, in a manner similar to (but many orders of magnitude slower than) inflation.

    Universe map Sloan Digital Sky Survey (SDSS) 2dF Galaxy Redshift Survey

    Physicists think that dark energy—a mysterious repulsive force that currently accounts for about 70% of the energy in our universe—is most likely driving that accelerated expansion.

    Dark energy depiction. Image: Volker Springle/Max Planck Institute for Astrophysics/SP)

    See the full article here .


    five-ways-keep-your-child-safe-school-shootings

    Please help promote STEM in your local schools.


    Stem Education Coalition

    Symmetry is a joint Fermilab/SLAC publication.


     
  • richardmitnick 11:56 am on April 30, 2019 Permalink | Reply
    Tags: "A common language", , , “These are the most brilliant students who just happen not to hear ” Nordhaus says. “It’s wrong that these barriers exist.”, Jason Nordhaus teaches physics at Rochester Institute of Technology, National Technical Institute for the Deaf at the Rochester Institute of Technology, , Reduce barriers to STEM for deaf and hard-of-hearing students., , Symmetry Magazine   

    From Symmetry: “A common language” 

    Symmetry Mag
    From Symmetry

    04/30/19
    Emily Ayshford

    Physics professor Jason Nordhaus is working to reduce barriers to STEM for deaf and hard-of-hearing students.

    4
    Physics professor Jason Nordhaus

    1
    Illustration by Sandbox Studio, Chicago with Ana Kova

    When Jason Nordhaus teaches physics at Rochester Institute of Technology, he doesn’t just consider how to best explain topics like thermodynamics and quantum mechanics.

    Much of what he thinks about involves the physical space of his classroom: He arranges his students’ desks so everyone has a line of sight to him. He positions his body to ensure that students can see both of his hands. He displays important information on the board or on a PowerPoint slide, and he always pauses his lecture to give students time to read it.

    That’s because many of Nordhaus’s students are deaf or hard of hearing. Nordhaus communicates with them by both speaking and using American Sign Language.

    “I had a student thank me once for not having a beard,” he says. “He was able to see my expressions and my lips and then better understand me. I never would have considered that before.”

    In 2009 and 2010, a national survey by Gallaudet Research Institute identified about 28,500 deaf or hard-of-hearing students under age 18 in the United States. Deaf and hard-of-hearing students face numerous barriers when trying to study technical STEM fields like physics. Significantly, many find it difficult to learn complex topics from lesson plans and classrooms designed for hearing students.

    Physicists like Nordhaus, who is an assistant professor of physics and a theoretical astrophysicist at the National Technical Institute for the Deaf at the Rochester Institute of Technology, are trying to change all that with specialized programs, classes, and interpreter training, all aimed at reducing barriers in STEM.

    “These are the most brilliant students who just happen not to hear,” Nordhaus says. “It’s wrong that these barriers exist.”

    2
    Illustration by Sandbox Studio, Chicago with Ana Kova

    Understanding the barriers

    Asher Kirschbaum, who attended the Maryland School for the Deaf from 1997 to 2012, got into physics the way many kids do—by reading books about space and science and becoming fascinated with learning how the universe worked.

    After graduation, he enrolled at the National Technical Institute for the Deaf. NTID, one of the Rochester Institute of Technology’s nine colleges, has an active partnership with RIT’s Astrophysical Sciences and Technology program. Together they offer unique opportunities for deaf and hard-of-hearing students, including a doctorate program in astrophysics.

    At RIT, the majority of STEM professors do not know sign language, so interpreters translate their lectures for deaf and hard-of-hearing students. Kirschbaum found watching interpreters try to translate technical concepts was not equivalent to learning the concepts in ASL, as he had in Maryland.

    “I visited my professors’ offices for hours with a pen and paper and tried to better understand topics such as differential equations and heat transfer,” he writes in an email to Symmetry. Though professors were always willing to work with him, he writes, it took a lot of extra work on his part as well. “I became very good at learning from textbooks and teaching myself concepts.”

    Outside the classroom, Kirschbaum found himself left out of peer support systems. “When I found a study group, it was still inaccessible to me because most of the time, my peers did not know sign language,” he writes.

    Gabriela Santos, a hearing-impaired student interested in cosmology, had a similar experience at RIT/NTID. “In the science classroom setting, if there is a hearing professor, interpreters and [deaf or hard-of-hearing] students both have a harder time being able to understand and translate the science concepts well,” she writes. “Some hearing professors tend to talk rapidly and explain concepts while simultaneously writing notes on the board, or going through slides,” visual cues that are impossible to fully absorb while also watching a signed translation.

    Creating a pipeline

    Nordhaus entered into the sphere of deaf and hard-of-hearing physics students in 2008, when he was a postdoctoral fellow at Princeton. While walking through the airport one day, he noticed two people communicating in ASL. Intrigued, he signed up for a class at the Katzenbach School for the Deaf in West Trenton, New Jersey, to learn the language. He kept up his studies and eventually applied for a National Science Foundation fellowship at RIT to develop an astronomy course to be taught in ASL.

    After he was hired, Nordhaus began collaborating with faculty in deaf education to develop four physics courses and two astronomy courses taught in ASL. He also became a co-principal investigator on a Research Experience for Undergraduates that is specifically for deaf students.

    “We want to set up the pipeline to give students as much opportunity as possible,” he says.

    Kirschbaum conducted an independent study with Nordhaus. He studied a triple star system, learning how to code in Python to create simulations to better understand the stars’ gravitational force. He presented his results through an interpreter at the poster session for the conference of the American Astronomical Society.

    “The experience was fantastic,” Kirschbaum writes. “I do not think people were expecting to see a deaf person presenting his research. I am glad that I had the opportunity to show the STEM community that deaf people are capable of achieving the same things as anyone else in the field.”

    Santos conducted research with Nordhaus as part of the Research Experience program, examining the qualities exoplanets would need to be livable. She presented her work at an undergraduate conference. Santos, who uses cochlear implants to receive sound, both spoke and signed her presentation.

    4
    Illustration by Sandbox Studio, Chicago with Ana Kova

    Developing a technical vocabulary

    When interpreters translate physics lessons, they often do so conceptually, creating their own ways of communicating the idea.

    This can put deaf and hard-of-hearing students at a disadvantage. At the undergraduate conference where Santos presented, Nordhaus watched a hearing student give a presentation on the n-body problem—in this case, calculations of how three stars and their planets move together. The interpreter signed “Three people have a problem,” in ASL.

    “Deaf students watching the presentation would have been totally confused, because they would have thought that three people are having some sort of issue,” Nordhaus says.

    Several years ago, a group created signs for 119 physics terms in British Sign Language. Some groups working to build the ASL vocabulary for STEM topics have included physics in their efforts, but no one set of physics signs has been formally adopted in the US.

    Nordhaus recently won a grant from the Gordon and Betty Moore Foundation to tackle this problem. He and collaborator Jessica Trussell, an assistant professor in the Master of Science in Secondary Education program at NTID, are developing training videos for interpreters and students for each unit of a physics course. The videos will teach a consistent set of vocabulary to both interpreters and students in physics courses.

    “That way, when we get to that unit in class, everyone will be on the same page,” Nordhaus says.

    Nordhaus says he plans to evaluate whether the videos improve student outcomes.

    “Then we have a template for doing the same thing in areas like biology, chemistry and engineering. Over time we can do things like this to reduce barriers, to get more deaf and hard-of-hearing students in STEM.”

    _______________________________________________________

    Tips for hearing professors for teaching deaf and hard-of-hearing students

    Work with ASL interpreters to ensure they understand the concepts. Throughout the semester, professors should meet with interpreters before new concepts are introduced to ensure they understand and have the vocabulary to translate.
    Manage classroom discussions. Only one person should be speaking at a time, so the interpreter can translate everything.
    Use visuals when possible. Deaf students are visual learners—if they can see it, they can understand it better.
    Pause to give students time to read what is on PowerPoint slides. Students cannot watch an interpreter and read important information at the same time.

    See the full article here .


    five-ways-keep-your-child-safe-school-shootings

    Please help promote STEM in your local schools.


    Stem Education Coalition

    Symmetry is a joint Fermilab/SLAC publication.


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

    From Symmetry: “Falsifiability and physics” 

    Symmetry Mag
    From Symmetry

    04/23/19
    Matthew R. Francis

    1
    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.”

    LHC

    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.

    2
    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?

    Inflation

    4
    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:
    5

    “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 .


    five-ways-keep-your-child-safe-school-shootings

    Please help promote STEM in your local schools.


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

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

    Symmetry Mag
    From Symmetry

    04/18/19
    Caitlyn Buongiorno

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

    1
    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 .


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    Please help promote STEM in your local schools.


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  • richardmitnick 11:37 am on April 16, 2019 Permalink | Reply
    Tags: , , , , , , , Symmetry Magazine   

    From Symmetry: “A collision of light” 

    Symmetry Mag
    From Symmetry

    04/16/19
    Sarah Charley

    1
    Natasha Hartono

    One of the latest discoveries from the LHC takes the properties of photons beyond what your electrodynamics teacher will tell you in class.

    Professor Anne Sickles is currently teaching a laboratory class at the University of Illinois in which her students will measure what happens when two photons meet.

    What they will find is that the overlapping waves of light get brighter when two peaks align and dimmer when a peak meets a trough. She tells her students that this is process called interference, and that—unlike charged particles, which can merge, bond and interact—light waves can only add or subtract.

    “We teach undergraduates the classical theory,” Sickles says. “But there are situations where effects forbidden in the classical theory are allowed in the quantum theory.”

    Sickles is a collaborator on the ATLAS experiment at CERN and studies what happens when particles of light meet inside the Large Hadron Collider.

    CERN ATLAS Credit CERN SCIENCE PHOTO LIBRARY

    LHC

    CERN map


    CERN LHC Tunnel

    CERN LHC particles

    For most of the year, the LHC collides protons, but for about a month each fall, the LHC switches things up and collides heavy atomic nuclei, such as lead ions. The main purpose of these lead collisions is to study a hot and dense subatomic fluid called the quark-gluon plasma, which is harder to create in collisions of protons. But these ion runs also enable scientists to turn the LHC into a new type of machine: a photon-photon collider.

    “This result demonstrates that photons can scatter off each other and change each other’s direction,” says Peter Steinberg, and ATLAS scientist at Brookhaven National Laboratory.

    When heavy nuclei are accelerated in the LHC, they are encased within an electromagnetic aura generated by their large positive charges.

    As the nuclei travel faster and faster, their surrounding fields are squished into disks, making them much more concentrated. When two lead ions pass closely enough that their electromagnetic fields swoosh through one another, the high-energy photons which ultimately make up these fields can interact. In rare instances, a photon from one lead ion will merge with a photon from an oncoming lead ion, and they will ricochet in different directions.

    However, according to Steinberg, it’s not as simple as two solid particles bouncing off each other. Light particles are both chargeless and massless, and must go through a quantum mechanical loophole (literally called a quantum loop) to interact with one another.

    “That’s why this process is so rare,” he says. “They have no way to bounce off of each other without help.”

    When the two photons see each other inside the LHC, they sometimes overreact with excitement and split themselves into an electron and positron pair. These electron-positron pairs are not fully formed entities, but rather unstable quantum fluctuations that scientists call virtual particles. The four virtual particles swirl into each other and recombine to form two new photons, which scatter off at weird angles into the detector.

    “It’s like a quantum-mechanical square dance,” Steinberg says.

    When ATLAS first saw hints of this process in 2017, they had only 13 candidate events with the correct characteristics (collisions that resulted in two low-energy photons inside the detector and nothing else).

    After another two years of data taking, they have now collected 59 candidate events, bumping this original observation into the statistical certainty of a full-fledged discovery.

    Steinberg sees this discovery as a big win for quantum electrodynamics, a theory about the quantum behavior of light that predicted this interaction. “This amazingly precise theory, which was developed in the first half of the 20th century, made a prediction that we are finally able to confirm many decades later.”

    Sickles says she is looking forward to exploring these kinds of light-by-light interactions and figuring out what else they could teach us about the laws of physics. “It’s one thing to see something,” she says. “It’s another thing to study it.”

    See the full article here .


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    Symmetry is a joint Fermilab/SLAC publication.


     
  • richardmitnick 3:21 pm on April 9, 2019 Permalink | Reply
    Tags: , , , , , , , , Symmetry Magazine   

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

    Symmetry Mag
    From Symmetry

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

    1
    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.

    2
    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.

    3
    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.

    4
    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.

    5
    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.

    6
    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 .


    five-ways-keep-your-child-safe-school-shootings

    Please help promote STEM in your local schools.


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    Symmetry is a joint Fermilab/SLAC publication.


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

    From Symmetry: “All hands on deck” 

    Symmetry Mag
    From Symmetry

    04/09/19
    Ali Sundermier

    1
    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.

    3

    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.

    2
    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 .


    five-ways-keep-your-child-safe-school-shootings

    Please help promote STEM in your local schools.


    Stem Education Coalition

    Symmetry is a joint Fermilab/SLAC publication.


     
  • richardmitnick 2:14 pm on March 26, 2019 Permalink | Reply
    Tags: , , , Symmetry Magazine, To PhD or not to PhD   

    From Symmetry: “To PhD or not to PhD” 

    Symmetry Mag
    From Symmetry

    1
    Illustration by Sandbox Studio, Chicago with Corinne Mucha

    03/26/19
    By Laura Dattaro

    Respondents to Symmetry’s survey about what it’s like to earn a PhD in particle physics or astrophysics offer their views of the experience.

    In the mid-1980s, Nigel Smith was a graduate student at Leeds University. He was interested in the mysterious origins of cosmic rays, bursts of radiation that collide with our atmosphere and produce showers of high-energy particles. He was settling in to work toward his doctorate degree running an array of cosmic-ray detectors with his advisor in nearby West Yorkshire, England.

    And then the South Pole called.

    His adviser decided to partner with a pair of American physicists to build another array in Antarctica, where higher ground and a constant view of the Milky Way galactic center made for better observing. The collaboration needed someone on location to operate the detector, and Smith leapt at the opportunity. He stayed at the South Pole the months between Valentine’s Day and Halloween—what’s called “wintering over”—collecting the data that would earn him his PhD.

    One of the great things about exploring the natural world is that it can take you out of your usual habitat, says Smith, who is now the executive director of SNOLAB, a physics laboratory that sits more than a mile underground in a working nickel mine in Ontario, Canada.

    SNOLAB, a Canadian underground physics laboratory at a depth of 2 km in Vale’s Creighton nickel mine in Sudbury, Ontario


    “You’ve got to go where you need to to undertake that exploration,” he says. “That’s just as true in physics as it in other natural sciences.”

    While Smith’s specific experience was unique, it demonstrates many of the general themes reported in a recent Symmetry survey about the experience of getting a PhD in particle physics or astrophysics. To get his degree, he had to work hard, collaborate with other people, adapt to new situations, and pick up unexpected skills.

    Nearly 2000 people worldwide complete the scientific rite of passage of earning a physics PhD each year. Yet for many people, particularly those outside of science, the process remains mysterious.

    “I don’t think most people even have a conception, much less a misconception” about earning a physics PhD, wrote survey respondent Charlie Cosse.

    The more than 300 responses to Symmetry’s survey described a challenging, multifaceted experience that goes far beyond job training, and even beyond the scientific goal of studying the fundamental nature of the universe.

    What it takes

    Many survey respondents felt that stereotypes about the field—particularly the idea of it being filled with Big Bang Theory-style science nerds—didn’t accurately reflect their experiences.

    In fact, many survey takers said they would like to retire the notion that people who study physics are inherently smarter than others—a stereotype that can intimidate current and would-be physics students. In reality, they said, success depends more on hard work and passion for the subject.

    Holly Mein, for example, says that when people hear about her job—she works for NASA’s International Space Station program—and her education—she’s getting her master’s degree in physics—they say she “must be really smart.” But, she says, “I don’t think it requires extra smarts at all. I really believe it’s how much you love the subject, how much time and effort you put into studying and how motivated you are.”

    Motivation can be key to getting through a degree process that, in many countries, usually takes at least five years. In the United States, doctoral students spend their first couple of years taking classes before going on to conduct research and write a dissertation. According to Symmetry’s survey results, day-to-day life generally includes programming, data analysis and experimental work, combined with teaching, writing and meetings—particularly for people who work on large collaborations. One thing essentially all respondents agreed on: It is a lot of hard work.

    “You never expect the resilience that you will need to have to get through the whole process,” says Stefano Tognini, a respondent who recently finished his degree at Federal University of Goiás in Brazil. “I think the PhD was a very good personal achievement, not just professional.”

    Tognini started his graduate work in 2010, directly after earning his undergraduate degree, and defended his dissertation in 2018. In Brazil, the federal government provides funds for doctoral degrees, which must be paid back if the student doesn’t graduate, adding extra pressure to finish the program. Tognini worked for seven years without taking an extended vacation, which he says made the process particularly stressful.

    Different countries have different requirements and processes; in the UK, for example, students don’t start with classes, shortening the PhD process by several years.

    Nearly all survey respondents agreed that the experience of earning a PhD is mentally taxing and that finding support among fellow students is a key to success.

    “A lot of people who wind up in this field are people who are used to feeling like they have to know what’s going on,” or at least give others the impression that things come easily to them, says Kate Pachal, who completed her doctorate in 2015. “If you feel like you have to look tough and cool in front of your friends, you don’t have the emotional support network you need.”

    2
    Illustration by Sandbox Studio, Chicago with Corinne Mucha

    Where it takes you

    Pachal is in her second postdoctoral appointment, a temporary academic or research position for PhD graduates. With its large experimental collaborations, particle physics in particular relies on a high number of postdoctoral researchers—often called postdocs—to do the day-to-day work needed to keep experiments operating.

    But permanent positions as researchers or tenured professors are rare. For example, CERN Director-General Fabiola Gianotti mentioned at a recent talk that 90 percent of students who earn their PhDs at CERN don’t go on to work in physics. That leads to another common, if paradoxical, piece of advice from Symmetry’s survey: Don’t get a physics PhD because you want to be a physicist; get a physics PhD because you want the experience of earning a physics PhD.

    “Very few people become professors, and it is a very long and arduous path even if you are one of those people,” Pachal says. “So if you don’t enjoy the path, there’s a very good chance you’re just having a few years of no fun at all for no reason.

    “If you feel like you would be able to walk away and say, ‘I’ve had some fun, I wrote a few papers, I’ve had a nice time,’ it will be a lot more positive overall.”

    Fortunately, many physics doctorates who responded to the survey found that their experiences prepared them well for work in other fields. The most commonly cited skills were technical ones like coding, data analysis and programming, which are in high demand in industry. (In 2017, technology magazine WIRED published a story called Move over, coders—physicists will soon rule Silicon Valley.)

    Recent graduate Daniel Klein, who completed his PhD in 2018 at the University of California, San Diego working on CERN’s CMS experiment, says he’s found that much of the work he did processing large datasets for CMS is directly applicable to data science. He’s currently a fellow with Insight, a for-profit company that provides training, mentoring and industry connections for doctoral graduates seeking to enter the data science field.

    “It provides a lot of the same satisfaction as doing research in particle physics, because there are a lot of really interesting conclusions out there that can be made,” Klein says. “Doing a PhD in physics will set you up for success in other endeavors in life, so if it’s something you think you’re really excited about, do it.”

    Survey respondents shared that earning a physics PhD also helps with skills that are useful in nearly any job, like presenting your work clearly, working with large collaborations and across cultures, managing projects, thinking critically and recovering from failures. (The survey also turned up some extremely unexpected skills: underground mining, surviving without sleep, playing cribbage, building a cheese catapult.)

    About one-fifth of the survey’s respondents were not working in physics. Although many of that group worked in data or finance, they also included teachers, a US Navy researcher, an artist, a literary agent and an orchestra manager. “The breadth of skills you can pick up with a well-placed PhD in physics is quite extraordinary,” Smith says.

    Not all respondents agreed that their degree prepared them well for life after physics, and some noted that those skills could also be obtained more easily—and more cheaply—with an undergraduate or master’s degree or other training. It’s clear a physics PhD is not something to be undertaken lightly, and most respondents suggested speaking with others in the field and thinking through career options before making a final decision.

    For nearly everyone who answered, earning a doctoral degree in physics was a unique experience that provided them the opportunity to work with people from all over the globe while attempting to answer some of the universe’s most challenging questions.

    “This may be the most exciting and the most intellectually stimulating time of your life,” wrote respondent Wes Gohn. “Do not dwell on where you will go after, but appreciate what you are doing.”

    See the full article here .


    five-ways-keep-your-child-safe-school-shootings

    Please help promote STEM in your local schools.


    Stem Education Coalition

    Symmetry is a joint Fermilab/SLAC publication.


     
  • richardmitnick 1:19 pm on March 14, 2019 Permalink | Reply
    Tags: "The potential of plasma wakefield acceleration", , , , , , , , Symmetry Magazine   

    From Symmetry: “The potential of plasma wakefield acceleration” 

    Symmetry Mag
    From Symmetry

    03/14/19
    Daniel Garisto

    1
    Illustration by Sandbox Studio, Chicago with Ana Kova

    Scientists around the world are testing ways to further boost the power of particle accelerators while drastically shrinking their size.

    At least when it comes to particle accelerators, bigger is usually better. The bigger the particle accelerator, the more energetic its particle collisions; the more energetic the collision, the greater the variety of particles produced.

    Before CERN’s Large Hadron Collider, the world’s most powerful accelerator was the Tevatron, a circular collider 4 miles long. Scientists used it to discover the last and most massive of the quarks, the top quark. To discover the Higgs boson, the LHC had to be larger still— almost 17 miles around. Scientists are discussing ideas for even bigger accelerators, such as the proposed Future Circular Collider, which would have a colossal circumference of more than 62 miles.

    LHC

    CERN map


    CERN LHC Tunnel

    CERN LHC particles

    FNAL Tevatron

    FNAL/Tevatron map


    FNAL/Tevatron DZero detector


    FNAL/Tevatron CDF detector

    [Don’t forget that the U.S. was going to build the Supercoducting Super Collider [SSC], cancelled by our idiot Congress for having “no immediate economic benefit. If we had built the SSC, Higgs would have been found in the U.S., which instead simply ceded High Energy Physics [HEP] to Europe.

    3

    Its planned ring circumference was 87.1 kilometers (54.1 mi) with an energy of 20 TeV per proton and was set to be the world’s largest and most energetic. It would have greatly surpassed the current record held by the Large Hadron Collider which has ring circumference 27 km (17 mi) and energy of 14 TeV per proton. The project’s director was Roy Schwitters, a physicist at the University of Texas at Austin. Dr. Louis Ianniello served as its first Project Director for 15 months. The project was cancelled in 1993 due to budget problems (read:ignorance and stupidity.]

    CERN FCC Future Circular Collider details of proposed 100km-diameter successor to LHC

    Bigger colliders (and the bigger price tags that come with them) have been essential for advances in particle physics. But what if there were a way to scale down their immense size? What if you could accelerate particles to even higher energies in only a few meters?

    This is the alluring potential of an up-and-coming technology called plasma wakefield acceleration.

    Let’s break down the name. “Plasma” is often referred to as the “fourth state of matter.” It’s created when atoms in a gas are stripped of their electrons, often via a laser. This mixture of ions and free floating electrons behaves like a gas, except that it’s extremely sensitive to electric and magnetic fields.

    A “wake” is created when something is quickly pushed through a fluid or gaseous substance, like a boat cutting through water. In this case, the substance is plasma.

    And “acceleration” simply refers to the effect: When a bunch of particles is placed behind a plasma wake, it accelerates, like a wake surfer.

    There are a variety of ways to create plasma wakefield acceleration, or PWFA. Generally, these can be broken down into “laser wakefield acceleration” and “beam wakefield acceleration.” Both rely on plasma as a medium, but to “drive” the wake, one technique uses lasers while the other uses a beam of particles. Current efforts using this beam technique rely on electrons, protons, or positrons.

    This month, PWFA turns 40. The concept was developed in an audacious 1979 paper by scientists Toshiki Tajima and John Dawson [Physical Review Letters], both then at the University of California, Los Angeles. Today, several hundred physicists at institutes around the world study PWFA.

    In the past few years, advances in the field have turned heads in the larger physics community. Studies have corroborated the technique’s ability to accelerate particles and increased its prospects of practical application. But even if PWFA is as promising as its proponents claim, it will be years if not decades before it begins to succeed traditional accelerating technology.

    RF cavities vs. PWFAs

    Conventional accelerators rely on hollow metal chambers called radio-frequency cavities, or RF cavities. An electric field inside an RF cavity accelerates particles that pass through it.

    “In simple terms, it works like a battery,” says Edda Gschwendtner, a particle physicist who heads the AWAKE plasma wakefield accelerator R&D collaboration at CERN.

    CERN AWAKE schematic


    CERN AWAKE


    CERN AWAKE

    “You have a positive end and a negative end, and then particles … are attracted by the field and get accelerated.”

    This technology is extremely reliable, and used in the nearly 30,000 accelerators around the world. For decades, improvements to the design of RF cavities and larger machines using more and more of them allowed accelerator energy to double about every six years. Recently, however, this trend has been leveling off.

    That’s because RF cavities can sustain electric fields only up to a certain strength—too high and the metal can ionize, releasing electrons that contaminate the vacuum inside the cavities, destroying the RF field inside the cavity.

    Today’s cavities have an acceleration gradient, or increase in energy, of about 10 GeV—10 million electronvolts—per meter. Proposed colliders like the International Linear Collider aim to investigate physics at the Higgs scale—around 125,000 GeV. To reach that energy, electrons and positrons would each have to travel through about 8 miles of cavities. Unless the accelerating technology improves, machines will have to get larger and larger to reach higher energies where physics beyond the Standard Model may be hidden.

    PWFA has the potential to blow these numbers away.

    When he gives talks, physicist Spencer Gessner of the AWAKE team likes to give people an idea of how potent plasma is. “The air in the room that we’re breathing has a particle density of 2.7×1019 particles per cubic centimeter,” Gessner says.

    So what? Well, if you plug that density into an equation that tells you how much acceleration a plasma can support, you get a big number. A really big number, one that puts highly engineered, state-of-the art RF cavities to shame: 500,000 GeV per meter. That’s enough force to produce a Higgs boson in an accelerator the size of a shoe box.

    “We just have to kind of light the air on fire and then drive a wake in that, and you have something a thousand times higher gradient than these finely engineered devices,” Gessner says. It’s a simplification of the process, but his point is clear: Plasma has potential.

    “The beauty of plasma is that it’s basically giving you this enormous acceleration gradient,” Gessner says. “Of course, the complication is harnessing that.”

    And it is certainly easier said than done. The basic principle, though, is easy enough to grasp.

    “Imagine you have a boat which crosses a lake,” Gschwendtner says. “In our case the lake is the plasma, and the boat is what we call the ‘drive beam.’ The drive beam goes into the lake and creates waves, and these are the wakefields.”

    Behind the drive beam sits a “trailing beam,” which in this analogy is like a wake surfer, riding behind a wake.

    “Now what you do is sit electrons onto these wakes, and then they get accelerated,” Gschwendtner says. Why? Wake surfers accelerate because they effectively ride down a watery hill; they’re pulled along by gravity. Electrons or other particles accelerate because they’re pulled by an electric field.

    How do you create an electric field? Plasma is what’s known as “quasi-neutral.” As a whole, the positive charges of its ions are cancelled out by the negative charges of its electrons. But these free-floating electrons are easily pushed around, and a difference smaller than 1 percent in electron density can create a sizeable electric field.

    The strength of the electric field is proportional to the square root of the density; as plasma gets denser, the field can get a bit stronger. A stronger electric field creates more acceleration.

    But how you get that acceleration depends on the type of boat you use.

    Laser wakefield acceleration

    All PWFA experiments require lasers to create a plasma—that’s how they ionize gas. But laser wakefield accelerators also use a laser as a drive beam. The radiation pressure from the laser pushes electrons out of the way. Ions, which are much heavier, remain essentially motionless, while bubbles of electron-free areas propagate forward through the plasma.

    This difference in electron density creates an electric field that accelerates particles placed precisely at the back of a bubble.

    Beam wakefield acceleration

    Beam wakefield acceleration techniques use a beam of particles as a drive beam instead of a laser. Though they’re called “beams,” particle beams aren’t continuous and long like lasers, but instead are short bursts of particles fired in a straight line.

    Plasma wakefield acceleration using electrons

    Using a beam of electrons as the drive beam is similar to using a laser. A bundle of electrons is fired into the plasma; this time it pushes aside other electrons because they are both negatively charged. Again, the ions remain in place so that a positively charged bubble is formed. Particles at the back of the bubble are accelerated because of a strong electric field created by differences in electron density.

    Plasma wakefield acceleration using positrons

    Ideally, physicists would like to be able to use plasma wakefield acceleration to accelerate both electrons and positrons. Because both are fundamental units of matter and matter-antimatter partners, they annihilate cleanly on contact. Compared to the proton-proton collisions of the LHC, electron-positron collisions are incredibly clean and easy to interpret.

    Unfortunately, positrons are trickier to work with. When a bunch of positrons are fired through plasma, they suck in electrons instead of expelling them. Sucking in electrons also creates a similar bubble of mostly electron-free space, but it doesn’t stay electron-free for long—electrons rush down the center to catch up with the positrons. With electrons in the center of the bubble, the electric field can get defocused, so that positrons aren’t accelerated uniformly forward. Physicists have put forward possible solutions that rely on lasers to shape the plasma so that the defocusing effect is mitigated.

    Still, physicists have had some success with positrons, accelerating them to 5000 GeV in about a meter.

    Plasma wakefield acceleration using protons

    Like positrons, protons have a positive charge, which makes them tricky to work with, because they don’t create completely electron-free bubbles. So why work with them? Their energy.

    “The way we accelerate is that we take energy from whatever beam we put in. We give it to the plasma, and the plasma gives it to the charge that we accelerate,” says Diana Amorim, a physicist at Stony Brook University.

    While a bunch of electrons or a laser might hit the plasma with 60 joules of energy, a more massive bunch of protons can have 20,000 joules. Here, it’s again helpful to use the boat and wake surfer analogy.

    “A laser beam or electron beam has little petrol stored. So in these beams, the boat stops on the lake. You cannot accelerate particles for a very long distance,” Gschwendtner says.

    Each joule is about 6 trillion GeV, but most of the energy is inefficiently lost. If scientists could extract the massive energy stored in the bunches of protons, their boat could go for dozens of meters, allowing the particles in their wake to accelerate all along the way.

    Last year, AWAKE successfully used a drive beam of protons to accelerate electrons to 2000 GeV.

    Type of acceleration Experiments
    Laser wakefield acceleration BELLA, TREX, CLF, LUX
    Plasma wakefield acceleration FACET, FACET II, DESY FLASHForward
    using electrons
    Plasma wakefield acceleration FACET, FACET II
    using positrons
    Plasma wakefield acceleration AWAKE
    using protons

    Future questions

    Each of these PWFA techniques has pros and cons, but they’re all still in development and all need to answer one question, Gessner says: Can you have high efficiency, high quality acceleration at the same time?

    High efficiency means that particles in the wake actually get the energy from the drive beam, so it’s not wasted. High quality refers to features of a beam, like the energy spread among the particles in a beam—physicists want all of their accelerated particles to have about the same energy.

    Physicists at FACET accelerator facility at SLAC National Accelerator Laboratory, for example, have already created high efficiency, low quality beams. But getting both features is tricky, because higher energy beams want to misbehave more—they’re more likely to wiggle up or down instead of simply going straight.


    SLAC FACET-II upgrading its Facility for Advanced Accelerator Experimental Tests (FACET) – a test bed for new technologies that could revolutionize the way we build particle accelerators


    SLAC FACET

    To someday replace existing accelerator technology, achieving both is a must for PWFA.

    With experiments at DESY in Germany, CERN in France and Switzerland, and SLAC, Argonne National Laboratory and Lawrence Berkeley National Laboratory in the United States, physicists studying PWFA are confident they’ll continue to take steps toward that goal the next few years.

    “It was easy for the community to be skeptical when you have not shown any results,” Gschwendtner says. “Of course, they are now more convinced because we’ve shown these results.”

    “The amazing thing about plasma accelerators is that the naysayers have been coming up with why things wouldn’t work at every stage of the program,” says Chan Joshi, a particle physicist at UCLA who helped found the field of PWFA.

    In the beginning, he says, naysayers doubted plasma accelerator researchers could reach the high gradient they predicted they could reach, a thousand times larger than the conventional cavity.

    “Well that turned out to be not the case,” he says.

    After that, the doubters thought plasma accelerator researchers would never achieve a narrow-energy-spread beam.

    “Well that turned out not to be the case,” he says.

    Challenges remain, but scientists around the world continue to push the technology forward in the hopes of showing that, while bigger has historically been better, in the future smaller can be best.

    See the full article here .


    five-ways-keep-your-child-safe-school-shootings

    Please help promote STEM in your local schools.


    Stem Education Coalition

    Symmetry is a joint Fermilab/SLAC publication.


     
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