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  • richardmitnick 10:59 am on August 5, 2021 Permalink | Reply
    Tags: "How particle detectors capture matter’s hidden and beautiful reality", , , , , Fermi National Accelerator Laboratory DUNE/LBNF experiment (US)., , , Higgs boson, , , , , W and Z bosons   

    From “Science News (US) : “How particle detectors capture matter’s hidden and beautiful reality” 

    From “Science News (US)

    Emily Conover

    Subatomic particles become visible as graceful arcs and whorls in bubble chambers (this image from 1978) and other detectors. Credit: DOE (US) Fermi National Accelerator Laboratory.

    At every moment, subatomic particles stream in unfathomable numbers through your body. Each second, about 100 billion neutrinos from the sun pass through your thumbnail, and you’re bathed in a rain of muons, birthed in Earth’s atmosphere. Even humble bananas emit positrons, the electron’s antimatter counterpart. A whole universe of particles exists, and we are mostly oblivious, largely because these particles are invisible.

    When I first learned, as a teenager, that this untold world of particles existed, I couldn’t stop thinking about it. And when I thought about it, I could barely breathe. I was, to steal a metaphor from writer David Foster Wallace, a fish who has only just noticed she’s swimming in water. The revelation that we’re stewing in a particle soup is why I went on to study physics, and eventually, to write about it.

    To truly fathom matter at its most fundamental level, people must be able to visualize this hidden world. That’s where particle detectors come in. They spot traces of the universe’s most minuscule constituents, making these intangible concepts real. What’s more, particle detectors reveal beauty: Particles leave behind graceful spirals of bubbles, flashes of light and crisp lines of sparks.

    Tracks from bubble chambers and cloud chambers typically had to be inspected by eye. In this June 1984 image, Renee Jones, a bubble chamber scanner working at Fermilab, measures the details of the tracks, including length and curvature.Credit: David Parker/Science Source(US).

    As a physics student, I spent hours examining these stunning pictures in my textbooks. I went on to build particle detectors in graduate school, and to make my own images of particles wending their way through our world.

    As a particle moves through a material, it drops bread crumbs that can give away its path. Those bread crumbs come in a variety of forms: light, heat or electric charge. “Basically, every particle detector that exists is looking for one or more of those three things,” says particle physicist Jennifer Raaf of Fermilab in Batavia, Ill. Particle detectors translate the bread crumbs into signals that can be recorded and analyzed. Such signals helped reveal the physics of the standard model, a crowning achievement of science that describes the particles and forces of nature. They’re also likely to be key in the discovery of physics beyond the standard model.

    As time has passed, technologies for detecting particles have vastly improved. Here are a few types of detectors that have made the invisible visible.

    Through a cloud

    One of the first ways scientists visualized particle tracks was with cloud chambers. Developed more than a century ago, cloud chambers are filled with a gas — often a vapor of alcohol — on the verge of condensing into liquid. When a charged particle passes through the chamber, it strips electrons from the air within, creating an electric charge that initiates condensation. A wispy line forms along the particle’s path, like a miniature contrail.

    A particle track in a cloud chamber in the early 1930s was the first evidence of a positron, a positively charged particle with the mass of an electron. In 1928, Paul Dirac published a paper proposing that electrons can have both a positive and negative charge. This paper introduced the Dirac equation. The track is curved due to a magnetic field that surrounded the chamber. Credit: C. D. Anderson, courtesy of Emilio Segrè Visual Archives | American Institute of Physics (US)

    Scientists often surround cloud chambers and other detectors with a strong magnetic field, which bends particles’ paths into curves or spirals. Negatively charged particles curve in one direction, positive particles go the opposite way. Other details further characterize the particle: The amount of curvature indicates a particle’s momentum, for example.

    Cloud chambers revealed a variety of previously unknown particles, including the positron and the muon, a heavy cousin of the electron, in the 1930s. These particles were mostly unexpected. At the time, physicists were barely coming to grips with the fact that particles besides electrons and protons existed.

    In this 1948 image, physicist Clifford Butler (center) is adjusting the instruments on a cloud chamber intended to track particles in cosmic rays. These showers of particles are produced when a high-energy particle from space slams into Earth’s atmosphere. Credit: Picture Post/Hulton Archive/Getty Images

    Bubble trails

    The 1950s were all about bubble chambers.

    When charged particles pass through liquid in a bubble chamber, they leave tiny vapor bubbles, like iridescent orbs trailing a soap bubble wand. Although the chambers are typically filled with liquid hydrogen, a variety of liquids can be used; one early prototype even used beer. Bubble chambers could be made bigger than cloud chambers, and produced sharper tracks, making it possible to observe more particles in more detail.

    A subatomic particle called a kaon decays into other particles that leave distinct spirals in this bubble chamber image from the 1970s. Credit: European Organization for Nuclear Research [Organisation européenne pour la recherche nucléaire] [Europäische Organisation für Kernforschung](CH) [CERN].

    In the same decade, particle accelerators came to the fore. These accelerators produce energetic beams of particles that scientists can crash into other particles or into targets. Those collisions whip up a flurry of new particles. Scientists sent those beams into bubble chambers to watch what happened.

    The Big European Bubble Chamber, pictured during installation of the vessel, started up at CERN near Geneva in 1973.Credit: CERN.

    The resulting images were not only scientifically illuminating, they were stunning: If Raaf were going to get a tattoo, she says, it might be a bubble chamber image. I’ve so far resisted the temptation to get ink.

    Going digital

    Cloud chambers and bubble chambers had a drawback. Tracks were typically recorded with photographs, and each had to be inspected by eye for anything of interest. That process was too slow; it held physicists back from discovering the particles that might show up in only one or two out of myriad photographs, if that. To find the rarest of particles, “you can’t really be looking at pictures. You want to have that information digitized in a smart way,” says Sam Zeller, a particle physicist at Fermilab.

    In the UA1 detector at CERN near Geneva, high-voltage wires recorded the electric charge produced when incoming particles dislodged electrons from atoms in a gas-filled chamber. In this computer display, a proton and antiproton have collided and annihilated, producing new particles that traced out paths throughout the detector.Credit: Peter I.P. Kalmus, UA1 Experiment/Science Source.

    Enter the multiwire proportional chamber. Invented in 1968, this technology relies on a fine array of high-voltage wires, which record charge produced when incoming particles dislodge electrons from atoms in a gas-filled chamber. This technique could capture millions of particle tracks per second, much more than bubble chambers could achieve. And the data went directly to a computer for analysis. Multiwire proportional chambers and their descendants revolutionized particle physics, and led to discoveries of particles such as the charm quark and the gluon in the 1970s, and the W and Z bosons in the 1980s.

    CERN’s UA1 detector was active from 1981 until 1990; its most notable discoveries were the W and Z bosons, together with the UA2 experiment. This image shows a section of the experiment, strung with many fine wires, on display at the CERN museum. Credit: Mark Williamson/Wikimedia Commons (CC BY-SA 4.0).

    Some of the most advanced modern detectors trace their lineage back to multiwire proportional chambers, such as liquid argon time projection chambers. These detectors are high-resolution, meaning that researchers can zoom in on the details of an interaction and visualize it in 3-D. Liquid argon time projection chambers will be key to one of the biggest upcoming particle physics experiments in the United States, the Fermi National Accelerator Laboratory DUNE/LBNF experiment (US) in South Dakota. Because neutrinos very rarely interact with matter, the experiment demands such advanced detection techniques.

    Shining a light

    Scientists have also devised methods to detect particles via light. When a particle moves above a certain speed limit for a given material, it emits light, known as Čerenkov light. It’s analogous to an airplane passing the sound-speed barrier and creating a sonic boom. Charged particles can also emit light when passing through materials laced with certain chemicals, called scintillators.

    The NOvA experiment at Fermilab (US) uses tubes of liquid scintillator to spot neutrinos interacting inside the detector. In this image of data from the detector, a neutrino, which enters from the left, produces a spurt of charged particles. The neutrino is not visible, due to its lack of electric charge. Credit: NOvA/Fermilab.

    To spot the small amounts of light left behind by individual particles, scientists use photomultiplier tubes, originally invented in the 1930s, which convert light into electrical signals. These tubes could be used to pick up either Čerenkov light or scintillator light.

    Scintillator detectors began to prove their worth in 1956 when a tank of liquid scintillator was used to discover the neutrino — once thought to be entirely undetectable. Liquid scintillator detectors are still common — used in the NOvA neutrino experiment at Fermilab, for example — as are detectors made of solid plastic strips with scintillator mixed in.

    The NOvA neutrino experiment at Fermilab uses two detectors, this one located in Minnesota, made up of hundreds of thousands of PVC tubes filled with liquid scintillator. Credit: Justinvasel/Wikimedia Commons (CC BY-SA 4.0).

    Putting it all together

    The Tevatron was a circular particle accelerator (active until 2011) in the United States, at the Fermi National Accelerator Laboratory (also known as Fermilab), east of Batavia, Illinois, and is the second highest energy particle collider ever built, after the Large Hadron Collider (LHC) of the European Organization for Nuclear Research (CERN) near Geneva, Switzerland. The Tevatron was a synchrotron that accelerated protons and antiprotons in a 6.28 km (3.90 mi) ring to energies of up to 1 TeV, hence its name. The Tevatron was completed in 1983 at a cost of $120 million and significant upgrade investments were made during its active years of 1983–2011.

    The main achievement of the Tevatron was the discovery in 1995 of the top quark—the last fundamental fermion predicted by the Standard Model of particle physics. On July 2, 2012, scientists of the CDF and DØ collider experiment teams at Fermilab announced the findings from the analysis of around 500 trillion collisions produced from the Tevatron collider since 2001, and found that the existence of the suspected Higgs boson was highly likely with a confidence of 99.8%, later improved to over 99.9%.

    The Tevatron ceased operations on 30 September 2011, due to budget cuts and because of the completion of the LHC, which began operations in early 2010 and is far more powerful (planned energies were two 7 TeV beams at the LHC compared to 1 TeV at the Tevatron). The main ring of the Tevatron will probably be reused in future experiments, and its components may be transferred to other particle accelerators.



    Modern detectors at the world’s major particle colliders, like the detectors at the Large Hadron Collider at CERN near Geneva, throw in a bit of everything. “It’s this onion of different types of detectors; every layer is a different thing,” Raaf says.









    Standing multiple stories tall, these massive machines include an assortment of technologies — plastic scintillator detectors, Cherenkov detectors, descendants of multiwire proportional chambers. They also typically include detectors made from silicon that can precisely measure particle tracks based on small electric currents produced when particle pass through. These detectors all work in concert within a very strong magnet. After particles collide at the center of the detector, computers crunch the data from all the parts and reconstruct what happened in the collision, tracing out the paths the particles took.

    No matter the technique, the mesmerizing subatomic hieroglyphs allow physicists to decipher the native language of matter, unveiling its constituents and the forces by which they communicate. “It’s pretty amazing that you can see the invisible,” says Zeller.

    See the full article here .


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  • richardmitnick 12:54 pm on July 13, 2021 Permalink | Reply
    Tags: "Plasma Particle Accelerators Could Find New Physics", , Accelerators come in two shapes: circular (synchrotron) or linear (linac)., At the start of the 20th century scientists had little knowledge of the building blocks that form our physical world., , By the end of the century they had discovered not just all the elements that are the basis of all observed matter but a slew of even more fundamental particles that make up our cosmos., CERN CLIC collider, CERN is proposing a 100-kilometer-circumference electron-positron and proton-proton collider called the Future Circular Collider., , , , Higgs boson, International Linear Collider (ILC), , , , Plasma is often called the fourth state of matter., , ,   

    From Scientific American (US) : “Plasma Particle Accelerators Could Find New Physics” 

    From Scientific American (US)

    July 2021
    Chandrashekhar Joshi

    Credit: Peter and Maria Hoey.

    At the start of the 20th century scientists had little knowledge of the building blocks that form our physical world. By the end of the century they had discovered not just all the elements that are the basis of all observed matter but a slew of even more fundamental particles that make up our cosmos, our planet and ourselves. The tool responsible for this revolution was the particle accelerator.

    The pinnacle achievement of particle accelerators came in 2012, when the Large Hadron Collider (LHC) uncovered the long-sought Higgs boson particle.

    The LHC is a 27-kilometer accelerating ring that collides two beams of protons with seven trillion electron volts (TeV) of energy each at CERN near Geneva.

    It is the biggest, most complex and arguably the most expensive scientific device ever built. The Higgs boson was the latest piece in the reigning theory of particle physics called the Standard Model. Yet in the almost 10 years since that discovery, no additional particles have emerged from this machine or any other accelerator.

    Have we found all the particles there are to find? Doubtful. The Standard Model of particle physics does not account for dark matter—particles that are plentiful yet invisible in the universe. A popular extension of the Standard Model called supersymmetry predicts many more particles out there than the ones we know about.

    And physicists have other profound unanswered questions such as: Are there extra dimensions of space? And why is there a great matter-antimatter imbalance in the observable universe? To solve these riddles, we will likely need a particle collider more powerful than those we have today.

    Many scientists support a plan to build the International Linear Collider (ILC), a straight-line-shaped accelerator that will produce collision energies of 250 billion (giga) electron volts (GeV).

    Though not as powerful as the LHC, the ILC would collide electrons with their antimatter counterparts, positrons—both fundamental particles that are expected to produce much cleaner data than the proton-proton collisions in the LHC. Unfortunately, the design of the ILC calls for a facility about 20 kilometers long and is expected to cost more than $10 billion—a price so high that no country has so far committed to host it.

    In the meantime, there are plans to upgrade the energy of the LHC to 27 TeV in the existing tunnel by increasing the strength of the superconducting magnets used to bend the protons. Beyond that, CERN is proposing a 100-kilometer-circumference electron-positron and proton-proton collider called the Future Circular Collider.

    Such a machine could reach the unprecedented energy of 100 TeV in proton-proton collisions. Yet the cost of this project will likely match or surpass the ILC. Even if it is built, work on it cannot begin until the LHC stops operation after 2035.

    But these gargantuan and costly machines are not the only options. Since the 1980s physicists have been developing alternative concepts for colliders. Among them is one known as a plasma-based accelerator, which shows great promise for delivering a TeV-scale collider that may be more compact and much cheaper than machines based on the present technology.

    The Particle Zoo

    The story of particle accelerators began in 1897 at the Cavendish physics laboratory at the University of Cambridge (UK).

    There J. J. Thomson created the earliest version of a particle accelerator using a tabletop cathode-ray tube like the ones used in most television sets before flat screens. He discovered a negatively charged particle—the electron.

    Soon physicists identified the other two atomic ingredients—protons and neutrons—using radioactive particles as projectiles to bombard atoms. And in the 1930s came the first circular particle accelerator—a palm-size device invented by Ernest Lawrence called the cyclotron, which could accelerate protons to about 80 kilovolts.

    Ernest Lawrence’s First Cyclotron, 1930 Stock Photo – Alamy.

    Thereafter accelerator technology evolved rapidly, and scientists were able to increase the energy of accelerated charged particles to probe the atomic nucleus. These advances led to the discovery of a zoo of hundreds of subnuclear particles, launching the era of accelerator-based high-energy physics. As the energy of accelerator beams rapidly increased in the final quarter of the past century, the zoo particles were shown to be built from just 17 fundamental particles predicted by the Standard Model [above]. All of these, except the Higgs boson, had been discovered in accelerator experiments by the late 1990s. The Higgs’s eventual appearance [above] at the LHC made the Standard Model the crowning achievement of modern particle physics.

    Aside from being some of the most successful instruments of scientific discovery in history, accelerators have found a multitude of applications in medicine and in our daily lives. They are used in CT scanners, for x-rays of bones and for radiotherapy of malignant tumors. They are vital in food sterilization and for generating radioactive isotopes for myriad medical tests and treatments. They are the basis of x-ray free-electron lasers, which are being used by thousands of scientists and engineers to do cutting-edge research in physical, life and biological sciences.

    Scientist tests a prototype plasma accelerator at the Facility for Advanced Accelerator Experimental Tests (FACET) at the DOE’s SLAC National Accelerator Laboratory (US) in California. Credit: Brad Plummer and SLAC National Accelerator Laboratory.

    Accelerator Basics

    Accelerators come in two shapes: circular (synchrotron) or linear (linac). All are powered by radio waves or microwaves that can accelerate particles to near light speed. At the LHC, for instance, two proton beams running in opposite directions repeatedly pass through sections of so-called radio-frequency cavities spaced along the ring.

    Radio waves inside these cavities create electric fields that oscillate between positive and negative to ensure that the positively charged protons always feel a pull forward. This pull speeds up the protons and transfers energy to them. Once the particles have gained enough energy, magnetic lenses focus the proton beams to several very precise collision points along the ring. When they crash, they produce extremely high energy densities, leading to the birth of new, higher-mass particles.

    When charged particles are bent in a circle, however, they emit “synchrotron radiation.” For any given radius of the ring, this energy loss is far less for heavier particles such as protons, which is why the LHC is a proton collider. But for electrons the loss is too great, particularly as their energy increases, so future accelerators that aim to collide electrons and positrons must either be linear colliders or have very large radii that minimize the curvature and thus the radiation the electrons emit.

    The size of an accelerator complex for a given beam energy ultimately depends on how much radio-frequency power can be pumped into the accelerating structure before the structure suffers electrical breakdown. Traditional accelerators have used copper to build this accelerating structure, and the breakdown threshold has meant that the maximum energy that can be added per meter is between 20 million and 50 million electron volts (MeV). Accelerator scientists have experimented with new types of accelerating structures that work at higher frequencies, thereby increasing the electrical breakdown threshold. They have also been working on improving the strength of the accelerating fields within superconducting cavities that are now routinely used in both synchrotrons and linacs. These advances are important and will almost certainly be implemented before any paradigm-changing concepts disrupt the highly successful conventional accelerator technologies.

    Eventually other strategies may be necessary. In 1982 the U.S. Department of Energy’s program on high-energy physics started a modest initiative to investigate entirely new ways to accelerate charged particles. This program generated many ideas; three among them look particularly promising.

    The first is called two-beam acceleration. This scheme uses a relatively cheap but very high-charge electron pulse to create high-frequency radiation in a cavity and then transfers this radiation to a second cavity to accelerate a secondary electron pulse. This concept is being tested at CERN on a machine called the Compact Linear Collider (CLIC).

    Another idea is to collide muons, which are much heavier cousins to electrons. Their larger mass means they can be accelerated in a circle without losing as much energy to synchrotron radiation as electrons do. The downside is that muons are unstable particles, with a lifetime of two millionths of a second. They are produced during the decay of particles called pions, which themselves must be produced by colliding an intense proton beam with a special target. No one has ever built a muon accelerator, but there are die-hard proponents of the idea among accelerator scientists.

    Finally, there is plasma-based acceleration. The notion originated in the 1970s with John M. Dawson of the University of California-Los Angeles (US), who proposed using a plasma wake produced by an intense laser pulse or a bunch of electrons to accelerate a second bunch of particles 1,000 or even 10,000 times faster than conventional accelerators can. This concept came to be known as the plasma wakefield accelerator.


    It generated a lot of excitement by raising the prospect of miniaturizing these gigantic machines, much like the integrated circuit miniaturized electronics starting in the 1960s.

    The Fourth State of Matter

    Most people are familiar with three states of matter: solid, liquid and gas. Plasma is often called the fourth state of matter. Though relatively uncommon in our everyday experience, it is the most common state of matter in our universe. By some estimates more than 99 percent of all visible matter in the cosmos is in the plasma state—stars, for instance, are made of plasma. A plasma is basically an ionized gas with equal densities of electrons and ions. Scientists can easily form plasma in laboratories by passing electricity through a gas as in a common fluorescent tube.

    A plasma wakefield accelerator takes advantage of the kind of wake you can find trailing a motorboat or a jet plane. As a boat moves forward, it displaces water, which moves out behind the boat to form a wake. Similarly, a tightly focused but ultraintense laser pulse moving through a plasma at the speed of light can generate a relativistic wake (that is, a wake also propagating nearly at light speed) by exerting radiation pressure and displacing the plasma electrons out of its way. If, instead of a laser pulse, a high-energy, high-current electron bunch is sent through the plasma, the negative charge of these electrons can expel all the plasma electrons, which feel a repulsive force. The heavier plasma ions, which are positively charged, remain stationary. After the pulse passes by, the expelled electrons are attracted back toward the ions by the force between their negative and positive charges. The electrons move so quickly they overshoot the ions and then again feel a backward pull, setting up an oscillating wake. Because of the separation of the plasma electrons from the plasma ions, there is an electric field inside this wake.

    If a second “trailing” electron bunch follows the first “drive” pulse, the electrons in this trailing bunch can gain energy from the wake much in the same way an electron bunch is accelerated by the radio-frequency wave in a conventional accelerator. If there are enough electrons in the trailing bunch, they can absorb sufficient energy from the wake so as to dampen the electric field. Now all the electrons in the trailing bunch see a constant accelerating field and gain energy at the same rate, thereby reducing the energy spread of the beam.

    The main advantage of a plasma accelerator over other schemes is that electric fields in a plasma wake can easily be 1,000 times stronger than those in traditional radio-frequency cavities. Plus, a very significant fraction of the energy that the driver beam transfers to the wake can be extracted by the trailing bunch. These effects make a plasma wakefield-based collider potentially both more compact and cheaper than conventional colliders.

    The Future of Plasma

    Both laser- and electron-driven plasma wakefield accelerators have made tremendous progress in the past two decades. My own team at U.C.L.A. has carried out prototype experiments with SLAC National Accelerator Laboratory physicists at their Facility for Advanced Accelerator Experimental Tests (FACET) in Menlo Park, Calif.

    We injected both drive and trailing electron bunches with an initial energy of 20 GeV and found that the trailing electrons gained up to 9 GeV after traveling through a 1.3-meter-long plasma. We also achieved a gain of 4 GeV in a positron bunch using just a one-meter-long plasma in a proof-of-concept experiment. Several other labs around the world have used laser-driven wakes to produce multi-GeV energy gains in electron bunches.

    Plasma accelerator scientists’ ultimate goal is to realize a linear accelerator that collides tightly focused electron and positron, or electron and electron, beams with a total energy exceeding 1 TeV. To accomplish this feat, we would likely need to connect around 50 individual plasma accelerator stages in series, with each stage adding an energy of 10 GeV.

    Yet aligning and synchronizing the drive and the trailing beams through so many plasma accelerator stages to collide with the desired accuracy presents a huge challenge. The typical radius of the wake is less than one millimeter, and scientists must inject the trailing electron bunch with submicron accuracy. They must synchronize timing between the drive pulse and the trailing beam to less than a hundredth of a trillionth of one second. Any misalignment would lead to a degradation of the beam quality and a loss of energy as well as charge caused by oscillation of the electrons about the plasma wake axis. This loss shows up in the form of hard x-ray emission, known as betatron emission, and places a finite limit on how much energy we can obtain from a plasma accelerator.

    Other technical hurdles also stand in the way of immediately turning this idea into a collider. For instance, the primary figure of merit for a particle collider is the luminosity—basically a measure of how many particles you can squeeze through a given space in a given time. The luminosity multiplied by the cross section—or the chances that two particles will collide— tells you how many collisions of a particular kind per second you are likely to observe at a given energy. The desired luminosity for a 1-TeV electron-positron linear collider is 10^34 cm^–2s^–1. Achieving this luminosity would require the colliding beams to have an average power of 20 megawatts each—10^10 particles per bunch at a repetition rate of 10 kilohertz and a beam size at the collision point of tens of a billionth of a meter. To illustrate how difficult this is, let us focus on the average power requirement. Even if you could transfer energy from the drive beam to the accelerating beam with 50 percent efficiency, 20 megawatts of power will be left behind in the two thin plasma columns. Ideally we could partially recover this power, but it is far from a straightforward task.

    And although scientists have made substantial progress on the technology needed for the electron arm of a plasma-based linear collider, positron acceleration is still in its infancy. A decade of concerted basic science research will most likely be needed to bring positrons to the same point we have reached with electrons. Alternatively, we could collide electrons with electrons or even with protons, where one or both electron arms are based on a plasma wakefield accelerator. Another concept that scientists are exploring at CERN is modulating a many-centimeters-long proton bunch by sending it through a plasma column and using the accompanying plasma wake to accelerate an electron bunch.

    The future for plasma-based accelerators is uncertain but exciting. It seems possible that within a decade we could build 10-GeV plasma accelerators on a large tabletop for various scientific and commercial applications using existing laser and electron beam facilities. But this achievement would still put us a long way from realizing a plasma-based linear collider for new physics discoveries. Even though we have made spectacular experimental progress in plasma accelerator research, the beam parameters achieved to date are not yet what we would need for just the electron arm of a future electron-positron collider that operates at the energy frontier. Yet with the prospects for the International Linear Collider and the Future Circular Collider uncertain, our best bet may be to persist with perfecting an exotic technology that offers size and cost savings. Developing plasma technology is a scientific and engineering grand challenge for this century, and it offers researchers wonderful opportunities for taking risks, being creative, solving fascinating problems—and the tantalizing possibility of discovering new fundamental pieces of nature.

    See the full article here .

    Please help promote STEM in your local schools.

    Stem Education Coalition

    Scientific American (US) , the oldest continuously published magazine in the U.S., has been bringing its readers unique insights about developments in science and technology for more than 160 years.

  • richardmitnick 3:16 pm on June 14, 2021 Permalink | Reply
    Tags: "Leptoquarks; the Higgs boson; and the muon’s magnetism", "muon anomaly", , , , , , Higgs boson, , ,   

    From European Organization for Nuclear Research [Organisation européenne pour la recherche nucléaire] [Europäische Organisation für Kernforschung](CH) [CERN]: “Leptoquarks; the Higgs boson; and the muon’s magnetism” 

    Cern New Bloc

    Cern New Particle Event

    From European Organization for Nuclear Research [Organisation européenne pour la recherche nucléaire] [Europäische Organisation für Kernforschung](CH) [CERN]

    14 June, 2021
    Ana Lopes

    Displays of candidate events for a Higgs boson decaying into two muons, as recorded by CMS (left) and ATLAS (right). (Image: CERN)

    Zoom into an online particle physics conference, and the chances are you’ll hear the term muon anomaly. This is a longstanding tension with the Standard Model of particle physics, seen in the magnetism of a heavier cousin of the electron called a muon, that has recently been strengthened by measurements made at DOE’s Fermi National Accelerator Laboratory (US) in the US.

    In a paper accepted for publication in Physical Review Letters, a trio of theorists including Andreas Crivellin of CERN shows that a class of new unknown particles that could account for the muon anomaly, known as leptoquarks, also affects the transformation, or “decay”, of the Higgs boson into muons.

    Leptoquarks are hypothetical particles that connect quarks and leptons, the two types of particles that make up matter at the most fundamental level. They are a popular explanation for the muon anomaly and other anomalies seen in certain decays of particles called B mesons.

    In their new study, Crivellin and his colleagues explored how two kinds of leptoquarks that could explain the muon anomaly would affect the rare decay of the Higgs boson into muons, of which the ATLAS and CMS experiments recently obtained the first indications.

    They found that one of the two kinds of leptoquarks increases the rate at which this Higgs decay takes place, while the other one decreases it.

    “The current measurements of the Higgs decay to muons are not sufficient to see this increase or decrease, and the muon anomaly has yet to be confirmed,” says Crivellin. “But if future measurements, at the LHC or future colliders, display such a change, and the muon anomaly is confirmed, it will be possible to pick out which of the two kinds of leptoquarks would be more likely to explain the muon anomaly.”

    See the full article here.

    Please help promote STEM in your local schools.

    Stem Education Coalition

    Meet CERN in a variety of places:

    Quantum Diaries

    Cern Courier








    European Organization for Nuclear Research (Organisation européenne pour la recherche nucléaire)(EU)[CERN] AEGIS.

    CERN FASER is designed to study the interactions of high-energy neutrinos and search for new as-yet-undiscovered light and weakly interacting particles. Such particles are dominantly produced along the beam collision axis and may be long-lived particles, travelling hundreds of metres before decaying. The existence of such new particles is predicted by many models beyond the Standard Model that attempt to solve some of the biggest puzzles in physics, such as the nature of dark matter and the origin of neutrino masses.

  • richardmitnick 10:23 am on January 20, 2021 Permalink | Reply
    Tags: "Light-controlled Higgs modes found in superconductors; potential sensor; computing uses", , , , , , Higgs boson, , , , , , , Quantum terahertz spectroscopy, The Higgs mode sensors on the table-top have the potential help us discover the hidden secrets of quantum states of matter., This understanding could advance a new “quantum revolution” for high-speed computing and information technologies.   

    From Iowa State University: “Light-controlled Higgs modes found in superconductors; potential sensor; computing uses” 

    From Iowa State University

    Jigang Wang, Physics and Astronomy,
    Ames Laboratory

    Mike Krapfl
    News Service

    This illustration shows light at trillions of pulses per second (red flash) accessing and controlling Higgs modes (gold balls) in an iron-based superconductor. Even at different energy bands, the Higgs modes interact with each other (white smoke). Credit: Jigang Wang.

    Even if you weren’t a physics major, you’ve probably heard something about the Higgs boson.

    CERN CMS Higgs Event May 27, 2012.

    CERN ATLAS Higgs Event>June 12, 2012.

    There was the title of a 1993 book by Nobel laureate Leon Lederman that dubbed the Higgs The God Particle. There was the search for the Higgs particle that launched after 2009’s first collisions inside the Large Hadron Collider in Europe.


    CERN map

    CERN LHC Maximilien Brice and Julien Marius Ordan

    CERN LHC particles



    CERN ATLAS Image Claudia Marcelloni CERN/ATLAS


    CERN/ALICE Detector




    CERN/LHCb detector

    There was the 2013 announcement that Peter Higgs and Francois Englert won the Nobel Prize in Physics for independently theorizing in 1964 that a fundamental particle – the Higgs – is the source of mass in subatomic particles, making the universe as we know it possible.

    Peter Higgs

    (Plus, there are the Iowa State University physicists on the author list of a 2012 research paper describing how the ATLAS Experiment at the collider observed a new particle later confirmed to be the Higgs.)

    And now Jigang Wang, a professor of physics and astronomy at Iowa State and a senior scientist at the U.S. Department of Energy’s Ames Laboratory, and a team of researchers have discovered a form of the famous particle within a superconductor, a material capable of conducting electricity without resistance, generally at very cold temperatures.

    Wang and his collaborators – including Chang-Beom Eom, the Raymond R. Holton Chair for Engineering and Theodore H. Geballe Professor at the University of Wisconsin-Madison; Ilias Perakis, professor and chair of physics at the University of Alabama at Birmingham; and Eric Hellstrom, professor and interim chair of mechanical engineering at Florida State University – report the details in a paper recently published online by the journal Nature Communications.

    They write that in lab experiments they’ve found a short-lived “Higgs mode” within iron-based, high-temperature (but still very cold), multi-energy band, unconventional superconductors.

    A quantum discovery

    This Higgs mode is a state of matter found at the quantum scale of atoms, their electronic states and energetic excitations. The mode can be accessed and controlled by laser light flashing on the superconductor at terahertz frequencies of trillions of pulses per second. The Higgs modes can be created within different energy bands and still interact with each other.

    Wang said this Higgs mode within a superconductor could potentially be used to develop new quantum sensors.

    “It’s just like the Large Hadron Collider can use the Higgs particle to detect dark energy or antimatter to help us understand the origin of the universe,” Wang said. “And our Higgs mode sensors on the table-top have the potential help us discover the hidden secrets of quantum states of matter.”

    That understanding, Wang said, could advance a new “quantum revolution” for high-speed computing and information technologies.

    “It’s one way this exotic, strange, quantum world can be applied to real life,” Wang said.

    Light control of superconductors

    The project takes a three-pronged approach to accessing and understanding the special properties, such as this Higgs mode, hidden within superconductors:

    Wang’s research group uses a tool called quantum terahertz spectroscopy to visualize and steer pairs of electrons moving through a superconductor. The tool uses laser flashes as a control knob to accelerate supercurrents and access new and potentially useful quantum states of matter.

    Eom’s group developed the synthesis technique that produces crystalline thin films of the iron-based superconductor with high enough quality to reveal the Higgs mode. Hellstrom’s group developed deposition sources for the iron-based superconducting thin film development.

    Perakis’ group led the development of quantum models and theories to explain the results of the experiments and to simulate the salient features that come from the Higgs mode.

    The work has been supported by a grant to Wang from the National Science Foundation and grants to Eom and Perakis from the U.S. Department of Energy.

    “Interdisciplinary science is the key here,” Perakis said. “We have quantum physics, materials science and engineering, condensed matter physics, lasers and photonics with inspirations from fundamental, high-energy and particle physics.”

    There are good, practical reasons for researchers in all those fields to work together on the project. In this case, students from the four research groups worked together with their advisors to accomplish this discovery.

    “Scientists and engineers,” Wang wrote in a research summary, “have recently come to realize that certain materials, such as superconductors, have properties that can be exploited for applications in quantum information and energy science, e.g., processing, recording, storage and communication.”

    See the full article here .


    Please help promote STEM in your local schools.

    Stem Education Coalition

    Iowa State University is a public, land-grant university, where students get a great academic start in learning communities and stay active in 800-plus student organizations, undergrad research, internships and study abroad. They learn from world-class scholars who are tackling some of the world’s biggest challenges — feeding the hungry, finding alternative fuels and advancing manufacturing.

    Iowa Agricultural College and Model Farm (now Iowa State University) was officially established on March 22, 1858, by the legislature of the State of Iowa. Story County was selected as a site on June 21, 1859, and the original farm of 648 acres was purchased for a cost of $5,379. The Farm House, the first building on the Iowa State campus, was completed in 1861, and in 1862, the Iowa legislature voted to accept the provision of the Morrill Act, which was awarded to the agricultural college in 1864.

    Iowa State University Knapp-Wilson Farm House. Photo between 1911-1926

    Iowa Agricultural College (Iowa State College of Agricultural and Mechanic Arts as of 1898), as a land grant institution, focused on the ideals that higher education should be accessible to all and that the university should teach liberal and practical subjects. These ideals are integral to the land-grant university.

    The first official class entered at Ames in 1869, and the first class (24 men and 2 women) graduated in 1872. Iowa State was and is a leader in agriculture, engineering, extension, home economics, and created the nation’s first state veterinary medicine school in 1879.

    In 1959, the college was officially renamed Iowa State University of Science and Technology. The focus on technology has led directly to many research patents and inventions including the first binary computer (the ABC), Maytag blue cheese, the round hay baler, and many more.

    Beginning with a small number of students and Old Main, Iowa State University now has approximately 27,000 students and over 100 buildings with world class programs in agriculture, technology, science, and art.

    Iowa State University is a very special place, full of history. But what truly makes it unique is a rare combination of campus beauty, the opportunity to be a part of the land-grant experiment, and to create a progressive and inventive spirit that we call the Cyclone experience. Appreciate what we have here, for it is indeed, one of a kind.

  • richardmitnick 5:21 pm on December 1, 2020 Permalink | Reply
    Tags: , , , , , Higgs boson, "Naturalness Hits a Snag with Higgs"   

    From “Physics”: “Naturalness Hits a Snag with Higgs” 

    About Physics

    From “Physics”

    November 24, 2020
    Nathaniel Craig

    A theoretical approach called naturalness has helped physicists understand several particle physics puzzles—but the Higgs boson’s unsuitably small mass is currently foiling this strategy.

    Figure 1: The Higgs mass is 10^17 times smaller than the Planck mass, but quantum corrections from Higgs interactions with other particles should cause the two masses to be nearly equal. This dilemma is deemed the electroweak hierarchy problem.
    Credit: APS/Alan Stonebraker.

    The discovery of the Higgs boson at the Large Hadron Collider (LHC) in 2012 illuminated the mechanism of electroweak symmetry breaking, through which the electromagnetic and weak nuclear forces emerge from a unified electroweak force.

    CERN CMS Higgs Event May 27, 2012.

    CERN ATLAS Higgs Event
    June 12, 2012.

    But if the Higgs discovery answered the question of how electroweak symmetry breaking occurs, it left open the further question of why this breaking occurs at around 250 GeV—an energy that is far removed from other energy scales in particle physics. This energy mismatch goes by the name of the electroweak hierarchy problem, and it is one of the great mysteries in physics. The central question is often reframed in terms of the Higgs mass: why does the Higgs weigh 125GeV∕c2 when a naïve prediction (Fig. 1) would place it 17 orders of magnitude higher, near the Planck mass?

    Particle physicists have previously faced incongruities like this, in which parameters that seem closely related have values that are far from each other. One way of dealing with these large gaps, or hierarchies, is to use the “naturalness” strategy. Naturalness is a theoretical perspective, which assumes that nature actually does choose values that are close to each other, but this only becomes evident when one identifies a symmetry or other mechanism that explains the apparent discrepancy. This strategy has successfully explained the values of the electron mass and the pion mass, as well as predicted the existence of the charm quark. One would assume the same strategy could explain the Higgs mass. But so far it hasn’t. Physicists are therefore proposing alternative strategies that go beyond the usual naturalness paradigm.

    The Higgs Paradigm

    One of the first physicists to address hierarchy problems was Paul Dirac, who was struck by the enormous difference between the proton mass ( ∼1GeV∕c^2) and the Planck mass ( ∼1019GeV∕c^2). Dirac’s desire to understand this hierarchy motivated him to create an elaborate cosmology in which fundamental constants varied as a function of time [1]. Although Dirac’s proposed answer turned out to be wrong, his interest in the question was justified. Decades after his work on the subject, the explanation for the proton mass emerged from considerations of the strong force, specifically its increase in strength at long range—opposite to what occurs for other forces. This behavior, which is called asymptotic freedom, sets the mass scale for the proton and other quark bound states.

    It is tempting to posit the same asymptotic-freedom mechanism to explain the energy scale of electroweak symmetry breaking—a proposal known as technicolor [2]. However, the discovery of the Higgs boson (Fig. 2) suggests that a qualitatively different mechanism is at play. The Higgs boson is the particle excitation of a scalar field, whose value in the vacuum is what breaks the unified electroweak force into its low-energy remnants and gives masses to the fundamental particles of the standard model (see Viewpoint: A Fuller Picture of the Higgs Boson). But if electroweak symmetry is indeed broken by the Higgs field’s behavior, the mystery only deepens. The problem can be stated like this: the Higgs gives mass to all other particles, but all other particles give mass to the Higgs—through quantum corrections to the mass term of the scalar field. This pile-on effect should drive the Higgs mass to the Planck scale, but the Higgs has resisted this expected weight gain in view of the electroweak hierarchy problem.

    The essence of the hierarchy problem is familiar to anyone who has taken undergraduate electrodynamics. The electron is surrounded by an electric field that diverges at small distances from the electron’s point-like charge. The energy in this field is called the self-energy, and it should contribute to the electron’s mass (just as quantum corrections contribute to the Higgs mass). But if one uses the current bounds on the radius of the electron, re<10^−18cm, then the self-energy contribution to the mass is greater than 100GeV∕c^2—which is a million times the measured rest mass of 511keV∕c^2 for the electron. One could assume that the electron’s “bare” mass—the mass not coming from the electric field—somehow cancels most of the self-energy contribution, but that kind of balancing act would require these mass values to be bewilderingly close to each other—to a precision of one part in a million.

    Such fine tuning seems unnatural—as if the electron were put together like a delicate watch. But this picture can be avoided by making the self-energy naturally smaller through a change in the electric field. This change comes out of the relativistic quantum theory of electrodynamics, which predicts that the strong electric field around the electron’s charge causes the spontaneous formation and rapid annihilation of virtual electron-positron pairs. These so-called quantum loops screen the electron charge, thus modifying the field at radii near to re (Fig. 3). As a result, the self-energy tunes itself down to precisely the same order as the observed rest energy, rendering the outcome “natural.”

    Figure 3: Quantum electrodynamics predicts that the electric charge of an electron is screened by virtual electron-positron pairs that pop in and out from the vacuum. Credit: APS/Alan Stonebraker.

    If one digs deeper, one finds that this electron-mass explanation relies on a symmetry of nature. Specifically, the electron and positron obey chiral symmetry at energies greater than the electron mass. At lower energies, chiral symmetry is broken, which fixes the self-energy quantum corrections to a value near the electron mass. Symmetry-based explanations are prime examples of the naturalness approach. But there are other ways by which naturalness can solve a hierarchy or fine-tuning problem. For example, the mass of the charged pion can appear fine tuned, but the problem disappears once you treat the pion as a composite particle made of two quarks. In general, the naturalness strategy tells physicists to be on the lookout for the appearance of new physics (new symmetries, new particles) whenever nature seems to be fine tuning its parameters.

    Higgs Hitch

    So, where should new physics appear to solve the electroweak hierarchy problem? Calculations of the Higgs mass suggest the picture should change at energies around 500 GeV, well within the range of the energies being probed at the LHC.

    As for what might show up at this scale, the analogy with other states in the standard model suggests two obvious options: compositeness and supersymmetry. In the first case, the Higgs is considered to be like the pion, a bound state of lighter particles. Such a composite Higgs would be held together by new strong interactions, which would entail the appearance of additional particles. Alternatively, the Higgs could be like the electron. This explanation is a bit trickier, since the chiral symmetry explaining the electron mass is unique to fermions. But a new symmetry relating the Higgs boson to a new fermion would allow a screening mechanism to develop around the Higgs and this fermion, which would explain the lightness of both particles. Such a symmetry relating bosons and fermions is known as supersymmetry [3, 4].

    These two options closely follow the logic of previous naturalness solutions. As such, they have been the dominant paradigms for solving the electroweak hierarchy problem over the past 40 years. But both frameworks predict an abundance of new particles that have—thus far, at least—not turned up at the LHC. While it is entirely possible that the new particles predicted by compositeness or supersymmetry lie just around the corner, the null results may be telling us that these ideas are akin to Dirac’s explanation for the proton mass: they are motivated, but do not reflect the path that nature has chosen. But if not supersymmetry or compositeness, what else could render the Higgs mass natural?


    One possibility is that symmetries are still at play but in an unexpected form. In this case, the unexpected symmetry relates the standard model to an identical twin, which has its own particles and interactions that are mirror reflections of those in the standard model [5]. In these “twin Higgs” scenarios, the only connection between the standard model and its mirror twin are the Higgs bosons of the two sectors (Fig. 4). The particles of the standard model and their mirror counterparts act in tandem to control the self-energy of the Higgs, explaining at least some (but not all) of the hierarchy between the weak scale and the Planck scale.

    Twin Higgs models could offer unique experimental signatures. Although these models predict a host of new particles—an entire standard-model’s-worth near the weak scale—the mirror particles do not interact via standard model forces, so they could have evaded detection at the LHC. Even so, they may not be entirely invisible. The strong interactions of the mirror sector lead to bound states, just like in the standard model, giving rise to a zoo of mirror mesons and baryons. Some of these particles can mix with the Higgs, providing a portal through which mirror particles can be produced and eventually decay back into standard model particles. Remarkably, these processes are slow enough that mirror particles, if produced at the LHC, would travel distances ranging from centimeters to kilometers before decaying. Detecting such long-lived particles requires a dedicated approach to recording and analyzing data at the LHC, which the CMS and ATLAS collaborations are energetically pursuing.

    Figure 4: The twin Higgs model assumes that the particles of the standard model (left) have mirror counterparts in a hidden sector (right). The two sets of particles could communicate through a coupling between our standard-model Higgs and the mirror Higgs. Credit: N. Craig; adapted from Symmetry Magazine.

    Another possibility is that symmetries play no decisive role, but rather, the Higgs mass is determined dynamically by the evolution of other fields in the early Universe—similar in spirit to Dirac’s proposed solution for the proton mass. Specifically, this idea assumes a new field, called the relaxion field [6], that behaves like the hypothetical axion field, which theorists have proposed as a fix to a fine-tuning problem in nuclear physics. The amplitude of the relaxion field, which evolves along a gently sloping potential in the early Universe, helps to control the mass of the Higgs. In other words, the Higgs mass is determined by the combination of self-energy from known standard model fields and the background value of the relaxion—both of which may be very large in the early Universe. Only when the total mass of the Higgs becomes small, do features appear in the relaxion potential that cause the evolution to stop, thus fixing the Higgs mass at its observed value.

    In this dynamic scenario, the relaxion is the only new particle associated with the value of the weak scale, leaving few possibilities of detectable signals at the LHC but allowing for a wealth of possible signatures in other experiments. Depending on the mass of the relaxion, such signatures might show up as new long-range forces, energy density in dark radiation, rare meson decays at beam dump experiments, or exotic Higgs decays at the LHC [7]. Current axion searches could be sensitive to relaxions, which means any limits placed on axions, such as recent results from the CASPEr experiment [8], would also apply to relaxions.

    Finally, it may be that the difficulty of finding a naturalness solution to the hierarchy problem is symptomatic of something larger: the omission of gravity in the standard model. Perhaps the Higgs mass fine-tuning problem will disappear in a theory that unifies quantum mechanics and gravity. We don’t yet have such a theory, but researchers are able to identify quantum field theories that don’t fit with quantum gravity expectations. These gravity-inconsistent theories are said to belong to the “swampland” (see Trend: Cosmic Predictions from the String Swampland). By studying limits of the swampland, researchers can use gravity as a guide for finding a viable theory that goes beyond the standard model. If the Higgs mass happens to be fixed by these sorts of gravity constraints, then one would expect new long-range forces and light particles that couple to the Higgs. These phenomena might be observable in a future Higgs factory (see Opinion: Exploring Futures for Particle Physics).


    P. A. M. Dirac, “A new basis for cosmology,” Proc. R. Soc. London A 165, 199 (1938).
    S. Weinberg, “Implications of dynamical symmetry breaking,” Phys. Rev. D 13, 974 (1976).
    P. Fayet, “Spontaneously broken supersymmetric theories of weak, electromagnetic and strong interactions,” Phys. Lett. B 69, 489 (1977).
    S. Dimopoulos and H. Georgi, “Softly broken supersymmetry and SU(5),” Nucl. Phys. B 193, 150 (1981).
    Z. Chacko et al., “Natural electroweak breaking from a mirror symmetry,” Phys. Rev Lett. 96, 231802 (2006).
    P. W. Graham et al., “Cosmological Relaxation of the Electroweak Scale,” Phys. Rev. Lett. 115, 221801 (2015).
    T. Flacke et al., “Phenomenology of relaxion-Higgs mixing,” J. High Energy Phys. 2017, 50 (2017).
    T. Wu et al., “Search for axionlike dark matter with a liquid-state nuclear spin comagnetometer,” Phys. Rev. Lett. 122, 191302 (2019).

    See the full article here .


    Please help promote STEM in your local schools.

    Stem Education Coalition

    Physicists are drowning in a flood of research papers in their own fields and coping with an even larger deluge in other areas of physics. How can an active researcher stay informed about the most important developments in physics? Physics highlights a selection of papers from the Physical Review journals. In consultation with expert scientists, the editors choose these papers for their importance and/or intrinsic interest. To highlight these papers, Physics features three kinds of articles: Viewpoints are commentaries written by active researchers, who are asked to explain the results to physicists in other subfields. Focus stories are written by professional science writers in a journalistic style and are intended to be accessible to students and non-experts. Synopses are brief editor-written summaries. Physics provides a much-needed guide to the best in physics, and we welcome your comments (physics@aps.org).

  • richardmitnick 12:49 pm on July 21, 2019 Permalink | Reply
    Tags: "A golden era of exploration: ATLAS highlights from EPS-HEP 2019", , , , Higgs boson, , ,   

    From CERN ATLAS: “A golden era of exploration: ATLAS highlights from EPS-HEP 2019” 

    CERN/ATLAS detector

    CERN ATLAS Higgs Event

    CERN ATLAS another view Image Claudia Marcelloni ATLAS CERN



    20th July 2019
    Katarina Anthony

    Event display of a Higgs boson candidate decaying in the four-lepton channel. (Image: ATLAS Collaboration/CERN)

    Eight years of operation. Over 10,000 trillion high-energy proton collisions. One critical new particle discovery. Countless new insights into our universe. The Large Hadron Collider (LHC) has been breaking records since data-taking began in 2010 – and yet, for ATLAS and its fellow LHC experiments, a golden era of exploration is only just beginning.

    Figure 1: New ATLAS measurement of the Higgs boson decaying in the four-lepton channel, using the full LHC Run-2 dataset. The distribution of the invariant mass of the four leptons (m4l) is shown. The Higgs boson corresponds to the excess of events (blue) over the non-resonant ZZ* background (red) at 125 GeV. (Image: ATLAS Collaboration/CERN)

    This week, the ATLAS Collaboration presented 25 new results at the European Physical Society’s High-Energy Physics conference (EPS-HEP) in Ghent, Belgium. The new analyses examine the largest-ever proton–proton collision dataset from the LHC, recorded during Run 2 of the accelerator (2015–2018) at the 13 TeV energy frontier.

    The new data have been fertile ground for ATLAS. New precision measurements of the Higgs boson, observations of key electroweak processes and high-precision tests of the Standard Model are among the highlights described below; find the full list of ATLAS public results using the full Run-2 dataset here.

    Studying the Higgs discovery channels

    Just over seven years ago, the Higgs boson was an elusive particle, out of reach from physicists for nearly five decades. Today, not only is the Higgs boson frequently observed, it is studied with such precision as to become a powerful tool for exploration.

    Key to these accomplishments are the so-called “Higgs discovery channels”: H→γγ, where the Higgs boson decays into two photons, and H→ZZ*→4l, where it decays via two Z bosons into four leptons. Though rare, these decays are easily identified in the ATLAS detector, making them essential to both the particle’s discovery and study.

    ATLAS presented new explorations of the Higgs boson in these channels (Figures 1 and 2), yielding greater insight into its behaviour. The new results benefit from the large full Run-2 dataset, as well as a number of new improvements to the analysis techniques. For example, ATLAS physicists now utilise Deep-Learning Neural Networks to assign the Higgs-boson events to specific production modes.

    All four Higgs-boson production modes can now be clearly identified in a single decay channel. ATLAS’ studies of the Higgs boson have advanced so quickly, in fact, that rare processes – such as its production in association with a top-quark pair, observed only just last year – can now been seen in just a single decay channel. The new sensitivity allowed physicists to measure kinematic properties of the Higgs boson with unprecedented precision (Figure 3). These are sensitive to new physics processes, making their exploration of particular interest to the collaboration.

    All four Higgs-boson production modes can now be clearly identified in a single decay channel. ATLAS’ studies of the Higgs boson have advanced so quickly, in fact, that rare processes – such as its production in association with a top-quark pair, observed only just last year – can now been seen in just a single decay channel. The new sensitivity allowed physicists to measure kinematic properties of the Higgs boson with unprecedented precision (Figure 3). These are sensitive to new physics processes, making their exploration of particular interest to the collaboration.

    Figure 2: Distribution of the invariant mass of the two photons in the ATLAS measurement of H→γγ using the full Run-2 dataset. The Higgs boson corresponds to the excess of events observed at 125 GeV with respect to the non-resonant background (dashed line). (Image: ATLAS Collaboration/CERN)

    Figure 3: Differential cross section for the transverse momentum (pT,H) of the Higgs boson from the two individual channels (H→ZZ*→4ℓ, H→γγ) and their combination. (Image: ATLAS Collaboration/CERN)

    Searching unseen properties of the Higgs boson

    Having accomplished the observation of Higgs boson interactions with third-generation quarks and leptons, ATLAS physicists are turning their focus to the lighter, second-generation of fermions: muons, charm quarks and strange quarks. While their interactions with the Higgs boson are described by the Standard Model, they have – so far – remained relegated to theory. Results from the ATLAS Collaboration are backing up these theories with real data.

    At EPS-HEP, ATLAS presented a new search for the Higgs boson decaying into muon pairs. This already-rare process is made all the more difficult to detect by background Standard Model processes, which produce muon pairs in abundance.

    Figure 4: ATLAS search for the Higgs boson decaying to two muons. The plot shows the weighted muon pair invariant mass spectrum (muu) summed over all categories. (Image: ATLAS Collaboration/CERN)

    The new result utilised novel machine learning techniques to provide ATLAS’ most sensitive result yet, with a moderate excess of 1.5 standard deviations expected for the predicted signal. In agreement with this prediction, only a small excess of 0.8 standard deviations is present around the Higgs-boson mass in the data (Figure 4).

    “This result shows that we are now close to the sensitivity required to test the Standard Model’s predictions for this very rare decay of the Higgs boson,” said ATLAS spokesperson Karl Jakobs from the University of Freiburg, Germany. “However, a definitive statement on the second generation will require the larger datasets that will be provided by the LHC in Run 3 and by the High-Luminosity LHC.”

    ATLAS’ growing sensitivity was also clearly on display in the collaboration’s new “di-Higgs” search, where two Higgs bosons are formed via the fusion of two vector bosons. Though one of the rarest Standard Model processes explored by ATLAS, its study gives unique insight into the previously-untested relationship between vector boson and Higgs-boson pairs. A small variation of this coupling relative to the Standard Model value would result in a dramatic rise in the measured cross section. The new search, despite being negative, successfully sets the first constraints on this relationship.

    Entering the Higgs sector

    The Higgs mechanism, giving mass to all elementary particles, is directly connected with profound questions about our universe, including the stability and energy of the vacuum, the “naturalness” of a world described by the Standard Model, and more. As such, the exploration of the Higgs sector is not limited to direct measurements of the Higgs boson – it instead requires a broad experimental programme that will extend over decades.

    A perfect example of this came in ATLAS’ new observation of the electroweak production of two jets in association with a pair of Z bosons. The Z and W bosons are the force carriers of weak interactions and, as they both have a spin of 1, are known as “vector bosons”. The Higgs boson is a vital mediator in “vector-boson scattering”, an electroweak process that contributes to the pair production of vector bosons (WW, WZ and ZZ) with jets. Measurements of these production processes are key for the study of electroweak symmetry breaking via the Higgs mechanism.

    The new ATLAS result – with a statistical significance of 5.5 standard deviations (Figure 5) – completes the experiment’s observation of vector-boson scattering in these critical processes, and sparks new ways to test the Standard Model.

    Figure 5: Observed and predicted distributions (BDT) in the signal regions of Z-boson pairs decaying to four leptons. The electroweak production of the Z-boson pair is shown in red; the error bars on the data points (black) show the statistical uncertainty on data. (Image: ATLAS Collaboration/CERN)

    Figure 6: Summary of the mass limits on supersymmetry models set by the ATLAS searches for Supersymmetry. Results are quoted for the nominal cross section in both a region of near-maximal mass reach and a demonstrative alternative scenario, in order to display the range in model space of search sensitivity. (Image: ATLAS Collaboration/CERN)

    Probing new physics

    As the community enters the tenth year of supersymmetry searches at the LHC, the ATLAS Collaboration continues to take a broad approach to the hunt. ATLAS is committed to providing results that are theory-independent as well as signature-based searches, in addition to the highly-targeted, model-dependent ones.

    Along with new, updated limits on various supersymmetry searches using the full Run-2 dataset (Figure 6), ATLAS once again highlighted new searches (first presented at the LHCP2019 conference) for superpartners produced through the electroweak interaction. Generated at extremely low rates at the LHC and decaying into Standard Model particles that are themselves difficult to reconstruct, such supersymmetry searches can only be described by the iconic quote: “not because it is easy, but because it is hard”.

    Overall, the results place strong constraints on important supersymmetric scenarios, which will inform theory developments and future ATLAS searches. Further, they provide examples of how advanced reconstruction techniques can help improve the ATLAS’ sensitivity of new physics searches.

    Asymmetric top-quark production

    The Standard Model continued to show its strength in ATLAS’ new precision measurement of charge asymmetry in top-quark pairs (Figure 7). This intriguing imbalance – where top and antitop quarks are not produced equally at all angles with respect to the proton beam direction – is among the most subtle, difficult and yet vital properties to measure in the study of top quarks.

    The effect of this asymmetry is predicted to be extremely small, however new physics processes interfering with the known production modes can lead to larger (or even smaller) values. ATLAS found evidence of this imbalance, with a significance of four standard deviations, with a value compatible with the Standard Model. The result marks an important milestone for the field, following decades of measurements which began at the Tevatron proton–antiproton collider, the predecessor of the LHC in the USA.


    FNAL/Tevatron map

    Figure 7: Measured values of the charge asymmetry (Ac) as a function of the invariant mass of the top quark pair system (mtt) in data. (Image: ATLAS Collaboration/CERN)

    Following the data

    As EPS-HEP 2019 drew to a close, it was clear that exploration of the high-energy frontier remains far from complete. With the LHC – and its upcoming HL-­LHC upgrade – set to continue apace, the future of high-energy physics will be guided by the results of ATLAS and its fellow experiments at the energy frontier.

    “Our community is living through data-driven times,” said ATLAS Deputy Spokesperson Andreas Hoecker from CERN. “Experimental results must guide the high-energy physics community to the next stage of exploration. This requires a broad and diverse particle physics research programme. The ATLAS Collaboration is up to taking this challenge!”

    See the full article here .

    Please help promote STEM in your local schools.

    Stem Education Coalition

    CERN Courier

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  • richardmitnick 12:10 pm on July 15, 2019 Permalink | Reply
    Tags: , , , , , , Higgs boson, , ,   

    From CERN: “Exploring the Higgs boson “discovery channels” 

    Cern New Bloc

    Cern New Particle Event

    From CERN

    12th July 2019
    ATLAS Collaboration

    Event display of a two-electron two-muon ZH candidate. The Higgs candidate can be seen on the left with the two leading electrons represented by green tracks and green EM calorimeter deposits (pT = 22 and 120 GeV), and two subleading muons indicated by two red tracks (pT = 34 and 43 GeV). Recoiling against the four lepton candidate in the left hemisphere is a dimuon pair in the right hemisphere indicated by two red tracks (pT = 139 and 42 GeV) and an invariant mass of 91.5 GeV, which agrees well with the mass of the Z boson. (Image: ATLAS Collaboration/CERN)

    At the 2019 European Physical Society’s High-Energy Physics conference (EPS-HEP) taking place in Ghent, Belgium, the ATLAS and CMS collaborations presented a suite of new results. These include several analyses using the full dataset from the second run of CERN’s Large Hadron Collider (LHC), recorded at a collision energy of 13 TeV between 2015 and 2018. Among the highlights are the latest precision measurements involving the Higgs boson. In only seven years since its discovery, scientists have carefully studied several of the properties of this unique particle, which is increasingly becoming a powerful tool in the search for new physics.

    The results include new searches for transformations (or “decays”) of the Higgs boson into pairs of muons and into pairs of charm quarks. Both ATLAS and CMS also measured previously unexplored properties of decays of the Higgs boson that involve electroweak bosons (the W, the Z and the photon) and compared these with the predictions of the Standard Model (SM) of particle physics. ATLAS and CMS will continue these studies over the course of the LHC’s Run 3 (2021 to 2023) and in the era of the High-Luminosity LHC (from 2026 onwards).

    The Higgs boson is the quantum manifestation of the all-pervading Higgs field, which gives mass to elementary particles it interacts with, via the Brout-Englert-Higgs mechanism. Scientists look for such interactions between the Higgs boson and elementary particles, either by studying specific decays of the Higgs boson or by searching for instances where the Higgs boson is produced along with other particles. The Higgs boson decays almost instantly after being produced in the LHC and it is by looking through its decay products that scientists can probe its behaviour.

    In the LHC’s Run 1 (2010 to 2012), decays of the Higgs boson involving pairs of electroweak bosons were observed. Now, the complete Run 2 dataset – around 140 inverse femtobarns each, the equivalent of over 10 000 trillion collisions – provides a much larger sample of Higgs bosons to study, allowing measurements of the particle’s properties to be made with unprecedented precision. ATLAS and CMS have measured the so-called “differential cross-sections” of the bosonic decay processes, which look at not just the production rate of Higgs bosons but also the distribution and orientation of the decay products relative to the colliding proton beams. These measurements provide insight into the underlying mechanism that produces the Higgs bosons. Both collaborations determined that the observed rates and distributions are compatible with those predicted by the Standard Model, at the current rate of statistical uncertainty.

    Since the strength of the Higgs boson’s interaction is proportional to the mass of elementary particles, it interacts most strongly with the heaviest generation of fermions, the third. Previously, ATLAS and CMS had each observed these interactions. However, interactions with the lighter second-generation fermions – muons, charm quarks and strange quarks – are considerably rarer. At EPS-HEP, both collaborations reported on their searches for the elusive second-generation interactions.
    ATLAS presented their first result from searches for Higgs bosons decaying to pairs of muons (H→μμ) with the full Run 2 dataset. This search is complicated by the large background of more typical SM processes that produce pairs of muons. “This result shows that we are now close to the sensitivity required to test the Standard Model’s predictions for this very rare decay of the Higgs boson,” says Karl Jakobs, the ATLAS spokesperson. “However, a definitive statement on the second generation will require the larger datasets that will be provided by the LHC in Run 3 and by the High-Luminosity LHC.”
    CMS presented their first result on searches for decays of Higgs bosons to pairs of charm quarks (H→cc). When a Higgs boson decays into quarks, these elementary particles immediately produce jets of particles. “Identifying jets formed by charm quarks and isolating them from other types of jets is a huge challenge,” says Roberto Carlin, spokesperson for CMS. “We’re very happy to have shown that we can tackle this difficult decay channel. We have developed novel machine-learning techniques to help with this task.”

    An event recorded by CMS showing a candidate for a Higgs boson produced in association with two top quarks. The Higgs boson and top quarks decay leading to a final state with seven jets (orange cones), an electron (green line), a muon (red line) and missing transverse energy (pink line) (Image: CMS/CERN)

    The Higgs boson also acts as a mediator of physics processes in which electroweak bosons scatter or bounce off each other. Studies of these processes with very high statistics serve as powerful tests of the Standard Model. ATLAS presented the first-ever measurement of the scattering of two Z bosons. Observing this scattering completes the picture for the W and Z bosons as ATLAS has previously observed the WZ scattering process and both collaborations the WW processes. CMS presented the first observation of electroweak-boson scattering that results in the production of a Z boson and a photon.
    “The experiments are making big strides in the monumental task of understanding the Higgs boson,” says Eckhard Elsen, CERN’s Director of Research and Computing. “After observation of its coupling to the third-generation fermions, the experiments have now shown that they have the tools at hand to address the even more challenging second generation. The LHC’s precision physics programme is in full swing.”

    See the full article here .

    Please help promote STEM in your local schools.

    Stem Education Coalition

    Meet CERN in a variety of places:

    Quantum Diaries

    Cern Courier



    CERN ATLAS Image Claudia Marcelloni CERN/ATLAS


    CERN/ALICE Detector

    CERN CMS New

    CERN LHCb New II


    CERN map

    CERN LHC Grand Tunnel

    CERN LHC particles

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

    From Symmetry: “Casting a wide net” 

    Symmetry Mag
    From Symmetry

    Jim Daley

    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 .


    Please help promote STEM in your local schools.

    Stem Education Coalition

    Symmetry is a joint Fermilab/SLAC publication.

  • richardmitnick 3:02 pm on May 2, 2019 Permalink | Reply
    Tags: , , , , Higgs boson, , , ,   

    From University of Chicago: “Scientists invent way to trap mysterious ‘dark world’ particle at Large Hadron Collider” 

    U Chicago bloc

    From University of Chicago

    Apr 17, 2019 [Just found this via social media]
    Louise Lerner

    Courtesy of Zarija Lukic/Berkeley Lab

    A new paper outlines a method to directly detect particles from the ‘dark world’ using the Large Hadron Collider. Until now we’ve only been able to make indirect measurements and simulations, such as the visualization of dark matter above.

    CERN LHC Maximilien Brice and Julien Marius Ordan

    Higgs boson could be tied with dark particle, serve as ‘portal to the dark world’.

    Now that they’ve identified the Higgs boson, scientists at the Large Hadron Collider have set their sights on an even more elusive target.

    All around us is dark matter and dark energy—the invisible stuff that binds the galaxy together, but which no one has been able to directly detect. “We know for sure there’s a dark world, and there’s more energy in it than there is in ours,” said LianTao Wang, a University of Chicago professor of physics who studies how to find signals in large particle accelerators like the LHC.

    Wang, along with scientists from the University and UChicago-affiliated Fermilab, think they may be able to lead us to its tracks; in a paper published April 3 in Physical Review Letters, they laid out an innovative method for stalking dark matter in the LHC by exploiting a potential particle’s slightly slower speed.

    While the dark world makes up more than 95% of the universe, scientists only know it exists from its effects—like a poltergeist you can only see when it pushes something off a shelf. For example, we know there’s dark matter because we can see gravity acting on it—it helps keep our galaxies from flying apart.

    Theorists think there’s one particular kind of dark particle that only occasionally interacts with normal matter. It would be heavier and longer-lived than other known particles, with a lifetime up to one tenth of a second. A few times in a decade, researchers believe, this particle can get caught up in the collisions of protons that the LHC is constantly creating and measuring.

    “One particularly interesting possibility is that these long-lived dark particles are coupled to the Higgs boson in some fashion—that the Higgs is actually a portal to the dark world,” said Wang, referring to the last holdout particle in physicists’ grand theory of how the universe works, discovered at the LHC in 2012.

    Standard Model of Particle Physics

    CERN CMS Higgs Event

    CERN ATLAS Higgs Event

    “It’s possible that the Higgs could actually decay into these long-lived particles.”

    The only problem is sorting out these events from the rest; there are more than a billion collisions per second in the 27-kilometer LHC, and each one of these sends subatomic chaff spraying in all directions.

    Wang, UChicago postdoctoral fellow Jia Liu and Fermilab scientist Zhen Liu (now at the University of Maryland) proposed a new way to search by exploiting one particular aspect of such a dark particle. “If it’s that heavy, it costs energy to produce, so its momentum would not be large—it would move more slowly than the speed of light,” said Liu, the first author on the study.

    That time delay would set it apart from all the rest of the normal particles. Scientists would only need to tweak the system to look for particles that are produced and then decay a bit more slowly than everything else.

    The difference is on the order of a nanosecond—a billionth of a second—or smaller. But the LHC already has detectors sophisticated enough to catch this difference; a recent study using data collected from the last run and found the method should work, plus the detectors will get even more sensitive as part of the upgrade that is currently underway.

    “We anticipate this method will increase our sensitivity to long-lived dark particles by more than an order of magnitude—while using capabilities we already have at the LHC,” Liu said.

    Experimentalists are already working to build the trap: When the LHC turns back on in 2021, after boosting its luminosity by tenfold, all three of the major detectors will be implementing the new system, the scientists said. “We think it has great potential for discovery,” Liu said.


    CERN/CMS Detector

    CERN/ALICE Detector

    “If the particle is there, we just have to find a way to dig it out,” Wang said. “Usually, the key is finding the question to ask.”

    See the full article here .


    Please help promote STEM in your local schools.

    Stem Education Coalition

    U Chicago Campus

    An intellectual destination

    One of the world’s premier academic and research institutions, the University of Chicago has driven new ways of thinking since our 1890 founding. Today, UChicago is an intellectual destination that draws inspired scholars to our Hyde Park and international campuses, keeping UChicago at the nexus of ideas that challenge and change the world.

    The University of Chicago is an urban research university that has driven new ways of thinking since 1890. Our commitment to free and open inquiry draws inspired scholars to our global campuses, where ideas are born that challenge and change the world.

    We empower individuals to challenge conventional thinking in pursuit of original ideas. Students in the College develop critical, analytic, and writing skills in our rigorous, interdisciplinary core curriculum. Through graduate programs, students test their ideas with UChicago scholars, and become the next generation of leaders in academia, industry, nonprofits, and government.

    UChicago research has led to such breakthroughs as discovering the link between cancer and genetics, establishing revolutionary theories of economics, and developing tools to produce reliably excellent urban schooling. We generate new insights for the benefit of present and future generations with our national and affiliated laboratories: Argonne National Laboratory, Fermi National Accelerator Laboratory, and the Marine Biological Laboratory in Woods Hole, Massachusetts.

    The University of Chicago is enriched by the city we call home. In partnership with our neighbors, we invest in Chicago’s mid-South Side across such areas as health, education, economic growth, and the arts. Together with our medical center, we are the largest private employer on the South Side.

    In all we do, we are driven to dig deeper, push further, and ask bigger questions—and to leverage our knowledge to enrich all human life. Our diverse and creative students and alumni drive innovation, lead international conversations, and make masterpieces. Alumni and faculty, lecturers and postdocs go on to become Nobel laureates, CEOs, university presidents, attorneys general, literary giants, and astronauts.

  • richardmitnick 12:32 pm on April 18, 2019 Permalink | Reply
    Tags: "When Beauty Gets in the Way of Science", , , , Higgs boson, , , , , , ,   

    From Nautilus: “When Beauty Gets in the Way of Science” 


    From Nautilus

    April 18, 2019
    Sabine Hossenfelder

    Insisting that new ideas must be beautiful blocks progress in particle physics.

    When Beauty Gets in the Way of Science. Nautilus

    The biggest news in particle physics is no news. In March, one of the most important conferences in the field, Rencontres de Moriond, took place. It is an annual meeting at which experimental collaborations present preliminary results. But the recent data from the Large Hadron Collider (LHC), currently the world’s largest particle collider, has not revealed anything new.


    CERN map

    CERN LHC Tunnel

    CERN LHC particles

    Forty years ago, particle physicists thought themselves close to a final theory for the structure of matter. At that time, they formulated the Standard Model of particle physics to describe the elementary constituents of matter and their interactions.

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

    After that, they searched for the predicted, but still missing, particles of the Standard Model. In 2012, they confirmed the last missing particle, the Higgs boson.

    CERN CMS Higgs Event

    CERN ATLAS Higgs Event

    The Higgs boson is necessary to make sense of the rest of the Standard Model. Without it, the other particles would not have masses, and probabilities would not properly add up to one. Now, with the Higgs in the bag, the Standard Model is complete; all Pokémon caught.

    HIGGS HANGOVER: After the Large Hadron Collider (above) confirmed the Higgs boson, which validated the Standard Model, it’s produced nothing newsworthy, and is unlikely to, says physicist Sabine Hossenfelder.Shutterstock

    The Standard Model may be physicists’ best shot at the structure of fundamental matter, but it leaves them wanting. Many particle physicists think it is simply too ugly to be nature’s last word. The 25 particles of the Standard Model can be classified by three types of symmetries that correspond to three fundamental forces: The electromagnetic force, and the strong and weak nuclear forces. Physicists, however, would rather there was only one unified force. They would also like to see an entirely new type of symmetry, the so-called “supersymmetry,” because that would be more appealing.

    Supersymmetry builds on the Standard Model, with many new supersymmetric particles, represented here with a tilde (~) on them. ( From the movie “Particle fever” reproduced by Mark Levinson)

    Oh, and additional dimensions of space would be pretty. And maybe also parallel universes. Their wish list is long.

    It has become common practice among particle physicists to use arguments from beauty to select the theories they deem worthy of further study. These criteria of beauty are subjective and not evidence-based, but they are widely believed to be good guides to theory development. The most often used criteria of beauty in the foundations of physics are presently simplicity and naturalness.

    By “simplicity,” I don’t mean relative simplicity, the idea that the simplest theory is the best (a.k.a. “Occam’s razor”). Relying on relative simplicity is good scientific practice. The desire that a theory be simple in absolute terms, in contrast, is a criterion from beauty: There is no deep reason that the laws of nature should be simple. In the foundations of physics, this desire for absolute simplicity presently shows in physicists’ hope for unification or, if you push it one level further, in the quest for a “Theory of Everything” that would merge the three forces of the Standard Model with gravity.

    The other criterion of beauty, naturalness, requires that pure numbers that appear in a theory (i.e., those without units) should neither be very large nor very small; instead, these numbers should be close to one. Exactly how close these numbers should be to one is debatable, which is already an indicator of the non-scientific nature of this argument. Indeed, the inability of particle physicists to quantify just when a lack of naturalness becomes problematic highlights that the fact that an unnatural theory is utterly unproblematic. It is just not beautiful.

    Anyone who has a look at the literature of the foundations of physics will see that relying on such arguments from beauty has been a major current in the field for decades. It has been propagated by big players in the field, including Steven Weinberg, Frank Wilczek, Edward Witten, Murray Gell-Mann, and Sheldon Glashow. Countless books popularized the idea that the laws of nature should be beautiful, written, among others, by Brian Greene, Dan Hooper, Gordon Kane, and Anthony Zee. Indeed, this talk about beauty has been going on for so long that at this point it seems likely most people presently in the field were attracted by it in the first place. Little surprise, then, they can’t seem to let go of it.

    Trouble is, relying on beauty as a guide to new laws of nature is not working.

    Since the 1980s, dozens of experiments looked for evidence of unified forces and supersymmetric particles, and other particles invented to beautify the Standard Model. Physicists have conjectured hundreds of hypothetical particles, from “gluinos” and “wimps” to “branons” and “cuscutons,” each of which they invented to remedy a perceived lack of beauty in the existing theories. These particles are supposed to aid beauty, for example, by increasing the amount of symmetries, by unifying forces, or by explaining why certain numbers are small. Unfortunately, not a single one of those particles has ever been seen. Measurements have merely confirmed the Standard Model over and over again. And a theory of everything, if it exists, is as elusive today as it was in the 1970s. The Large Hadron Collider is only the most recent in a long series of searches that failed to confirm those beauty-based predictions.

    These decades of failure show that postulating new laws of nature just because they are beautiful according to human standards is not a good way to put forward scientific hypotheses. It’s not the first time this has happened. Historical precedents are not difficult to find. Relying on beauty did not work for Kepler’s Platonic solids, it did not work for Einstein’s idea of an eternally unchanging universe, and it did not work for the oh-so-pretty idea, popular at the end of the 19th century, that atoms are knots in an invisible ether. All of these theories were once considered beautiful, but are today known to be wrong. Physicists have repeatedly told me about beautiful ideas that didn’t turn out to be beautiful at all. Such hindsight is not evidence that arguments from beauty work, but rather that our perception of beauty changes over time.

    That beauty is subjective is hardly a breakthrough insight, but physicists are slow to learn the lesson—and that has consequences. Experiments that test ill-motivated hypotheses are at high risk to only find null results; i.e., to confirm the existing theories and not see evidence of new effects. This is what has happened in the foundations of physics for 40 years now. And with the new LHC results, it happened once again.

    The data analyzed so far shows no evidence for supersymmetric particles, extra dimensions, or any other physics that would not be compatible with the Standard Model. In the past two years, particle physicists were excited about an anomaly in the interaction rates of different leptons. The Standard Model predicts these rates should be identical, but the data demonstrates a slight difference. This “lepton anomaly” has persisted in the new data, but—against particle physicists’ hopes—it did not increase in significance, is hence not a sign for new particles. The LHC collaborations succeeded in measuring the violation of symmetry in the decay of composite particles called “D-mesons,” but the measured effect is, once again, consistent with the Standard Model. The data stubbornly repeat: Nothing new to see here.

    Of course it’s possible there is something to find in the data yet to be analyzed. But at this point we already know that all previously made predictions for new physics were wrong, meaning that there is now no reason to expect anything new to appear.

    Yes, null results—like the recent LHC measurements—are also results. They rule out some hypotheses. But null results are not very useful results if you want to develop a new theory. A null-result says: “Let’s not go this way.” A result says: “Let’s go that way.” If there are many ways to go, discarding some of them does not help much.

    To find the way forward in the foundations of physics, we need results, not null-results. When testing new hypotheses takes decades of construction time and billions of dollars, we have to be careful what to invest in. Experiments have become too costly to rely on serendipitous discoveries. Beauty-based methods have historically not worked. They still don’t work. It’s time that physicists take note.

    And it’s not like the lack of beauty is the only problem with the current theories in the foundations of physics. There are good reasons to think physics is not done. The Standard Model cannot be the last word, notably because it does not contain gravity and fails to account for the masses of neutrinos. It also describes neither dark matter nor dark energy, which are necessary to explain galactic structures.

    So, clearly, the foundations of physics have problems that require answers. Physicists should focus on those. And we currently have no reason to think that colliding particles at the next higher energies will help solve any of the existing problems. New effects may not appear until energies are a billion times higher than what even the next larger collider could probe. To make progress, then, physicists must, first and foremost, learn from their failed predictions.

    So far, they have not. In 2016, the particle physicists Howard Baer, Vernon Barger, and Jenny List wrote an essay for Scientific American arguing that we need a larger particle collider to “save physics.” The reason? A theory the authors had proposed themselves, that is natural (beautiful!) in a specific way, predicts such a larger collider should see new particles. This March, Kane, a particle physicist, used similar beauty-based arguments in an essay for Physics Today. And a recent comment in Nature Reviews Physics about a big, new particle collider planned in Japan once again drew on the same motivations from naturalness that have already not worked for the LHC. Even the particle physicists who have admitted their predictions failed do not want to give up beauty-based hypotheses. Instead, they have argued we need more experiments to test just how wrong they are.

    Will this latest round of null-results finally convince particle physicists that they need new methods of theory-development? I certainly hope so.

    As an ex-particle physicist myself, I understand very well the desire to have an all-encompassing theory for the structure of matter. I can also relate to the appeal of theories such a supersymmetry or string theory. And, yes, I quite like the idea that we live in one of infinitely many universes that together make up the “multiverse.” But, as the latest LHC results drive home once again, the laws of nature care heartily little about what humans find beautiful.

    See the full article here .


    Please help promote STEM in your local schools.

    Stem Education Coalition

    Welcome to Nautilus. We are delighted you joined us. We are here to tell you about science and its endless connections to our lives. Each month we choose a single topic. And each Thursday we publish a new chapter on that topic online. Each issue combines the sciences, culture and philosophy into a single story told by the world’s leading thinkers and writers. We follow the story wherever it leads us. Read our essays, investigative reports, and blogs. Fiction, too. Take in our games, videos, and graphic stories. Stop in for a minute, or an hour. Nautilus lets science spill over its usual borders. We are science, connected.

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