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  • richardmitnick 11:26 am on April 12, 2019 Permalink | Reply
    Tags: , , , , , , , Supersymmetry   

    From Fermi National Accelerator Lab: “Quarks, squarks, stops and charm at this year’s Moriond conference” 

    FNAL Art Image
    FNAL Art Image by Angela Gonzales

    From Fermi National Accelerator Lab , an enduring source of strength for the US contribution to scientific research world wide.

    April 11, 2019
    Don Lincoln

    Fermilab RAs Kevin Pedro and Nadja Strobbe presented a variety of CMS and ATLAS research results at the 53rd annual Recontres de Moriond conference.

    This March, scientists from around the world gathered in LaThuile, Italy, for the 53rd annual Recontres de Moriond conference, one of the longest running and most prestigious conferences in particle physics. This conference is broken into two distinct weeks, with the first week usually covering electroweak physics and the second covering processes involving quantum chromodynamics. Fermilab and the LHC Physics Center were well represented at the conference.

    Fermilab research associates Kevin Pedro and Nadja Strobbe from the CMS group both presented talks on LHC physics result. Pedro spoke on searches for new physics with unconventional signatures at both the ATLAS and CMS experiments. The interest in unusual signatures is driven by the fact that many researchers have already searched for more commonly accepted physical processes. Looking for the unconventional opens up the possibility of unanticipated discoveries. Pedro covered long-lived particles emerging from a complex dark matter sector. The signature for this possible physics result is a jet that originates far from the interaction vertex. He also covered long-lived particles that disappear in the detector. This is a signature for a form of supersymmetry.

    Strobbe presented a thorough overview of searches for strong-force-produced signatures of supersymmetry. She covered both ATLAS and CMS results, covering a broad range of signatures, including the associated production of b quarks and Higgs bosons, diphotons, several stop squark analyses, and the associated production of three bottom quarks and missing transverse momentum. In total, she presented 12 distinct analyses. The phenomenology of strong-force-produced supersymmetry is diverse, and it provides a rich source for the possible discovery of new physics. This is Strobbe’s last Moriond presentation as a Fermilab research associate, as she has recently accepted a faculty position at the University of Minnesota, where she will be starting in the fall.

    Strobbe and Pedro were not the only people associated with the LHC Physics Center presenting or involved at Moriond. Fermilab Senior Scientist Boaz Klima has long been a member of the organizing committee. Meng Xiao (Johns Hopkins) and Greg Landsberg (Brown) also presented.

    More broadly, many interesting physics topics were covered at the conference. The LHCb experiment announced the discovery of new pentaquarks containing charm quarks. They also reported that a peak in the data that was previously thought to be a single pentaquark was actually two distinct particles. Studies of mesons containing both bottom and charm quarks were very well-represented, with ATLAS, CMS and LHCb all making presentations. In the first week of the Moriond conference, both ATLAS and LHCb announced studies in the matter-antimatter asymmetry in decays of mesons containing both bottom and strange quarks. And in an example of very quick inter-collaboration cooperation, the experiments presented a combined result in the second week.

    While the LHC is best known for colliding two beams of protons (studies of which were well represented at Moriond), the LHC also collides lead ions to study the behavior of superhot quark matter – what is called quark-gluon plasma. ALICE presented studies of charmed mesons called J/psi, which showed that charm quarks are affected in quark-gluon plasmas, just like lighter quarks. The ALICE experiment presented data gathered in a special run of proton-proton collisions at an energy unusual for the LHC an observation of charmed baryons in LHC collisions. These particles occur more often in proton-proton collisions than in electron-positron ones.

    The Moriond conference is a fascinating one. It is small and cozy and allows for conversations and collaboration between researchers, with a storied history of over half a century. In its 53rd year, researchers are showing that its second half century will be just as exciting.

    Don Lincoln is a Fermilab scientist on the CMS experiment.

    See the full article here.


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    Fermi National Accelerator Laboratory (Fermilab), located just outside Batavia, Illinois, near Chicago, is a US Department of Energy national laboratory specializing in high-energy particle physics. Fermilab is America’s premier laboratory for particle physics and accelerator research, funded by the U.S. Department of Energy. Thousands of scientists from universities and laboratories around the world collaborate at Fermilab on experiments at the frontiers of discovery.

    FNAL MINERvA front face Photo Reidar Hahn


    FNAL Muon g-2 studio

    FNAL Short-Baseline Near Detector under construction

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    Dark Energy Camera [DECam], built at FNAL

    FNAL DUNE Argon tank at SURF


    FNAL Don Lincoln


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    FNAL LBNF/DUNE from FNAL to SURF, Lead, South Dakota, USA

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  • richardmitnick 1:04 pm on August 14, 2018 Permalink | Reply
    Tags: , , Brute-force approach to particle hunt, , , , , , , Supersymmetry   

    From Nature: “LHC physicists embrace brute-force approach to particle hunt” 

    Nature Mag
    From Nature

    14 August 2018
    Davide Castelvecchi

    The world’s most powerful particle collider has yet to turn up new physics [since Higgs] — now some physicists are turning to a different strategy.

    The ATLAS detector at the Large Hadron Collider near Geneva, Switzerland.Credit: Stefano Dal Pozzolo/Contrasto /eyevine

    A once-controversial approach to particle physics has entered the mainstream at the Large Hadron Collider (LHC).


    CERN map

    CERN LHC Tunnel

    CERN LHC particles

    The LHC’s major ATLAS experiment has officially thrown its weight behind the method — an alternative way to hunt through the reams of data created by the machine — as the collider’s best hope for detecting behaviour that goes beyond the standard model of particle physics. Conventional techniques have so far come up empty-handed.

    So far, almost all studies at the LHC — at CERN, Europe’s particle-physics laboratory near Geneva, Switzerland — have involved ‘targeted searches’ for signatures of favoured theories. The ATLAS collaboration now describes its first all-out ‘general’ search of the detector’s data, in a preprint posted on the arXiv server last month and submitted to European Physics Journal C. Another major LHC experiment, CMS, is working on a similar project.

    “My goal is to try to come up with a really new way to look for new physics” — one driven by the data rather than by theory, says Sascha Caron of Radboud University Nijmegen in the Netherlands, who has led the push for the approach at ATLAS. General searches are to the targeted ones what spell checking an entire text is to searching that text for a particular word. These broad searches could realize their full potential in the near future, when combined with increasingly sophisticated artificial-intelligence (AI) methods.

    LHC researchers hope that the methods will lead them to their next big discovery — something that hasn’t happened since the detection of the Higgs boson in 2012, which put in place the final piece of the standard model. Developed in the 1960s and 1970s, the model describes all known subatomic particles, but physicists suspect that there is more to the story — the theory doesn’t account for dark matter, for instance. But big experiments such as the LHC have yet to find evidence for such behaviour. That means it’s important to try new things, including general searches, says Gian Giudice, who heads CERN’s theory department and is not involved in any of the experiments. “This is the right approach, at this point.”

    Collision course

    The LHC smashes together millions of protons per second at colossal energies to produce a profusion of decay particles, which are recorded by detectors such as ATLAS and CMS. Many different types of particle interaction can produce the same debris. For example, the decay of a Higgs might produce a pair of photons, but so do other, more common, processes. So, to search for the Higgs, physicists first ran simulations to predict how many of those ‘impostor’ pairs to expect. They then counted all photon pairs recorded in the detector and compared them to their simulations. The difference — a slight excess of photon pairs within a narrow range of energies — was evidence that the Higgs existed.

    ATLAS and CMS have run hundreds more of these targeted searches to look for particles that do not appear in the standard model.

    CERN/ATLAS detector

    CERN/CMS Detector

    The Standard Model of elementary particles (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

    Standard Model of Particle Physics from Symmetry Magazine

    Many searches have looked for various flavours of supersymmetry, a theorized extension of the model that includes hypothesized particles such as the neutralino, a candidate for dark matter. But these searches have come up empty so far.

    Standard model of Supersymmetry DESY

    This leaves open the possibility that there are exotic particles that produce signatures no one has thought of — something that general searches have a better chance of finding. Physicists have yet to look, for example, events that produced three photons instead of two, Caron says. “We have hundreds of people looking at Higgs decay and supersymmetry, but maybe we are missing something nobody thought of,” says Arnd Meyer, a CMS member at Aachen University in Germany.

    Whereas targeted searches typically look at only a handful of the many types of decay product, the latest study looked at more than 700 types at once. The study analysed data collected in 2015, the first year after an LHC upgrade raised the energy of proton collisions in the collider from 8 teraelectronvolts (TeV) to 13 TeV. At CMS, Meyer and a few collaborators have conducted a proof-of-principle study, which hasn’t been published, on a smaller set of data from the 8 TeV run.

    Neither experiment has found significant deviations so far. This was not surprising, the teams say, because the data sets were relatively small. Both ATLAS and CMS are now searching the data collected in 2016 and 2017, a trove tens of times larger.

    Statistical cons

    The approach “has clear advantages, but also clear shortcomings”, says Markus Klute, a physicist at the Massachusetts Institute of Technology in Cambridge. Klute is part of CMS and has worked on general searches in at previous experiments, but he was not directly involved in the more recent studies. One limitation is statistical power. If a targeted search finds a positive result, there are standard procedures for calculating its significance; when casting a wide net, however, some false positives are bound to arise. That was one reason that general searches had not been favoured in the past: many physicists feared that they could lead down too many blind alleys. But the teams say they have put a lot of work into making their methods more solid. “I am excited this came forward,” says Klute.

    Most of the people power and resources at the LHC experiments still go into targeted searches, and that might not change anytime soon. “Some people doubt the usefulness of such general searches, given that we have so many searches that exhaustively cover much of the parameter space,” says Tulika Bose of Boston University in Massachusetts, who helps to coordinate the research programme at CMS.

    Many researchers who work on general searches say that they eventually want to use AI to do away with standard-model simulations altogether. Proponents of this approach hope to use machine learning to find patterns in the data without any theoretical bias. “We want to reverse the strategy — let the data tell us where to look next,” Caron says. Computer scientists are also pushing towards this type of ‘unsupervised’ machine learning — compared with the supervised type, in which the machine ‘learns’ from going through data that have been tagged previously by humans.

    See the full article here .


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    Nature is a weekly international journal publishing the finest peer-reviewed research in all fields of science and technology on the basis of its originality, importance, interdisciplinary interest, timeliness, accessibility, elegance and surprising conclusions. Nature also provides rapid, authoritative, insightful and arresting news and interpretation of topical and coming trends affecting science, scientists and the wider public.

  • richardmitnick 1:16 pm on December 23, 2017 Permalink | Reply
    Tags: , Atlantic Magazine, , Causal fermion systems, Cosmic strings, M-theory, Noncommutative geometry, , Supersymmetry, The Best Explanation for Everything in the Universe   

    From The Atlantic Magazine: “The Best Explanation for Everything in the Universe” 

    Atlantic Magazine

    The Atlantic Magazine

    Dec 22, 2017
    Natalie Wolchover

    String theory is considered the leading “theory of everything,” but there’s still no empirical evidence for it.

    Olena Shmahalo / Quanta Magazine.

    It’s not easy being a “theory of everything.” A TOE has the very tough job of fitting gravity into the quantum laws of nature in such a way that, on large scales, gravity looks like curves in the fabric of space-time, as Albert Einstein described in his general theory of relativity. Somehow, space-time curvature emerges as the collective effect of quantized units of gravitational energy—particles known as gravitons. But naïve attempts to calculate how gravitons interact result in nonsensical infinities, indicating the need for a deeper understanding of gravity.

    String theory (or, more technically, M-theory) is often described as the leading candidate for the theory of everything in our universe. But there’s no empirical evidence for it, or for any alternative ideas about how gravity might unify with the rest of the fundamental forces. Why, then, is string/M-theory given the edge over the others?

    The theory famously posits that gravitons, as well as electrons, photons, and everything else, are not point particles but rather imperceptibly tiny ribbons of energy, or “strings,” that vibrate in different ways. Interest in string theory soared in the mid-1980s, when physicists realized that it gave mathematically consistent descriptions of quantized gravity. But the five known versions of string theory were all “perturbative,” meaning they broke down in some regimes. Theorists could calculate what happens when two graviton strings collide at high energies, but not when there’s a confluence of gravitons extreme enough to form a black hole.

    Then, in 1995, the physicist Edward Witten discovered the mother of all string theories. He found various indications that the perturbative string theories fit together into a coherent non-perturbative theory, which he dubbed M-theory. M-theory looks like each of the string theories in different physical contexts but does not itself have limits on its regime of validity—a major requirement for the theory of everything. Or so Witten’s calculations suggested. “Witten could make these arguments without writing down the equations of M-theory, which is impressive but left many questions unanswered,” explained David Simmons-Duffin, a theoretical physicist at the California Institute of Technology.

    Another research explosion ensued two years later, when the physicist Juan Maldacena discovered the AdS/CFT correspondence [International Journal of Theoretical Physics]: a hologram-like relationship connecting gravity in a space-time region called anti–de Sitter (AdS) space to a quantum description of particles (called a “conformal field theory”) moving around on that region’s boundary. AdS/CFT gives a complete definition of M-theory for the special case of AdS space-time geometries, which are infused with negative energy that makes them bend in a different way than our universe does. For such imaginary worlds, physicists can describe processes at all energies, including, in principle, black-hole formation and evaporation. The 16,000 papers that have cited Maldacena’s over the past 20 years mostly aim at carrying out these calculations in order to gain a better understanding of AdS/CFT and quantum gravity.

    This basic sequence of events has led most experts to consider M-theory the leading TOE candidate, even as its exact definition in a universe like ours remains unknown. Whether the theory is correct is an altogether separate question. The strings it posits—as well as extra, curled-up spatial dimensions that these strings supposedly wiggle around in—are 10 million billion times smaller than experiments like the Large Hadron Collider can resolve. And some macroscopic signatures of the theory that might have been seen, such as cosmic strings and supersymmetry, have not shown up.

    Other TOE ideas, meanwhile, are seen as having a variety of technical problems, and none have yet repeated string theory’s demonstrations of mathematical consistency, such as the graviton-graviton scattering calculation. (According to Simmons-Duffin, none of the competitors have managed to complete the first step, or first “quantum correction,” of this calculation.) One philosopher has even argued that string theory’s status as the only known consistent theory counts as evidence that the theory is correct.

    The distant competitors include asymptotically safe gravity, E8 theory, noncommutative geometry, and causal fermion systems. Asymptotically safe gravity, for instance, suggests that the strength of gravity might change as you go to smaller scales in such a way as to cure the infinity-plagued calculations. But no one has yet gotten the trick to work.

    See the full article here .

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  • richardmitnick 5:41 pm on December 18, 2017 Permalink | Reply
    Tags: , , , , Higgsinos?, , , Supersymmetry   

    From CERN ATLAS: “Searching for supersymmetric Higgs bosons on the compressed frontier” 

    CERN ATLAS Higgs Event


    18th December 2017
    ATLAS Collaboration

    Figure 1: The distribution of the di-electron or di-muon invariant mass (mll), where the signal events tend to cluster at low values of mll. Solid histograms indicate Standard Model background processes, points with error bars indicate the data, and the dashed lines indicate hypothetical Higgsino events. The bottom plot shows the ratio of the data to the total Standard Model background. (Image: ATLAS Collaboration/CERN)

    The Standard Model has a number of puzzling features. For instance, why does the Higgs boson have a relatively low mass? Could its mass arise from a hidden symmetry that keeps it from being extremely heavy? And what about dark matter? While the Standard Model has some (almost) invisible particles, like neutrinos, those particles can’t account for all of the dark matter observed by cosmological measurements.

    These puzzles could be solved by supersymmetry, a theory that provides a natural mechanism for protecting the Higgs mass and also has a dark matter candidate.

    Standard model of Supersymmetry DESY

    Supersymmetry predicts the existence of “super-partner” particles that are heavier than their Standard Model counterparts. As long as the supersymmetric partners of the Higgs boson, called “higgsinos”, aren’t too heavy, then supersymmetry can explain a Higgs mass consistent with current observations. The lightest higgsino, the “LSP” (for “lightest supersymmetric particle”), would be a dark matter candidate, while heavier higgsinos decay to the LSP along with other particles like electrons or muons.

    Detecting higgsinos can be difficult, especially if the heavier higgsinos and the LSP have very similar masses. In such “compressed” scenarios, the electrons and muons from the heavier higgsino decays have very low momenta, making them difficult to detect. In recent years, ATLAS has made significant progress in understanding these low-momentum particles, which has opened the door to new searches.

    Figure 2: Limits on Higgsino production from the soft-lepton analysis described here (in blue) and a separate search for “disappearing” tracks. The mass of the heavier charged Higgsino is on the horizontal axis, while the difference in mass between the heavier Higgsino and the LSP is shown on the vertical axis. The dashed lines and solid fill show the expected limits (assuming no signal) and observed limits, respectively, where models within the filled areas are excluded. The grey region represents the models excluded by the LEP experiments. (Image: ATLAS Collaboration/CERN)

    In December 2017, ATLAS presented new updates in the compressed supersymmetry search at the SUSY17 conference. These latest results exploit unique features of higgsino decays, most importantly how the small mass difference between the higgsinos causes the electron or muon pair to have a correspondingly small mass, as illustrated in Figure 1. The data are consistent with the Standard Model predictions and have thus been used to set limits on higgsino masses. The new limits are shown in Figure 2, along with limits from another recent ATLAS search that probes SUSY models with even more compressed spectra. For the first time, these LHC results surpass constraints set in 2004 by the Large Electron Positron (LEP) collider that was hosted in the same 27 km circumference tunnel that now holds the LHC.

    As many of the still viable supersymmetry scenarios have very small higgsino mass differences, there remains plenty of room for investigation. Look forward to new searches of the compressed frontier as ATLAS continues to collect and analyse data from the LHC.

    See the full article for further references with links.

    See the full article here .

    CERN LHC Map
    CERN LHC Grand Tunnel
    CERN LHC particles
    LHC at CERN

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    Quantum Diaries

  • richardmitnick 2:03 pm on September 16, 2016 Permalink | Reply
    Tags: , , , Supersymmetry,   

    From Symmetry: “The secret lives of long-lived particles” 

    Symmetry Mag


    Sarah Charley

    A theoretical species of particle might answer nearly every question about our cosmos—if scientists can find it.

    ATLAS collaboration

    The universe is unbalanced.

    Gravity is tremendously weak. But the weak force, which allows particles to interact and transform, is enormously strong. The mass of the Higgs boson is suspiciously petite. And the catalog of the makeup of the cosmos? Ninety-six percent incomplete.

    Almost every observation of the subatomic universe can be explained by the Standard Model of particle physics—a robust theoretical framework bursting with verifiable predictions.

    The Standard Model of elementary particles (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.
    The Standard Model of elementary particles (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

    But because of these unsolved puzzles, the math is awkward, incomplete and filled with restrictions.

    A few more particles would solve almost all of these frustrations. Supersymmetry (nicknamed SUSY for short) is a colossal model that introduces new particles into the Standard Model’s equations.

    Standard model of Supersymmetry DESY
    Standard model of Supersymmetry DESY

    It rounds out the math and ties up loose ends. The only problem is that after decades of searching, physicists have found none of these new friends.

    But maybe the reason physicists haven’t found SUSY (or other physics beyond the Standard Model) is because they’ve been looking through the wrong lens.

    “Beautiful sets of models keep getting ruled out,” says Jessie Shelton, a theorist at the University of Illinois, “so we’ve had to take a step back and consider a whole new dimension in our searches, which is the lifetime of these particles.”

    In the past, physicists assumed that new particles produced in particle collisions would decay immediately, almost precisely at their points of origin. Scientists can catch particles that behave this way—for example, Higgs bosons—in particle detectors built around particle collision points. But what if new particles had long lifetimes and traveled centimeters—even kilometers—before transforming into something physicists could detect?

    This is not unprecedented. Bottom quarks, for instance, can travel a few tenths of a millimeter before decaying into more stable particles. And muons can travel several kilometers (with the help of special relativity) before transforming into electrons and neutrinos. Many theorists are now predicting that there may be clandestine species of particles that behave in a similar fashion. The only catch is that these long-lived particles must rarely interact with ordinary matter, thus explaining why they’ve escaped detection for so long. One possible explanation for this aloof behavior is that long live particles dwell in a hidden sector of physics.

    “Hidden-sector particles are separated from ordinary matter by a quantum mechanical energy barrier—like two villages separated by a mountain range,” says Henry Lubatti from the University of Washington. “They can be right next to each other, but without a huge boost in energy to get over the peak, they’ll never be able to interact with each other.”

    High-energy collisions generated by the Large Hadron Collider could kick these hidden-sector particles over this energy barrier into our own regime. And if the LHC can produce them, scientists should be able to see the fingerprints of long-lived particles imprinted in their data.

    CERN/LHC Map
    CERN LHC Grand Tunnel
    CERN LHC particles
    LHC at CERN

    Long-lived particles jolted into our world by the LHC would most likely fly at close to the speed of light for between a few micrometers and a few hundred thousand kilometers before transforming into ordinary and measurable matter. This incredibly generous range makes it difficult for scientists to pin down where and how to look for them.

    But the lifetime of a subatomic particle is much like that of any living creature. Each type of particle has an average lifespan, but the exact lifetime of an individual particle varies. If these long-lived particles can travel thousands of kilometers before decaying, scientists are hoping that they’ll still be able to catch a few of the unlucky early-transformers before they leave the detector. Lubatti and his collaborators have also proposed a new LHC surface detector, which would extend their search range by many orders of magnitude.

    Because these long-lived particles themselves don’t interact with the detector, their signal would look like a stream of ordinary matter spontaneously appearing out of nowhere.

    “For instance, if a long lived particle decayed into quarks while inside the muon detector, it would mimic the appearance of several muons closely clustered together,” Lubatti says. “We are triggering on events like this in the ATLAS experiment.” After recording the events, scientists use custom algorithms to reconstruct the origins of these clustered particles to see if they could be the offspring of an invisible long-lived parent.

    If discovered, this new breed of matter could help answer several lingering questions in physics.

    “Long-lived particles are not a prediction of a single new theory, but rather a phenomenon that could fit into almost all of our frameworks for beyond-the-Standard-Model physics,” Shelton says.

    In addition to rounding out the Standard Model’s mathematics, inert long-lived particles could be cousins of dark matter—an invisible form of matter that only interacts with the visible cosmos through gravity. They could also help explain the origin of matter after the Big Bang.

    “So many of us have spent a lifetime studying such a tiny fraction of the universe,” Lubatti says. “We’ve understood a lot, but there’s still a lot we don’t understand—an enormous amount we don’t understand. This gives me and my colleagues pause.”

    See the full article here .

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    Symmetry is a joint Fermilab/SLAC publication.

  • richardmitnick 7:24 am on August 27, 2016 Permalink | Reply
    Tags: , , Supersymmetry   

    From Symmetry: “Winners declared in SUSY bet” 

    Symmetry Mag


    Kathryn Jepsen

    Peter Munch Andersen

    Physicists exchanged cognac in Copenhagen at the conclusion of a bet about supersymmetry and the LHC.

    As a general rule, theorist Nima Arkani-Hamed does not get involved in physics bets.

    “Theoretical physicists like to take bets on all kinds of things,” he says. “I’ve always taken the moral high ground… Nature decides. We’re all in pursuit of the truth. We’re all on the same side.”

    But sometimes you’re in Copenhagen for a conference, and you’re sitting in a delightfully unusual restaurant—one that sort of reminds you of a cave—and a fellow physicist gives you the opportunity to get in on a decade-old wager about supersymmetry and the Large Hadron Collider. Sometimes then, you decide to bend your rule. “It was just such a jovial atmosphere, I figured, why not?”

    That’s how Arkani-Hamed found himself back in Copenhagen this week, passing a 1000-Krone bottle of cognac to one of the winners of the bet, Director of the Niels Bohr International Academy Poul Damgaard.

    Arkani-Hamed had wagered that experiments at the LHC would find evidence of supersymmetry by the arbitrary date of June 16, 2016. Supersymmetry, SUSY for short, is a theory that predicts the existence of partner particles for the members of the Standard Model of particle physics.

    CERN/LHC Map
    CERN LHC Grand Tunnel
    CERN LHC particles
    LHC at CERN

    Standard model of Supersymmetry DESY
    Standard model of Supersymmetry DESY

    The deadline was not met. But in a talk at the Niels Bohr Institute, Arkani-Hamed pointed out that the end of the gamble does not equal the end of the theory.

    “I was not a good student in school,” Arkani-Hamed explained. “One of my big problems was not getting homework done on time. It was a constant battle with my teachers… Just give me another week! It’s kind of like the bet.”

    He pointed out that so far the LHC has gathered just 1 percent of the total amount of data it aims to collect.

    With that data, scientists can indeed rule out the most vanilla form of supersymmetry. But that’s not the version of supersymmetry Arkani-Hamed would expect the LHC to find anyway, he said.

    It is still possible LHC experiments will find evidence of other SUSY models—including the one Arkani-Hamed prefers, called split SUSY, which adds superpartners to just half of the Standard Model’s particles. And if LHC scientists don’t find evidence of SUSY, Arkani-Hamed pointed out, the theoretical problems it aimed to solve will remain an exciting challenge for the next generation of theorists to figure out.

    “I think Winston Churchill said that in victory you should be magnanimous,” Damgaard said after Arkani-Hamed’s talk. “I know also he said that in defeat you should be defiant. And that’s certainly Nima.”

    Arkani-Hamed shrugged. But it turned out he was not the only optimist in the room. Panelist Yonit Hochberg of the University of California, Berkeley conducted an informal poll of attendees. She found that the majority still think that in the next 20 years, as data continues to accumulate, experiments at the LHC will discover something new.

    See the full article here .

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    Symmetry is a joint Fermilab/SLAC publication.

  • richardmitnick 10:30 am on August 4, 2016 Permalink | Reply
    Tags: , Ghostly particles could be haunting our universe, , , Supersymmetry   

    From New Scientist: “Ghost particles may explain why gravity is so surprisingly weak” 


    New Scientist

    3 August 2016
    No writer credit found

    Spot the strange particles. ESO/T.Preibisch

    Ghostly particles could be haunting our universe. A new theory claims that the cosmos is full of unseen particle families that don’t interact with each other. If true, the model could explain why gravity is so puzzlingly weak.

    The idea is an alternative to supersymmetry, a theory in which every known particle has a heavier partner.

    Standard model of Supersymmetry DESY
    Standard model of Supersymmetry DESY

    Such superparticles would explain why the mass of our familiar set of particles is low enough to account for gravity’s weakness. But the particle-smashing Large Hadron Collider, near Geneva, Switzerland, still hasn’t seen any superparticles, despite years of searching.

    Maybe that’s because we need not just one set of partner particles, but many, says Nima Arkani-Hamed at Princeton University.

    “The idea is a little wild,” admits Tim Cohen at the University of Oregon in Eugene, who teamed up with Arkani-Hamed and others to work on the new theory. “The absence of new physics at the LHC has motivated us to – instead of introducing a few new particles – introduce 10^16 new particles.”

    The group began by tweaking the story of what happened in the big bang. After its birth, the universe inflated until it was cold and flat. Arkani-Hamed’s team believes that at this point, all the energy was locked up in the form of a single kind of particle, which they call the reheaton.

    Then, because particles always want to decay into something less energetic, the reheatons broke down into smaller particles – the ones that structure the universe today.

    Next, the physicists took our familiar standard model of particles, and introduced many copies of it into the story. These families live side-by-side, but scarcely interact with each other. Each family has the same kinds of particles, but a slightly different mass for its Higgs boson – the famous “missing piece” of the standard model, which was found to have a surprisingly light mass by the LHC in 2012.

    CERN CMS Higgs Event
    CERN CMS Higgs Event

    Because the Higgs gives mass to other particles, varying it leads to big differences in how much the particles in a group pull on each other.

    Extreme families

    If the universe was full of particles from families with very heavy or zero-mass Higgs bosons, then gravity between particles would be too strong or too weak for atoms to form. Only one family would have a Higgs boson just light enough to let planets and people develop – the Goldilocks amount of gravity. So how did our universe come to have more of the right sort of particles?

    The team says that for families with a Higgs mass just above zero, the reheatons would have decayed into one Higgs boson and another particle of the remaining energy. This wouldn’t have been the case for other families. Particles can only decay into things lighter than themselves, so for families with high Higgs boson masses, the reheatons would have decayed into two smaller entities – instead of the Higgs – plus the remainder particle. Particles prefer to decay into fewer entities, so most of the big bang’s energy would be funnelled into light-Higgs families, and only a bit to those with slightly heavier Higgs bosons.

    As a result, our familiar standard model of particles – which has the lightest possible non-zero-mass Higgs – reigns supreme.

    The Standard Model of elementary particles (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.
    The Standard Model of elementary particles (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

    That’s one step towards solving the weak-gravity problem. The next step comes from the team’s observation that the equation for gravity between particles can be fine-tuned by adjusting the number of particle sectors.

    The team calculated that about 10^16 particle families results in a small value for gravity’s strength. If you combine the effect of large numbers of particles on the gravity equation and the reheatons favouring light Higgs bosons, you get gravity just weak enough for playing basketball instead of collapsing into a singularity, says Cohen.

    Occam’s razor

    Some physicists are sceptical of this first incarnation of the new theory. Assuming that there are huge numbers of particle classes isn’t an appealing solution, says Peter Woit at Columbia University in New York. “It’s a huge violation of Occam’s razor,” he says.

    “This is not a bad idea – in fact, it’s a pretty clever idea in terms of how things might work,” says Matt Strassler at Harvard University. However, the mathematics used to flesh out the idea is pretty contrived, he says. Hopefully now that the idea is out there, the model can be improved, he adds.

    Paddy Fox at the Fermi National Accelerator Laboratory near Batavia, Illinois, says there’s a way to test this theory within the next 10 years. A few particles from the sets with slightly heavier Higgses probably got produced. We could see these ghostly particle families haunting the universe via their effects on the cosmic microwave background – leftover radiation from the universe’s birth.

    “There’s an exciting possibility that you could see such an imprint in the next round of experiments,” says Fox. “This is not something we have to wait around and one day hope to see.”

    This means the search for new particles could move out of underground colliders and into astronomy, says Cohen. So don’t call the Ghostbusters until the astronomy results are in.

    Journal reference: arxiv.org/abs/1607.06821

    See the full article here .

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  • richardmitnick 9:43 am on July 29, 2016 Permalink | Reply
    Tags: , Supersymmetry,   

    From Yale: “Yale leads research collaboration to explore origins of the universe” 

    Yale University bloc

    Yale University

    July 27, 2016
    No writer credit found

    A deformation in the shape of a nucleus along the direction of its spin axis can arise only due to the presence of new, exotic particles. To search for such a deformation, thallium (Tl) nuclei are exposed to the strong electric field inside a polar molecule (thallium fluoride, TlF) that has been polarized by an external electric field. The deformation leads to a torque on the nucleus, which causes its axis to rotate around the electric field, in the same way that the axis of a gyroscope rotates around the direction of gravity.

    Yale physics professor David DeMille has launched a pioneering investigation into the origins of the universe with support from the John Templeton Foundation and the Heising-Simons Foundation. DeMille plans to build a novel apparatus to sense the existence of never-before-seen subatomic particles thought to have a determining role in the formation of matter. Proving their existence — or absence — will provide a window into the earliest moments following the Big Bang.

    DeMille will undertake the project in partnership with collaborators David Kawall of the University of Massachusetts, Tanya Zelevinsky of Columbia University, and Steve Lamoreaux of Yale. The grants from the John Templeton Foundation and the Heising-Simons Foundation, totaling $3 million, will support project staff and laboratory equipment.

    “Our approach is a radical departure from the large particle accelerators that generally come to mind when you look for exotic particles,” DeMille said. “Building on work that has been done at Yale, we will conduct a new type of experiment to probe for new particles and forces responsible for the predominance of matter over antimatter in the universe. We are very grateful to the Templeton and Heising-Simons foundations for their support, which was critical for our team to launch this work.”

    Addressing a fundamental mystery in physics

    DeMille’s research partnership seeks to answer a persistent question in physics known as the matter-antimatter asymmetry: Why is the universe made entirely of matter, when an equal number of matter particles and antimatter particles were created just after the Big Bang? Astronomical observations show that the matter and antimatter mostly annihilated each other, turning back into energy. The antimatter was eliminated, but a tiny fraction of matter was somehow left over, forming all the objects in the universe today.

    The current model for all known fundamental forces and particles fails to explain how the excess matter survived. Recent mathematical theories seek to explain the matter-antimatter asymmetry by positing new, as-yet-undiscovered forces and particles, such as “supersymmetry particles.”

    Standard model of Supersymmetry DESY
    Standard model of Supersymmetry DESY

    In most of these theories, the new fundamental phenomena also cause a tiny, yet detectable, deformation in the distribution of electric charge in ordinary atomic nuclei, known as a Schiff moment. The nuclear Schiff moment arises only in the presence of new particles and forces with properties needed to explain the matter-antimatter asymmetry.

    A novel way to detect new particles

    Finding new subatomic particles is a notoriously difficult challenge. In the 1960s, physicists predicted the existence of the Higgs boson, an elementary particle in the Standard Model of particle physics, but it was not until 2013 that scientists at the CERN facility in Switzerland could prove its existence using the Large Hadron Collider — a facility measuring 17 miles in circumference and costing over $7.5 billion.

    CERN CMS Higgs Event
    CERN CMS Higgs Event

    CERN/LHC Map
    CERN LHC Grand Tunnel
    CERN LHC particles
    LHC at CERN

    In contrast, DeMille’s team will design and assemble an instrument, about 15 feet across, in an on-campus Yale physics laboratory. The device will be made up of roughly 100,000 custom-designed and fabricated parts; it will take a team of six postdoctoral fellows and graduate students three years to construct.

    “Our device will focus a cryogenic beam of diatomic molecules through an electric field to detect a nuclear Schiff moment,” DeMille said. “This technique will yield a 100-fold increase in sensitivity over the current state of the art, enough to say whether or not new particles with the properties posited by many theories to explain the matter-antimatter asymmetry actually exist. This determination will either validate 30 years of mainstream work in theoretical physics or send the field in another direction.”

    The experiment depends on a strange quirk of quantum mechanics, which posits that subatomic particles like electrons and protons must constantly spin out and reabsorb other particles — a phenomenon that DeMille and his colleagues have learned to observe in the laboratory. “I tell my students to imagine Pig Pen, the character from‘Peanuts,’” said DeMille. “Every proton is surrounded by an ever-cycling cloud of short-lived particles that pop in and out of existence. Theoretically, this cloud should include supersymmetry particles.”

    Direct observation of supersymmetry particles with the properties needed to explain the matter-antimatter asymmetry is beyond current technology, but DeMille believes he can record their influence on the proton itself — hence his search for the Schiff moment. “The forces associated with a supersymmetry particle should cause a small but observable deformation at the surface of the proton,” he said. “This will in turn cause a similar deformation in an atomic nucleus — the Schiff moment. We are looking for a minute dent on one side of the nucleus and a corresponding bulge on the other. If we find this deformation, we will have definitive proof that new particles exist.”

    The precision of this experiment will be unprecedented, notes DeMille. For comparison, if the proton were scaled up to the size of the Earth, the dent in its surface would be 1/30th the width of a human hair.
    Revising our understanding of the universe

    Steven Girvin, deputy provost for research and the Eugene Higgins Professor of Physics, has hailed this project as groundbreaking. “David DeMille, David Kawall, Tanya Zelevinsky, and Steve Lamoreaux are on the very cutting edge of physics,” he said. “Their experiment promises to detect particles 10 times more massive than what one might see in the Large Hadron Collider. If they succeed, this will be an extraordinary accomplishment, and one that revises our understanding of the universe. I am personally grateful to the Heising-Simons and Templeton foundations for their decision to work together to fund the different aspects of this large and complex project.”
    John Templeton Foundation

    Founded by Sir John Templeton, a 1934 graduate of Yale College, the John Templeton Foundation serves as a philanthropic catalyst for discoveries relating to the Big Questions of human purpose and ultimate reality. The foundation supports research on subjects ranging from complexity, evolution, and infinity to creativity, forgiveness, love, and free will, and encourages civil, informed dialogue among scientists, philosophers, theologians, and the public. More information is available at http://www.templeton.org.
    Heising-Simons Foundation

    The Heising-Simons Foundation is a family foundation located in Los Altos, California dedicated to advancing sustainable solutions in climate and clean energy, enabling groundbreaking research in science, enhancing the education of our youngest learners, and supporting human rights for all people. Learn more at http://www.heisingsimons.org.

    See the full article here .

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    Yale University Campus

    Yale University comprises three major academic components: Yale College (the undergraduate program), the Graduate School of Arts and Sciences, and the professional schools. In addition, Yale encompasses a wide array of centers and programs, libraries, museums, and administrative support offices. Approximately 11,250 students attend Yale.

  • richardmitnick 5:16 pm on June 26, 2016 Permalink | Reply
    Tags: , , , Supersymmetry   

    From particle bites: “Can’t Stop Won’t Stop: The Continuing Search for SUSY” 

    particlebites bloc


    June 19, 2016
    Julia Gonski


    Title: “Search for top squarks in final states with one isolated lepton, jets, and missing transverse momentum in √s = 13 TeV pp collisions with the ATLAS detector
    Author: The ATLAS Collaboration
    Publication: Submitted 13 June 2016, arXiv 1606.03903

    Things at the LHC are going great. Run II of the Large Hadron Collider is well underway, delivering higher energies and more luminosity than ever before. ATLAS and CMS also have an exciting lead to chase down– the diphoton excess that was first announced in December 2015. So what does lots of new data and a mysterious new excess have in common? They mean that we might finally get a hint at the elusive theory that keeps refusing our invitations to show up: supersymmetry.

    Standard model of Supersymmetry DESY
    Standard model of Supersymmetry DESY

    Figure 1: Feynman diagram of stop decay from proton-proton collisions.

    People like supersymmetry because it fixes a host of things in the Standard Model. But most notably, it generates an extra Feynman diagram that cancels the quadratic divergence of the Higgs mass due to the top quark contribution. This extra diagram comes from the stop quark. So a natural SUSY solution would have a light stop mass, ideally somewhere close to the top mass of 175 GeV. This expected low mass due to “naturalness” makes the stop a great place to start looking for SUSY. But according to the newest results from the ATLAS Collaboration, we’re not going to be so lucky.

    Using the full 2015 dataset (about 3.2 fb-1), ATLAS conducted a search for pair-produced stops, each decaying to a top quark and a neutralino, in this case playing the role of the lightest supersymmetric particle. The top then decays as tops do, to a W boson and a b quark. The W usually can do what it wants, but in this case the group chose to select for one W decaying leptonically and one decaying to jets (leptons are easier to reconstruct, but have a lower branching ratio from the W, so it’s a trade off.) This whole process is shown in Figure 1. So that gives a lepton from one W, jets from the others, and missing energy from the neutrino for a complete final state.

    Figure 2: Transverse mass distribution in one of the signal regions.

    The paper does report an excess in the data, with a significance around 2.3 sigma. In Figure 2, you can see this excess overlaid with all the known background predictions, and two possible signal models for various gluino and stop masses. This signal in the 700-800 GeV mass range is right around the current limit for the stop, so it’s not entirely inconsistent. While these sorts of excesses come and go a lot in particle physics, it’s certainly an exciting reason to keep looking.

    Figure 3 shows our status with the stop and neutralino, using 8 TeV data. All the shaded regions here are mass points for the stop and neutralino that physicists have excluded at 95% confidence. So where do we go from here? You can see a sliver of white space on this plot that hasn’t been excluded yet— that part is tough to probe because the mass splitting is so small, the neutralino emerges almost at rest, making it very hard to notice. It would be great to check out that parameter space, and there’s an effort underway to do just that. But at the end of the day, only more time (and more data) can tell.

    (P.S. This paper also reports a gluino search—too much to cover in one post, but check it out if you’re interested!)

    Figure 3: Limit curves for stop and neutralino masses, with 8 TeV ATLAS dataset.

    See the full article here .

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    What is ParticleBites?

    ParticleBites is an online particle physics journal club written by graduate students and postdocs. Each post presents an interesting paper in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.

    The papers are accessible on the arXiv preprint server. Most of our posts are based on papers from hep-ph (high energy phenomenology) and hep-ex (high energy experiment).

    Why read ParticleBites?

    Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.

    Our goal is to solve this problem, one paper at a time. With each brief ParticleBite, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in particle physics.

    Who writes ParticleBites?

    ParticleBites is written and edited by graduate students and postdocs working in high energy physics. Feel free to contact us if you’re interested in applying to write for ParticleBites.

    ParticleBites was founded in 2013 by Flip Tanedo following the Communicating Science (ComSciCon) 2013 workshop.

    Flip Tanedo UCI Chancellor’s ADVANCE postdoctoral scholar in theoretical physics. As of July 2016, I will be an assistant professor of physics at the University of California, Riverside

    It is now organized and directed by Flip and Julia Gonski, with ongoing guidance from Nathan Sanders.

  • richardmitnick 5:35 pm on March 18, 2016 Permalink | Reply
    Tags: , , , , , , Supersymmetry   

    From CERN: “CMS hunts for supersymmetry in uncharted territory” 

    Cern New Bloc

    Cern New Particle Event

    CERN New Masthead


    Mar 18, 2016

    The CMS collaboration is continuing its hunt for signs of supersymmetry (SUSY), a popular extension to the Standard Model that could provide a weakly interacting massive-particle candidate for dark matter, if the lightest supersymmetric particle (LSP) is stable.

    Standard model of Supersymmetry Illustration: CERN & IES de SAR
    The Standard Model of Supersymmetry Illustration: CERN & IES de SAR

    With the increase in the LHC centre-of-mass energy from 8 to 13 TeV, the production cross-section for hypothetical SUSY partners rises; the first searches to benefit are those looking for the strongly coupled SUSY partners of the gluon (gluino) and quarks (squarks) that had the most stringent mass limits from Run 1 of the LHC. By decaying to a stable LSP, which does not interact in the detector and instead escapes, SUSY particles can leave a characteristic experimental signature of a large imbalance in transverse momentum.

    Searches for new physics based on final states with jets (a bundle of particles) and large transverse-momentum imbalance are sensitive to broad classes of new-physics models, including supersymmetry. CMS has searched for SUSY in this final state using a variable called the “stransverse mass”, MT2, to measure the transverse-momentum imbalance, which strongly suppresses fake contributions due to potential hadronic-jet mismeasurement. This allows us to control the background from copiously produced QCD multi-jet events. The remaining background comes from Standard Model processes such as W, Z and top-quark pair production with decays to neutrinos, which also produce a transverse-momentum imbalance. We estimate our backgrounds from orthogonal control samples in data targeted to each. To cover a wide variety of signatures, we categorise our signal events according to the number of jets, the number of jets arising from bottom quarks, the sum of the transverse momenta of hadronic jets (HT), and MT2. Some SUSY scenarios predict spectacular signatures, such as four top quarks and two LSPs, which would give large values for all of these quantities, while others with small mass splittings produce much softer signatures.

    Unfortunately, we did not observe any evidence for SUSY in the 2015 data set. Instead, we are able to significantly extend the constraints on the masses of SUSY partners beyond those from the LHC Run 1. The gluino has the largest production cross-section and many potential decay modes. If the gluino decays to the LSP and a pair of quarks, we exclude gluino masses up to 1550–1750 GeV, depending on the quark flavour, extending our Run 1 limits by more than 300 GeV. We are also sensitive to squarks, with our constraints summarised in figure 1. We set limits on bottom-squark masses up to 880 GeV, top squarks up to 800 GeV, and light-flavour squarks up to 600–1260 GeV, depending on how many states are degenerate in mass.

    Even though SUSY was not waiting for us around the corner at 13 TeV, we look forward to the 2016 run, where a large increase in luminosity gives us another chance at discovery.

    See the full article here.

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