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  • richardmitnick 7:29 am on May 23, 2019 Permalink | Reply
    Tags: "Atom smasher could be making new particles that are hiding in plain sight", , , CERN FASER experiment, , Compact Detector for Exotics at LHCb, MATHUSLA- Massive Timing Hodoscope for Ultra Stable Neutral Particles,   

    From Science Magazine: “Atom smasher could be making new particles that are hiding in plain sight” 

    AAAS
    From Science Magazine

    May. 22, 2019
    Adrian Cho

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    In a simulated event, the track of a decay particle called a muon (red), displaced slightly from the center of particle collisions, could be a sign of new physics.
    ATLAS EXPERIMENT © 2019 CERN

    Are new particles materializing right under physicists’ noses and going unnoticed? The world’s great atom smasher, the Large Hadron Collider (LHC), could be making long-lived particles that slip through its detectors, some researchers say.

    LHC

    CERN map


    CERN LHC Maximilien Brice and Julien Marius Ordan


    CERN LHC particles

    Next week, they will gather at the LHC’s home, CERN, the European particle physics laboratory near Geneva, Switzerland, to discuss how to capture them.


    They argue the LHC’s next run should emphasize such searches, and some are calling for new detectors that could sniff out the fugitive particles.

    It’s a push born of anxiety. In 2012, experimenters at the $5 billion LHC discovered the Higgs boson, the last particle predicted by the standard model of particles and forces, and the key to explaining how fundamental particles get their masses.

    CERN CMS Higgs Event


    CERN ATLAS Higgs Event

    But the LHC has yet to blast out anything beyond the standard model.

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

    “We haven’t found any new physics with the assumptions we started with, so maybe we need to change the assumptions,” says Juliette Alimena, a physicist at Ohio State University (OSU) in Columbus who works with the Compact Muon Solenoid (CMS), one of the two main particle detectors fed by the LHC.

    CERN/CMS Detector


    For decades, physicists have relied on a simple strategy to look for new particles: Smash together protons or electrons at ever-higher energies to produce heavy new particles and watch them decay instantly into lighter, familiar particles within the huge, barrel-shaped detectors. That’s how CMS and its rival detector, A Toroidal LHC Apparatus (ATLAS), spotted the Higgs, which in a trillionth of a nanosecond can decay into, among other things, a pair of photons or two “jets” of lighter particles.

    CERN ATLAS Credit CERN SCIENCE PHOTO LIBRARY

    Long-lived particles, however, would zip through part or all of the detector before decaying. That idea is more than a shot in the dark, says Giovanna Cottin, a theorist at National Taiwan University in Taipei. “Almost all the frameworks for beyond-the-standard-model physics predict the existence of long-lived particles,” she says. For example, a scheme called supersymmetry posits that every standard model particle has a heavier superpartner, some of which could be long-lived. Long-lived particles also emerge in “dark sector” theories that envision undetectable particles that interact with ordinary matter only through “porthole” particles, such as a dark photon that every so often would replace an ordinary photon in a particle interaction.

    CMS and ATLAS, however, were designed to detect particles that decay instantaneously. Like an onion, each detector contains layers of subsystems—trackers that trace charged particles, calorimeters that measure particle energies, and chambers that detect penetrating and particularly handy particles called muons—all arrayed around a central point where the accelerator’s proton beams collide. Particles that fly even a few millimeters before decaying would leave unusual signatures: kinked or offset tracks, or jets that emerge gradually instead of all at once.

    Standard data analysis often assumes such oddities are mistakes and junk, notes Tova Holmes, an ATLAS member from the University of Chicago in Illinois who is searching for the displaced tracks of decays from long-lived supersymmetric particles. “It’s a bit of a challenge because the way we’ve designed things, and the software people have written, basically rejects these things,” she says. So Holmes and colleagues had to rewrite some of that software.

    More important is ensuring that the detectors record the odd events in the first place. The LHC smashes bunches of protons together 400 million times a second. To avoid data overload, trigger systems on CMS and ATLAS sift interesting collisions from dull ones and immediately discard data about 1999 of every 2000 collisions. The culling can inadvertently toss out long-lived particles. Alimena and colleagues wanted to look for particles that live long enough to get stuck in CMS’s calorimeter and decay only later. So they had to put in a special trigger that occasionally reads out the entire detector between the proton collisions.

    Long-lived particle searches had been fringe efforts, says James Beacham, an ATLAS experimenter from OSU. “It’s always been one guy working on this thing,” he says. “Your support group was you in your office.” Now, researchers are joining forces. In March, 182 of them released a 301-page white paper on how to optimize their searches.

    Some want ATLAS and CMS to dedicate more triggers to long-lived particle searches in the next LHC run, from 2021 through 2023. In fact, the next run “is probably our last chance to look for unusual rare events,” says Livia Soffi, a CMS member from the Sapienza University of Rome. Afterward, an upgrade will increase the intensity of the LHC’s beams, requiring tighter triggers.

    Others have proposed a half-dozen new detectors to search for particles so long-lived that they escape the LHC’s existing detectors altogether. Jonathan Feng, a theorist at the University of California, Irvine, and colleagues have won CERN approval for the Forward Search Experiment (FASER), a small tracker to be placed in a service tunnel 480 meters down the beamline from ATLAS.

    CERN FASER experiment schematic

    Supported by $2 million from private foundations and built of borrowed parts, FASER will look for low-mass particles such as dark photons, which could spew from ATLAS, zip through the intervening rock, and decay into electron-positron pairs.

    Another proposal calls for a tracking chamber in an empty hall next to the LHCb, a smaller detector fed by the LHC.

    CERN/LHCb detector

    The Compact Detector for Exotics at LHCb would look for long-lived particles, especially those born in Higgs decays, says Vladimir Gligorov, an LHCb member from the Laboratory for Nuclear Physics and High Energies in Paris.

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    The Compact Detector for Exotics at LHCb. https://indico.cern.ch/event/755856/contributions/3263683/attachments/1779990/2897218/PBC2019_CERN_CodexB_report.pdf

    Even more ambitious would be a detector called MATHUSLA, essentially a large, empty building on the surface above the subterranean CMS detector.

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    MATHUSLA. http://cds.cern.ch/record/2653848

    Tracking chambers in the ceiling would detect jets spraying up from the decays of long-lived particles created 70 meters below, says David Curtin, a theorist at the University of Toronto in Canada and project co-leader. Curtin is “optimistic” MATHUSLA would cost less than €100 million. “Given that it has sensitivity to this broad range of signatures—and that we haven’t seen anything else—I’d say it’s a no-brainer.”

    Physicists have a duty to look for the odd particles, Beacham says. “The nightmare scenario is that in 20 years, Jill Theorist says, ‘The reason you didn’t see anything is you didn’t keep the right events and do the right search.’”

    See the full article here .


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  • richardmitnick 12:47 pm on January 18, 2018 Permalink | Reply
    Tags: , , , Long-lived physics, MATHUSLA- Massive Timing Hodoscope for Ultra Stable Neutral Particles, ,   

    From CERN: “Long-lived physics” 

    Cern New Bloc

    Cern New Particle Event

    CERN New Masthead

    CERN

    18 Jan 2018
    Iva Raynova

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    The CMS experiment is looking for exotic long-lived particles that could get trapped in its detector layers (Image: Michael Hoch, Maximilien Brice/CERN)

    New particles produced in the LHC’s high-energy proton-proton collisions don’t hang around for long. A Higgs boson exists for less than a thousandth of a billionth of a billionth of a second before decaying into lighter particles, which can then be tracked or stopped in our detectors. Nothing rules out the existence of much longer-lived particles though, and certain theoretical scenarios predict that such extraordinary objects could get trapped in the LHC detectors, sitting there quietly for days.

    The CMS collaboration has reported new results [JHEP] in its search for heavy long-lived particles (LLPs), which could lose their kinetic energy and come to a standstill in the LHC detectors. Provided that the particles live for longer than a few tens of nanoseconds, their decay would be visible during periods when no LHC collisions are taking place, producing a stream of ordinary matter seemingly out of nowhere.

    The CMS team looked for these types of non-collision events in the densest detector materials of the experiment, where the long-lived particles are most likely to be stopped, based on LHC collisions in 2015 and 2016. Despite scouring data from a period of more than 700 hours, nothing strange was spotted. The results set the tightest cross-section and mass limits for hadronically-decaying long-lived particles that stop in the detector to date, and the first limits on stopped long-lived particles produced in proton-proton collisions at an energy of 13 TeV.

    The Standard Model, the theoretical framework that describes all the elementary particles, was vindicated in 2012 with the discovery of the Higgs boson.

    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

    CERN CMS Higgs Event

    CERN ATLAS Higgs Event

    But some of the universe’s biggest mysteries remain unexplained, such as why matter prevailed over antimatter in the early universe or what exactly dark matter is. Long-lived particles are among numerous exotic species that would help address these mysteries and their discovery would constitute a clear sign of physics beyond the Standard Model. In particular, the decays searched for in CMS concerned long-lived gluinos arising in a model called “split” supersymmetry (SUSY) and exotic particles called “MCHAMPs”.

    While the search for long-lived particles at the LHC is making rapid progress at both CMS and ATLAS, the construction of a dedicated LLP detector has been proposed for the high-luminosity era of the LHC. MATHUSLA (Massive Timing Hodoscope for Ultra Stable Neutral Particles) is planned to be a surface detector placed 100 metres above either ATLAS or CMS.

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    It would be an enormous (200 × 200 × 20 m) box, mostly empty except for the very sensitive equipment used to detect LLPs produced in LHC collisions.

    Since LLPs interact weakly with ordinary matter, they will experience no trouble travelling through the rocks between the underground experiment and MATHUSLA. This process is similar to how weakly interacting cosmic rays travel through the atmosphere and pass through the Earth to reach our underground detectors, only in reverse. If constructed, the experiment will explore many more scenarios and bring us closer to discovering new physics.

    See the full article here.

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