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  • richardmitnick 1:43 pm on September 20, 2022 Permalink | Reply
    Tags: "Catching neutrinos at the LHC", , , CERN FASER experiment, , , , , Scattering and Neutrino Detector or SND@LHC,   

    From “Symmetry”: “Catching neutrinos at the LHC” 

    Symmetry Mag

    From “Symmetry”

    9.20.22
    Chetna Krishna

    After the successful initiation of two new detectors, scientists have begun to envision an expanded suite of neutrino experiments at the Large Hadron Collider.

    CERN physicist Jamie Boyd enters a tunnel close to the ATLAS detector, an experiment at the largest particle accelerator in the world. From there, he turns into an underground space labeled TI12.

    “This is a very special tunnel,” Boyd says, “because this is where the old transfer line used to exist for the Large Electron-Positron Collider, before the Large Hadron Collider.” After the LHC was built, a new transfer line was added, “and this tunnel was then abandoned.”

    The tunnel is abandoned no more. Its new resident is an experiment much humbler in size than the neighboring ATLAS detector. Five meters in length, the ForwArd Search ExpeRiment, or FASER, detector sits in a shallow excavated trench in the floor, surrounded by low railings and cables.

    Scientists—including Boyd, who serves as co-spokesperson for FASER—installed the relatively small detector in 2021. Just in time before restarting the LHC in April, physicists nestled another small experiment, called Scattering and Neutrino Detector or SND@LHC, on the other side of ATLAS.

    2
    The SND@LHC experiment consists of an emulsion/tungsten target for neutrinos (yellow) interleaved with electronic tracking devices (grey), followed downstream by a detector (brown) to identify muons and measure the energy of the neutrinos. (Image: Antonio Crupano/SND@LHC)

    Both of the detectors are now running and have started collecting data. Scientists say they hope the two detectors represent the beginning of a new effort to catch and study particles that the LHC’s four main detectors can’t see.

    Hiding in plain sight

    Both FASER and SND@LHC detect particles called neutrinos. Not to be confused with neutrons—particles in the nuclei of atoms that are made up of quarks—neutrinos cannot be broken down into smaller constituents. Along with quarks, electrons, muons and taus, neutrinos are fundamental particles of matter in the Standard Model of physics.

    These light, neutral particles are abundant across the galaxy. Some have been around since the Big Bang; others are produced in particle collisions, such as those that happen when cosmic rays strike the atoms that make up Earth’s atmosphere. Every second, neutrinos pass through us in the trillions without leaving a trace—because they only rarely interact with other matter.

    Neutrinos are also produced in collisions at the LHC. Scientists are aware of their presence, but for more than a decade of LHC physics, neutrinos went undetected, as the ATLAS, CMS, LHCb and ALICE detectors were designed with other types of particles in mind.

    The four biggest LHC experiments cannot detect neutrinos directly, says Milind Diwan, a senior scientist at the US Department of Energy’s Brookhaven National Laboratory. Diwan was an original proponent of and spokesperson for what is now the Deep Underground Neutrino Experiment hosted by The DOE’s Fermi National Accelerator Laboratory.

    In 2021, FASER became the first detector to catch neutrinos at the LHC—or any particle collider.

    A new way of looking at neutrinos

    Neutrinos are the chameleons of the particle world. They come in three flavors, called muon, electron and tau neutrinos [above] for the particles associated with them. As they travel through the universe at nearly the speed of light, neutrinos shift between the three flavors. Both FASER and SND@LHC can detect all three flavors of neutrinos.

    The detectors will catch only a small fraction of the neutrinos that pass through them, but the high-energy collisions of the LHC should produce a staggering number of the particles. For example, during the current run of the LHC, which will last until the end of 2025, physicists estimate FASER and its new subdetector, called FASERv (pronounced FASERnu), will experience a flux of 200 billion electron neutrinos, 6 trillion muon neutrinos, and 4 billion tau neutrinos, along with a comparable number of anti-neutrinos of each flavor.

    “We are now guaranteed to see thousands of neutrinos at the LHC for the first time,” says Jonathan Feng, co-spokesperson for the FASER collaboration.

    Those neutrinos will be at the highest energies ever seen from a human-made source, says Tomoko Ariga, project leader for FASERv, who previously worked on the DONUT neutrino experiment. “At such extreme energies, FASERv will be able to probe neutrino properties in new ways.”

    The experiments will provide a new way of studying other particles as well, says Giovanni De Lellis, spokesperson for both SND@LHC and the OPERA neutrino experiment.

    Because a large fraction of the neutrinos produced in the range accessible to SND@LHC will come from the decays of particles made of charm quarks, SND@LHC can be used to study charm-quark particle production in a region that other LHC experiments cannot explore. This will help both physicists studying collisions at future colliders and physicists studying neutrinos from astrophysical sources.

    FASER and SND@LHC could also be used to detect dark matter, Diwan says. If dark-matter particles are produced in collisions at the LHC, they could slip away from the ATLAS detector alongside the beamline—right into FASER and SND@LHC.

    A proposal for the future

    These experiments could be just the beginning. Physicists have proposed five more experiments—including advanced versions of the FASER and SND@LHC detectors—to be built near the ATLAS detector. The experiments—FASERv2, Advanced SND, FASER2, FORMOSA and FLArE—could sit at a proposed Forward Physics Facility during the next phase of the LHC, the High-Luminosity LHC.

    The advanced FASERv and SND@LHC detectors would boost the experiments’ detection of neutrinos by a factor of 100, Feng says. “This means, for example, that instead of tens of tau neutrinos, they will detect thousands, allowing us to separate tau neutrinos from anti-tau neutrinos and do precision studies of these two independently for the first time.”

    The FLArE experiment, which would detect neutrinos in a different way from FASER and SND@LHC, could also be sensitive to light dark matter.

    Even without the proposed future experiments, scientists are poised to learn more about neutrinos from their studies at the LHC. FASERv and SND@LHC have already began taking physics data and are expected to present new results in 2023.

    “Neutrinos are amazing,” Feng says. “Every time we look at them from a new source, whether it is a nuclear reactor or the sun or the atmosphere, we learn something new. I am looking forward to seeing what surprises nature has in store.”

    See the full article here .


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  • richardmitnick 2:53 pm on November 27, 2021 Permalink | Reply
    Tags: "University of California-Irvine(US) of physicists detects signs of neutrinos at Large Hadron Collider", , , , CERN FASER experiment, , , , , The University of California-Irvine(US)   

    From The University of California-Irvine(US): “University of California-Irvine(US) of physicists detects signs of neutrinos at Large Hadron Collider” 

    UC Irvine bloc

    From The University of California-Irvine(US)

    November 24, 2021

    Brian Bell
    949-565-5533
    bpbell@uci.edu

    Scientific first at CERN facility a preview of upcoming 3-year research campaign.

    1
    The FASER particle detector that received CERN approval to be installed at the Large Hadron Collider in 2019 has recently been augmented with an instrument to detect neutrinos. The UCI-led FASER team used a smaller detector of the same type in 2018 to make the first observations of the elusive particles generated at a collider. The new instrument will be able to detect thousands of neutrino interactions over the next three years, the researchers say. Photo courtesy of CERN .

    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.

    The international Forward Search Experiment team, led by physicists at the University of California, Irvine, has achieved the first-ever detection of neutrino candidates produced by the Large Hadron Collider at the CERN facility near Geneva, Switzerland.

    In a paper published today in the journal Physical Review D, the researchers describe how they observed six neutrino interactions during a pilot run of a compact emulsion detector installed at the LHC in 2018.

    “Prior to this project, no sign of neutrinos has ever been seen at a particle collider,” said co-author Jonathan Feng, UCI Distinguished Professor of physics & astronomy and co-leader of the FASER Collaboration. “This significant breakthrough is a step toward developing a deeper understanding of these elusive particles and the role they play in the universe.”

    He said the discovery made during the pilot gave his team two crucial pieces of information.

    “First, it verified that the position forward of the ATLAS interaction point at the LHC is the right location for detecting collider neutrinos,” Feng said. “Second, our efforts demonstrated the effectiveness of using an emulsion detector to observe these kinds of neutrino interactions.”

    3
    The FASER experiment is situated 480 meters from the ATLAS interaction point at the Large Hadron Collider. According to Jonathan Feng, UCI Distinguished Professor of physics & astronomy and co-leader of the FASER Collaboration, this is a good location for detecting neutrinos that result from particle collisions at the facility. Photo courtesy of CERN.

    The pilot instrument was made up of lead and tungsten plates alternated with layers of emulsion. During particle collisions at the LHC, some of the neutrinos produced smash into nuclei in the dense metals, creating particles that travel through the emulsion layers and create marks that are visible following processing. These etchings provide clues about the energies of the particles, their flavors – tau, muon or electron – and whether they’re neutrinos or antineutrinos.

    According to Feng, the emulsion operates in a fashion similar to photography in the pre-digital camera era. When 35-millimeter film is exposed to light, photons leave tracks that are revealed as patterns when the film is developed. The FASER researchers were likewise able to see neutrino interactions after removing and developing the detector’s emulsion layers.

    “Having verified the effectiveness of the emulsion detector approach for observing the interactions of neutrinos produced at a particle collider, the FASER team is now preparing a new series of experiments with a full instrument that’s much larger and significantly more sensitive,” Feng said.

    Since 2019, he and his colleagues have been getting ready to conduct an experiment with FASER instruments to investigate dark matter at the LHC. They’re hoping to detect dark photons, which would give researchers a first glimpse into how dark matter interacts with normal atoms and the other matter in the universe through nongravitational forces.

    With the success of their neutrino work over the past few years, the FASER team – consisting of 76 physicists from 21 institutions in nine countries – is combining a new emulsion detector with the FASER apparatus. While the pilot detector weighed about 64 pounds, the FASERnu instrument will be more than 2,400 pounds, and it will be much more reactive and able to differentiate among neutrino varieties.

    “Given the power of our new detector and its prime location at CERN, we expect to be able to record more than 10,000 neutrino interactions in the next run of the LHC, beginning in 2022,” said co-author David Casper, FASER project co-leader and associate professor of physics & astronomy at UCI. “We will detect the highest-energy neutrinos that have ever been produced from a human-made source.”

    What makes FASERnu unique, he said, is that while other experiments have been able to distinguish between one or two kinds of neutrinos, it will be able to observe all three flavors plus their antineutrino counterparts. Casper said that there have only been about 10 observations of tau neutrinos in all of human history but that he expects his team will be able to double or triple that number over the next three years.

    “This is an incredibly nice tie-in to the tradition at the physics department here at UCI,” Feng said, “because it’s continuing on with the legacy of Frederick Reines, a UCI founding faculty member who won the Nobel Prize in physics for being the first to discover neutrinos.”

    “We’ve produced a world-class experiment at the world’s premier particle physics laboratory in record time and with very untraditional sources,” Casper said. “We owe an enormous debt of gratitude to the Heising-Simons Foundation and the Simons Foundation, as well as the Japan Society for the Promotion of Science and CERN, which supported us generously.”

    Savannah Shively and Jason Arakawa, UCI Ph.D. students in physics & astronomy, also contributed to the paper.

    See the full article here .

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    UC Irvine Campus.


    Since 1965, the University of California-Irvine (US) has combined the strengths of a major research university with the bounty of an incomparable Southern California location. UCI’s unyielding commitment to rigorous academics, cutting-edge research, and leadership and character development makes the campus a driving force for innovation and discovery that serves our local, national and global communities in many ways.

    With more than 29,000 undergraduate and graduate students, 1,100 faculty and 9,400 staff, UCI is among the most dynamic campuses in the University of California system. Increasingly a first-choice campus for students, UCI ranks among the top 10 U.S. universities in the number of undergraduate applications and continues to admit freshmen with highly competitive academic profiles.

    UCI fosters the rigorous expansion and creation of knowledge through quality education. Graduates are equipped with the tools of analysis, expression and cultural understanding necessary for leadership in today’s world.

    Consistently ranked among the nation’s best universities – public and private – UCI excels in a broad range of fields, garnering national recognition for many schools, departments and programs. Times Higher Education ranked UCI No. 1 among universities in the U.S. under 50 years old. Three UCI researchers have won Nobel Prizes – two in chemistry and one in physics.

    The university is noted for its top-rated research and graduate programs, extensive commitment to undergraduate education, and growing number of professional schools and programs of academic and social significance. Recent additions include highly successful programs in public health, pharmaceutical sciences and nursing science; an expanding education school; and a law school already ranked among the nation’s top 10 for its scholarly impact.

     
  • richardmitnick 10:09 am on August 18, 2020 Permalink | Reply
    Tags: "Long-lived particles get their moment", , ATLAS and CMS experiments at CERN LHC, , CERN FASER experiment, , , , ,   

    From Symmetry: “Long-lived particles get their moment” 

    Symmetry Mag
    From Symmetry

    08/18/20
    Sarah Charley

    Scientists on experiments at the LHC are redesigning their methods and building supplemental detectors to look for new particles that might be evading them.

    1
    Illustration by Sandbox Studio, Chicago with Ariel Davis.

    Duke University postdoc Katherine Pachal has spent the last ten years—from undergraduate on—searching for new particles with the ATLAS experiment at the Large Hadron Collider. “I’ve always been a search person,” she says.

    CERN ATLAS Image Claudia Marcelloni

    Physicists discovered the Higgs boson in 2012, but since then the list of known fundamental particles has remained static.

    CERN CMS Higgs Event May 27, 2012


    CERN ATLAS Higgs Event June 12, 2012

    This hasn’t dampened Pachal’s enthusiasm for the search for new particles. Rather, she sees it as an indication that physicists need to look for them in an innovative new way.

    When scientists designed detectors for the LHC, they wagered that new forces, fields and physics would come in the form of extremely short-lived particles that decay almost precisely at their points of origin. Scientists catch the particles that behave this way—such as the aforementioned Higgs bosons—in detectors surrounding the collision points.

    “The primary goal of ATLAS [above] and CMS was to find the Higgs, and we built darn good experiments to do that,” Pachal says.

    CERN/CMS Detector

    With many unanswered questions still looming in the field, LHC physicists are revisiting their original assumptions and reinventing their tools and techniques to reach for long-lived particles—ones that could travel long distances before becoming detectable.

    2
    Illustration by Sandbox Studio, Chicago with Ariel Davis.

    Long-lived particles

    “We already have long-lived particles in the Standard Model,” says Jingyu Luo, a graduate student at Princeton University.

    Standard Model of Particle Physics, Quantum Diaries

    Muons, for instance, can travel several kilometers before decaying (which is the main reason the particle detectors at CERN are so enormous). Protons and electrons may not decay at all.

    According to theorist Jonathan Feng at the University of California, Irvine, physicists were originally hesitant to search for additional long-lived particles because there seemed to be no real need for them in the theory.

    “If you want to come up with a theory with long-lived particles, it’s extremely easy,” he says. “You could add an arbitrarily long-lived particle to any theory and put it in by hand, but there was no rhyme or reason to it.”

    Feng’s feelings changed in 2003 when he was building upon a popular set of theories called supersymmetry, and a long-lived particle popped out of his equations. “There was no way around it, we needed long-lived particles,” he says. “This was different than putting it in by hand. It was coming out of a very well-structured theory.”

    But these theoretical particles seemed out of the grasp of the experiments running at the LHC.

    The detectors for the ATLAS and CMS experiments—funded by CERN member states and other contributing countries including the United States, via the US Department of Energy’s Office of Science and the National Science Foundation—generate about 50 terabytes of data a second. Most of this data comes from already well-understood subatomic processes, and a series of increasingly selective trigger systems evaluate the onslaught of hits and only pass along events that they pre-approve as “high quality and potentially interesting.” But a new type of long-lived particle wouldn’t necessarily have any of these pre-defined ‘interesting’ characteristics.

    “Our trigger systems are lacking a lot of the information that is core to many of our long-lived particle searchers,” Pachal says.

    These systems make snap judgments based on factors such as the amount of energy a collision leaves in the detector (a good indicator of the presence of a rare, massive particle). Scientists have already developed software that helps their trigger systems scan parts of the detector for signs of long-lived particles. But for a truly comprehensive search, they need to consider detailed particle tracks.

    “In the past, we were restricted by how time-consuming it is to reconstruct all the tracks,” Pachal says. In the next run of the LHC, “we’re improving our software so that we can use more of the detector to look for particle tracks in the trigger, and this will help us make these more subtle decisions.”

    3
    Illustration by Sandbox Studio, Chicago with Ariel Davis.

    Track-finder

    Even if long-lived particles are out there waiting to be found, there is still the question of whether scientists can find enough of them to claim a discovery.

    Traditional techniques to pick out possible sightings of new particles involve a series of strict cuts, removing giant chunks of the dataset at a time. “For instance, if I had a room full of people and wanted to find fans of the Italian composer Ennio Morricone, I could make a series of judgements such as, ‘people between 50 and 70 are good candidates to like this kind of music’ and focus my attention on them,” Luo says. “But in reality, it’s so much more complicated than that.”

    To separate long-lived particle candidates from an ocean of look-alikes, Luo is incorporating machine earning.

    Traditional techniques rely on a series of pre-programed “yes” or “no” check boxes to determine which events to keep. Machine-learning algorithms, on the contrary, examine thousands of collision events to build a deep understanding of how different variables interplay with one another to create the kind of particle signatures physicists are looking for.

    By the time physicists look at the data deemed “interesting,” their machine-learning framework is already a collision connoisseur. Like an expert judge scoring rhythmic gymnastics at the Olympics, it has built up enough specialized knowledge to rate each contender.

    The avoidance of strict cuts gives physicists increased flexibility to conduct these kinds of blue-sky searches.

    “There’s what we know and what we don’t know,” Luo says. “What we know is that there is a group of models that predict the existence of long-lived particles. But what we don’t know is which model is right.”

    Luo and his colleagues are working on model-independent searches at the LHC. Their goal is to stay sensitive to many types of potential long-lived particles, with a wide range of characteristics. “Leave no stone unturned,” he says.

    Particle escape artists

    While CMS and ATLAS search for long-lived particles inside their detectors, other teams of scientists are considering how to capture long-lived particles that could travel beyond them.

    “Upgrading existing experiments is one method,” Feng says. “The other method involves building supplemental detectors.”

    In fall of 2017, Feng and colleagues proposed building one such detector, which they named FASER.

    CERN FASER experiment schematic

    To catch long-lived particles that might escape the ATLAS experiment, FASER will sit in an unused tunnel that just happens to be right along the path they expect particles to follow, 480 meters from the ATLAS detector.

    Construction for FASER started in 2019. It is scheduled to start operation when collisions resume at the LHC, foreseen for late 2021 or early 2022.

    Teams of scientists are designing other, larger detectors—with names such as CODEX-b and MATHUSLA—to be built near other LHC collision points.

    With the help of these improved tools and techniques, the LHC physics community will be poised to jump on new physics. “There’s a moment for everything, and the moment for long-lived particles is starting,” Pachal says.

    See the full article here .


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  • richardmitnick 12:27 pm on November 27, 2019 Permalink | Reply
    Tags: "The plot thickens for a hypothetical “X17” particle", , Additional evidence of an unknown particle from a Hungarian lab, , CERN FASER experiment, , , , ,   

    From CERN: “The plot thickens for a hypothetical “X17” particle” 

    Cern New Bloc

    Cern New Particle Event


    From CERN

    27 November, 2019
    Ana Lopes

    Additional evidence of an unknown particle from a Hungarian lab gives a new impetus to NA64 searches.

    CERN NA64


    The NA64 experiment at CERN (Image: CERN)

    Fresh evidence of an unknown particle that could carry a fifth force of nature gives the NA64 collaboration at CERN a new incentive to continue searches.

    In 2015, a team of scientists spotted [Physical Review Letters] an unexpected glitch, or “anomaly”, in a nuclear transition that could be explained by the production of an unknown particle. About a year later, theorists suggested [Physical Review Letters] that the new particle could be evidence of a new fundamental force of nature, in addition to electromagnetism, gravity and the strong and weak forces. The findings caught worldwide attention and prompted, among other studies, a direct search [Physical Review Letters] for the particle by the NA64 collaboration at CERN.

    A new paper from the same team, led by Attila Krasznahorkay at the Atomki institute in Hungary, now reports another anomaly, in a similar nuclear transition, that could also be explained by the same hypothetical particle.

    The first anomaly spotted by Krasznahorkay’s team was seen in a transition of beryllium-8 nuclei. This transition emits a high-energy virtual photon that transforms into an electron and its antimatter counterpart, a positron. Examining the number of electron–positron pairs at different angles of separation, the researchers found an unexpected surplus of pairs at a separation angle of about 140º. In contrast, theory predicts that the number of pairs decreases with increasing separation angle, with no excess at a particular angle. Krasznahorkay and colleagues reasoned that the excess could be interpreted by the production of a new particle with a mass of about 17 million electronvolts (MeV), the “X17” particle, which would transform into an electron–positron pair.

    The latest anomaly reported by Krasznahorkay’s team, in a paper [.pdf above] that has yet to be peer-reviewed, is also in the form of an excess of electron–positron pairs, but this time the excess is from a transition of helium-4 nuclei. “In this case, the excess occurs at an angle 115º but it can also be interpreted by the production of a particle with a mass of about 17 MeV,” explained Krasznahorkay. “The result lends support to our previous result and the possible existence of a new elementary particle,” he adds.

    Sergei Gninenko, spokesperson for the NA64 collaboration at CERN, which has not found signs of X17 in its direct search, says: “The Atomki anomalies could be due to an experimental effect, a nuclear physics effect or something completely new such as a new particle. To test the hypothesis that they are caused by a new particle, both a detailed theoretical analysis of the compatibility between the beryllium-8 and the helium-4 results as well as independent experimental confirmation is crucial.”

    The NA64 collaboration searches for X17 by firing a beam of tens of billions of electrons from the Super Proton Synchrotron accelerator onto a fixed target.

    The Super Proton Synchrotron (SPS), CERN’s second-largest accelerator

    If X17 did exist, the interactions between the electrons and nuclei in the target would sometimes produce this particle, which would then transform into an electron–positron pair. The collaboration has so far found no indication that such events took place, but its datasets allowed them to exclude part of the possible values for the strength of the interaction between X17 and an electron. The team is now upgrading their detector for the next round of searches, which are expected to be more challenging but at the same time more exciting, says Gninenko.

    Among other experiments that could also hunt for X17 in direct searches are the LHCb experiment and the recently approved FASER experiment, both at CERN.

    CERN/LHCb detector

    CERN FASER experiment schematic

    Jesse Thaler, a theoretical physicist from the Massachusetts Institute of Technology, says: “By 2023, the LHCb experiment should be able to make a definitive measurement to confirm or refute the interpretation of the Atomki anomalies as arising from a new fundamental force. In the meantime, experiments such as NA64 can continue to chip away at the possible values for the hypothetical particle’s properties, and every new analysis brings with it the possibility (however remote) of discovery.”

    See the full article here.


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    Please help promote STEM in your local schools.

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    Meet CERN in a variety of places:

    Quantum Diaries
    QuantumDiaries

    Cern Courier

    THE FOUR MAJOR PROJECT COLLABORATIONS

    ATLAS

    CERN ATLAS Image Claudia Marcelloni CERN/ATLAS


    ALICE

    CERN/ALICE Detector


    CMS
    CERN CMS New

    LHCb
    CERN LHCb New II

    LHC

    CERN map

    CERN LHC Tunnel

    CERN LHC particles

     
  • 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, ,   

    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

    1
    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.

    3
    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.

    5
    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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    Please help promote STEM in your local schools.

    Stem Education Coalition

     
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