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  • richardmitnick 11:37 am on April 16, 2019 Permalink | Reply
    Tags: , CERN ATLAS, , , , , ,   

    From Symmetry: “A collision of light” 

    Symmetry Mag
    From Symmetry

    04/16/19
    Sarah Charley

    1
    Natasha Hartono

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

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

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

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

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

    CERN ATLAS Credit CERN SCIENCE PHOTO LIBRARY

    LHC

    CERN map


    CERN LHC Tunnel

    CERN LHC particles

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

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

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

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

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

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

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

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

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

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

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

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

    See the full article here .


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    Symmetry is a joint Fermilab/SLAC publication.


     
  • richardmitnick 11:55 am on April 8, 2019 Permalink | Reply
    Tags: "ATLAS sets strong constraints on supersymmetric dark matter", , , CERN ATLAS, , , ,   

    From CERN ATLAS: “ATLAS sets strong constraints on supersymmetric dark matter” 

    CERN/ATLAS detector

    CERN ATLAS Higgs Event

    CERN ATLAS another view Image Claudia Marcelloni ATLAS CERN


    CERN ATLAS New II Credit CERN SCIENCE PHOTO LIBRARY


    From CERN ATLAS

    8th April 2019

    1
    Figure 1: A comparison of the significance for the signal plus background hypothesis (vertical axis) of a chosen supersymmetric model obtained by selecting events using the new object-based ETmiss significance variable (black line), compared to the previous approximation (ETmiss/ET, cyan) or to selecting events using only the measured missing transverse energy (ETmiss, mauve). Higher significance is found for the new variable. (Image: ATLAS Collaboration/CERN)

    One of the most complete theoretical frameworks that includes a dark matter candidate is supersymmetry. Dark matter is an unknown type of matter present in the universe, which could be of particle origin. Many supersymmetric models predict the existence of a new stable, invisible particle – the lightest supersymmetric particle (LSP) – which has the right properties to be a dark matter particle. The ATLAS Collaboration has recently reported two new results on searches for an LSP where it exploited the experiment’s full “Run 2” data sample taken at 13 TeV proton-proton collision energy. The analyses looked for the pair production of two heavy supersymmetric particles, each of which decays to observable Standard Model particles and an LSP in the detector.

    Identifying missing energy

    2
    Figure 2: 95% exclusion limits on chargino pair production. The grey shaded region shows the results from Run 1 of the LHC. The new results substantially extend previous limits. (Image: ATLAS Collaboration/CERN)

    A central challenge of these searches is that dark matter candidate particles would escape the ATLAS detector without leaving a visible signal. Their presence can only be inferred through the magnitude of the collision’s missing transverse momentum (ETmiss) – an imbalance in the momenta of detected particles in the plane perpendicular to the colliding protons. In the dense environment of numerous overlapping LHC collisions it can be difficult to separate genuine ETmiss from fake ETmiss originating from mismeasurement of the visible collision debris in the detector.

    To resolve this difficulty, ATLAS developed a new ETmiss significance variable, which quantifies the likelihood that the observed ETmiss originates from undetectable particles rather than from mismeasured objects. Unlike previous calculations based entirely on the reconstructed event kinematics, the new variable also considers the resolution and misidentification probability of each of the reconstructed particles used in the calculation. This helps discriminate more effectively between events with genuine and fake ETmiss, respectively, as shown in Figure 1, thus improving ATLAS’ ability to identify and partially reconstruct dark matter particles.

    Applying new reconstruction techniques

    3
    Figure 3: Distribution of object-based significance discriminant in SRC (upper). The final state is also pictorially represented (lower). (Image: ATLAS Collaboration/CERN)

    Both of the new ATLAS searches implement this new reconstruction technique to the full Run 2 dataset. One search looks for the pair production of charginos (the charged superpartners of bosons) and sleptons (superpartners of leptons), respectively, which decay to either two electrons or muons and give rise to large ETmiss due to the escaping LSPs. These signals are very challenging to extract as they look similar to Standard Model diboson processes, where some (although less) ETmiss is produced from invisible neutrinos. Events were selected at high ETmiss significance together with several other variables that help discriminate signal from background. In absence of a significant excess in the data over the background expectation, strong limits were placed on the considered supersymmetric scenarios, as shown in Figure 2.

    The second new search targets the pair production of supersymmetric bottom squarks (superpartners of bottom quarks), which both decay to a final state involving a Higgs boson and an LSP (plus an additional b-quark). Then – targeting Higgs boson decays to two b-quarks, as it is predicted to occur 58% of the time – the final state measured in the ATLAS detector would have a unique signature: large ETmiss associated with up to six “jets” of hadronic particles, originating from b-quarks. The measured and expected ETmiss significance distribution and expected event topology are shown in Figure 3. Again, no significant excess in data was found in this search.

    Both results place strong constraints on important supersymmetric scenarios, which will guide future ATLAS searches. Further, they provide an example how novel reconstruction techniques can help improve the sensitivity of new physics searches at the LHC.

    See the full article here .


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  • richardmitnick 8:11 am on March 25, 2019 Permalink | Reply
    Tags: "Highlights from Moriond: ATLAS explores the full Run 2 dataset", , , CERN ATLAS, , , ,   

    From CERN ATLAS: “Highlights from Moriond: ATLAS explores the full Run 2 dataset” 

    CERN/ATLAS detector

    CERN ATLAS Higgs Event

    CERN ATLAS another view Image Claudia Marcelloni ATLAS CERN


    CERN ATLAS Credit CERN SCIENCE PHOTO LIBRARY

    From CERN ATLAS

    23rd March 2019
    Pierre Savard

    1
    Figure 1: The highest-mass dijet event measured by ATLAS (mass = 8.12 TeV). (Image: ATLAS Collaboration/CERN)

    This week, particle physicists from around the world gathered in La Thuile, Italy, for the annual Rencontres de Moriond conference on Electroweak Interactions and Unified Theories. It was one of the first major conferences to be held following the recent completion of the Large Hadron Collider’s (LHC) second operation period (Run 2). The ATLAS Collaboration unveiled a wide range of new results, including new analyses using the full Run 2 dataset, as well as some high-profile studies of Higgs, electroweak and heavy-ion physics.

    2
    Figure 2: The invariant mass spectrum of two electrons compared with the Standard Model prediction, and with putative signals from a Z’ boson. (Image: ATLAS Collaboration/CERN)

    First search results using the full Run 2 dataset

    Over the course of Run 2 of the LHC – from 2015 to 2018 – the ATLAS experiment collected 139 inverse femtobarn of proton-proton collision data for analysis. Though this data-taking period concluded just a few months ago, ATLAS physicists have already reported on a variety of new searches using the full Run 2 dataset. So far, all of these new searches are in agreement with the Standard Model expectation.

    The first of these analyses, released in a paper submitted to Physics Letters B, is a search for heavy neutral gauge bosons (denoted Z’) decaying into lepton pairs. The sensitivity of this analysis has increased significantly over the lifetime of the LHC, as seen in Figure 3. The new result sets exclusion limits on specific theoretical models up to a mass of 5.1 TeV.

    A similar search was also conducted by looking for new particles – or “resonances” – decaying to two jets of particles. These “dijet” searches reveal events with the highest energies observed at the LHC; an example of such an event can be seen in Figure 1. No evidence of significant resonant structures was observed in the mass spectrum probed with the Run 2 dataset.

    ATLAS physicists also presented a search for new particles decaying to two weak bosons (W of Z), where the weak bosons then decay to two jets each. As the resonance would be very heavy, the weak bosons produced would be highly energetic and would generate overlapping jets as they decay. Thus, identifying the weak bosons is particularly challenging, and required the development of new reconstruction and analysis techniques. These have substantially improved the sensitivity of the analysis, as illustrated in Figure 4, setting significantly improved constraints on the allowed parameter space for such heavy resonances decaying to W or Z bosons.

    New searches for supersymmetry were also presented. One analysis focused on electroweak production of supersymmetric particles called “charginos” and “sleptons” decaying into two electrons or muons, along with missing transverse momentum. A second analysis looked for long-lived supersymmetric partners of the top quark that decay further away from the collision point. Many other searches are ongoing at ATLAS that will probe vast regions of yet-unexplored supersymmetric parameter space.

    3
    Figure 3: Ratio of the observed limit to the Z’ cross section for the combination of the channels with two electrons and two muons. (Image: ATLAS Collaboration/CERN)

    4
    Figure 4: Comparison between the current and previous expected limits on the cross section times branching ratio for WW+WZ production. An extrapolation of the expected limits from the previous results to the current dataset size, assuming no change to the previous analysis strategy or its uncertainties, is also shown. (Image: ATLAS Collaboration/CERN)

    First measurement of the Higgs boson using the full Run 2 dataset

    ATLAS also released a new measurement of the rare production cross section of the Higgs boson in association with two top quarks (ttH), followed by the similarly rare decay of the Higgs boson to two photons. The ttH production process was first observed in 2018, though it required the combination of many Higgs decay channels. Using the full Run 2 dataset, the observation of ttH in a single Higgs decay channel – into a pair of photons – is now possible. This allowed for a measurement of the production rate with an uncertainty of 25%, with a central value that is compatible with the Standard Model prediction.

    An updated combination of Higgs analyses was also presented, setting new constraints on the Higgs coupling to other particles, as well as interesting indirect constraints on the elusive self-coupling of the Higgs boson with itself. This update includes analyses that use 80 fb-1 (the data taken from 2015 to 2017) and represents the most comprehensive and precise set of Higgs properties measurements presented by the collaboration to date. Other results reporting first evidence for the rare electroweak processes involving the production of three weak bosons were also shown at the conference.

    Observation of light-by-light scattering

    The scattering of light by light involves two incoming photons scattering off of each other and producing two outgoing photons. This is a purely quantum mechanical effect that is not predicted by the classical theory of electromagnetism. The scattering of light requires a very intense source of photons, which can be achieved by using the enormous electric fields generated by fully ionised lead ions. As the ions cross each other, the intense electric fields supply a beam of photons that can collide, effectively turning the Large Hadron Collider into a “Large Photon Collider”.

    Evidence for this process at the LHC was first reported by ATLAS in 2017 in Nature Physics, and was also seen by CMS. Using the much larger dataset collected in 2018, ATLAS was able to clearly observe this process with a significance of over 8 standard deviations and measure the cross section with an uncertainty of 19%. This was one of the first results presented at Moriond.

    New measurement of CP violation

    5
    Figure 5: The measured values of φs and ∆Γs, compared to measurements by other LHC experiments. (Image: ATLAS Collaboration/CERN)

    The observed asymmetry between matter and antimatter in the Universe (a symmetry breaking known as “CP violation”) is one the unresolved puzzles in particle physics. As the Standard Model is only able to explain part of this asymmetry, there is great motivation to search for additional sources in the form of new or larger CP violation phases. The LHC produces copious samples of B mesons that are used to measure CP violating processes. In a new analysis using 80 fb-1 of data, ATLAS investigated the decay of B-sub-s (Bs) mesons, which are composed of a bottom quark and a strange quark. Specifically, J/ψ φ decays were investigated to measure the CP-violating phase φs, the average decay width (Γs), and the width difference (∆Γs) between the physical Bs meson states.

    In the Standard Model, φs is predicted to be small. However, physics beyond the Standard Model could increase the size of the observed CP violation by enhancing the mixing phase φs with respect to the Standard Model value. The measured values of φs and ∆Γs are shown in Figure 5, and compared to measurements by other LHC experiments and to the Standard Model prediction.

    A week of rich and exciting results

    This week, ATLAS and other LHC experiments presented important new results, deepening our understanding of particle physics. The presentation of the first results with the full Run 2 dataset represent the first steps in the realisation of what will be a rich and exciting Run 2 physics programme. Though Moriond EW is now drawing to a close, the Moriond QCD conference will start on its heels on Sunday 24 March – expect more exciting new results.

    See the full article here .


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  • richardmitnick 12:25 pm on March 17, 2019 Permalink | Reply
    Tags: "ATLAS observes light scattering off light", , , CERN ATLAS, Heavy Ion, , HION group, , ,   

    From CERN ATLAS: “ATLAS observes light scattering off light” 

    CERN/ATLAS detector

    CERN ATLAS Higgs Event

    CERN ATLAS another view Image Claudia Marcelloni ATLAS CERN


    CERN ATLAS New II Credit CERN SCIENCE PHOTO LIBRARY


    From CERN ATLAS

    17th March 2019
    ATLAS Collaboration

    New result studies photons interacting at high energies.

    1
    Figure 1: ATLAS event display showing the energy deposits of two photons in the electromagnetic calorimeter (green) on opposite sides and no other activity in the detector, which is the clean signature of light-by-light scattering. The Feynman diagram of this process is shown in the lower right corner. (Image: ATLAS Collaboration/CERN)

    Light-by-light scattering is a very rare phenomenon in which two photons – particles of light – interact, producing again a pair of photons. This process was among the earliest predictions of quantum electrodynamics (QED), the quantum theory of electromagnetism, and is forbidden in classical physics (such as Maxwell’s theory of electrodynamics).

    Direct evidence for light-by-light scattering at high energy had proven elusive for decades, until the Large Hadron Collider (LHC) began its second data-taking period (Run 2). Collisions of lead ions in the LHC provide a uniquely clean environment to study light-by-light scattering. The bunches of lead ions that are accelerated to very high energy are surrounded by an enormous flux of photons. Indeed, the coherent action from the large number of 82 protons in a lead atom with all the electrons stripped off (as is the case for the lead ions in the LHC) give rise to an electromagnetic field of up to 1025 Volt per metre. When two lead ions pass close by each other at the centre of the ATLAS detector, but with a distance greater than twice the lead ion radius, those photons can still interact and scatter off one another without any further interaction between the lead ions, as the reach of the (much stronger) strong force is bound to the radius of a single proton. These interactions are known as ultra-peripheral collisions.

    In a result published in Nature Physics in 2017, the ATLAS Collaboration found thirteen candidate events for light-by-light scattering in lead-lead collision data recorded in 2015, for 2.6 events expected from background processes. The corresponding significance of this result was 4.4 standard deviations – making it the first direct evidence of high-energy light-by-light scattering.

    2
    Figure 2: Invariant mass distribution of the measured final state photon pairs (markers), compared to the expected light-by-light scattering signal (red line) and expected background contributions (shaded areas). (Image: ATLAS Collaboration/CERN)

    Today, at the Rencontres de Moriond conference (La Thuile, Italy), the ATLAS Collaboration reported the observation of light-by-light scattering with a significance of 8.2 standard deviations. The result utilises data from the most recent heavy-ion operation of the LHC, which took place in November 2018. About 3.6 times more events (1.73 nb−1) were collected compared to 2015. The increased dataset, in combination with improved analysis techniques, allowed the measurement of the scattering of light-by-light with greatly improved precision. A total of 59 candidate events were observed (see Figure 2), for 12 events expected from background processes. From these numbers, the cross section of this process, restricted to the kinematic region considered in the analysis, was calculated as 78 ± 15 nb.

    Curiously, the signature of this process – two photons in an otherwise empty detector (see the event display in Figure 1) – is almost the opposite of the tremendously rich and complex events typically observed in high-energy collisions of two lead nuclei. Observing it required the development of improved trigger algorithms for fast online event selection, as well as a specifically-adjusted photon-identification algorithm using a neural network, as the studied photons have about ten times less energy than the lowest energetic photons usually measured with the ATLAS detector. Being able to record these events demonstrates the power and flexibility of the ATLAS detector and its event reconstruction, which was designed for very different event topologies.

    This new measurement opens the door to further study the light-by-light scattering process, which is not only interesting in itself as a manifestation of an extremely rare QED phenomenon, but may be sensitive to contributions from particles beyond the Standard Model. It allows for a new generation of searches for hypothetical light and neutral particles.

    See the full article here .


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  • richardmitnick 11:57 am on March 17, 2019 Permalink | Reply
    Tags: "ATLAS finds evidence of three massive vector boson production", , , CERN ATLAS, , , ,   

    From CERN ATLAS: “ATLAS finds evidence of three massive vector boson production” 

    CERN/ATLAS detector

    CERN ATLAS Higgs Event

    CERN ATLAS another view Image Claudia Marcelloni ATLAS CERN

    CERN ATLAS New II Credit CERN SCIENCE PHOTO LIBRARY


    From CERN ATLAS

    17th March 2019
    ATLAS Collaboration

    Today, at the Rencontres de Moriond conference (La Thuile, Italy), the ATLAS collaboration released evidence for the simultaneous production of three W or Z bosons in proton–proton collisions at the Large Hadron Collider (LHC). The W and Z bosons are the mediator particles of the weak force – one of the four known fundamental forces – which is responsible for the phenomenon of radioactivity as well as an essential ingredient to our Sun’s thermonuclear process.

    A new window for exploration

    The new ATLAS result is based on data collected by ATLAS during 2015–2017 at a collision energy of 13 TeV. It provides evidence of “tri-boson” events with a significance of 4 standard deviations. This indication is but the latest chapter in a decades-long history of measurements with weak bosons. The W and the Z bosons were discovered in 1983 at CERN’s proton-antiproton collider.

    CERN Proton-Antiproton Collider

    In 1996, at CERN’s Large Electron-Positron (LEP) collider, events with two W bosons were first observed, and shortly thereafter ZZ events were found.

    CERN LEP Collider


    CERN LEP Collider

    A decade after that, WW, WZ and ZZ events were observed at Fermilab’s Tevatron collider.

    FNAL/Tevatron map


    FNAL/Tevatron

    Large rates of diboson events are now produced at the LHC, allowing for precise measurements.

    Tri-boson production are rare processes predicted by the Standard Model of particle physics.

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

    Their production involves self-interaction among the weak bosons, so-called triple and quartic gauge boson couplings, which are sensitive to possible contributions from yet unknown particles or forces.

    1
    Figure 1: Data compared to expectation for the distribution of the invariant mass of two jets. The signal (VVV, in yellow) is scaled to the measured value. (Image: ATLAS Collaboration/CERN)

    2
    Figure 2: Data compared to expectation for the distribution of the BDT in one of the WVZ channels. The signal (VVV, in yellow) is scaled to the measured value. (Image: ATLAS Collaboration/CERN)

    A result years in the making

    3
    Figure 3: Combination of two measurements (normalised to their Standard Model predictions) in the WWW and two in the WVZ channels, in final states with a different number of leptons. (Image: ATLAS Collaboration/CERN)

    Since weak bosons are unstable, they are reconstructed in the detector via their decays to pairs of leptons (including invisible neutrinos) or quarks – the latter forming sprays of particles, called “jets”. ATLAS physicists combined searches for different decay modes and different types of tri-boson production, including events with three W bosons (“WWW”), and events with one W boson, one Z boson and a third boson of either variety. The latter are known as “WVZ” events, where the “V” is a shorthand for “W or Z”.

    One technique employed by ATLAS physicists to search for “WWW” events used the calculated invariant mass of two jets and compared this to the mass of the W boson (Figure 1). This allowed them to determine whether the jets were the outcome of a W boson decay. Such techniques have been used by physicists for decades (including in the 2012 discovery of the Higgs boson).

    The WVZ analysis, on the other hand, employs machine learning techniques to identify tri-boson events. Several multivariate algorithms in form of boosted decision trees (BDTs) were trained to learn which events in data are from tri-boson production and which arise from other Standard Model processes (Figure 2). By considering various features of the event – such as the momenta of the leptons, the overall momentum imbalance and the number of jets – the BDTs are able to deduce (more efficiently than humans) the origin of the data. Ultimately, the BDTs identified some of the data as likely originating from WVZ production.

    Altogether, the resulting ATLAS measurement (Figure 3) is found to be in agreement with the Standard Model prediction, thus providing one more piece of the puzzle in our understanding of particle physics.

    See the full article here .


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  • richardmitnick 5:12 pm on March 5, 2019 Permalink | Reply
    Tags: As one of the main outstanding questions in fundamental physics the identification of the nature of dark matter is a key scientific driver for the future of particle physics., Because of gravitational lensing an effect related to Einstein’s general theory of relativity matter that stands between a light source and its observer can bend the light from the source so that th, CERN ATLAS, , From comparing the known position of the source (e.g. obtained through direct emission of visible particles from the source) to its distorted image one can reconstruct the distribution of the matter c, Invisible particles can be detected in ATLAS as they recoil against the visible ones (in this case the jet of particles), It is through the gravitational effect of dark matter on other matter in space that astronomers inferred its existence, Many astronomers had been observing the motion of galaxies and found a discrepancy with respect to their expectation that only accounted for matter that was emitting light. This was corroborated in th, More recently supercomputer simulations of the structure of our universe show that only including visible matter will not reproduce the structures that are observed in the universe while if dark matte, The first evidence for the existence of dark matter came as early as the 1930s, The most popular example of a more complete theory that includes a dark matter candidate is supersymmetry (SUSY), The presence of dark matter and its amount in the universe can also be inferred from the variations of temperature in the early universe. This leftover amount of dark matter is called its “relic den, Using WIMP models as our starting point for LHC searches doesn’t mean that we are bound to the idea that dark matter should be described with a single particle and a single interaction!   

    From CERN ATLAS: “Searching for Dark Matter with the ATLAS detector” 

    CERN/ATLAS detector

    CERN ATLAS Higgs Event

    From CERN ATLAS

    5th March 2019
    Caterina Doglioni
    Dan Tovey

    1
    Figure 1: An event with a highly energetic jet of particles and no other significant visible energy (monojet) recorded in 2016 by the ATLAS detector. This is how invisible particles can be detected in ATLAS, as they recoil against the visible ones (in this case, the jet of particles). The direction of the invisible particle is indicated by the dashed line. (Image: ATLAS Collaboration/CERN)

    When we look around us, at all the things we can touch and see – all of this is visible matter. And yet, this makes up less than 5% of the universe.

    We now know that the vast majority of matter is dark. This dark matter does not emit or reflect light, nor have we yet observed any known particle interacting with it. It is through the gravitational effect of dark matter on other matter in space that astronomers inferred its existence.

    The first evidence for the existence of dark matter came as early as the 1930s[1].

    Fritz Zwicky discovered Dark Matter when observing the movement of the Coma Cluster.

    Fritz Zwicky from http:// palomarskies.blogspot.com

    Coma cluster via NASA/ESA Hubble

    But most of the real work was done by Vera Rubin a Woman in STEM

    Astronomer Vera Rubin at the Lowell Observatory in 1965, worked on Dark Matter (The Carnegie Institution for Science)


    Vera Rubin measuring spectra, worked on Dark Matter (Emilio Segre Visual Archives AIP SPL)


    Vera Rubin, with Department of Terrestrial Magnetism (DTM) image tube spectrograph attached to the Kitt Peak 84-inch telescope, 1970. https://home.dtm.ciw.edu

    Many astronomers had been observing the motion of galaxies, and found a discrepancy with respect to their expectation that only accounted for matter that was emitting light. This was corroborated in the 70s through observations of the rotational velocity of galaxies made by Vera Rubin and collaborators.

    2
    Figure 2: Percentage of ordinary matter, dark matter and dark energy in the universe, as measured by the Planck satellite. (Image: E. Ward/ATLAS Collaboration, Credit: ESA and the Planck Collaboration)

    Because of gravitational lensing, an effect related to Einstein’s general theory of relativity, matter that stands between a light source and its observer can bend the light from the source so that the observed image is distorted.

    Gravitational Lensing NASA/ESA

    From comparing the known position of the source (e.g. obtained through direct emission of visible particles from the source) to its distorted image, one can reconstruct the distribution of the matter causing the distortion. Observations of gravitational lensing also pointed to additional matter with respect to what was visible.

    More recently, supercomputer simulations of the structure of our universe show that only including visible matter will not reproduce the structures that are observed in the universe, while if dark matter is included then a closer agreement is obtained between observations and simulations.

    The presence of dark matter and its amount in the universe can also be inferred from the variations of temperature in the early universe. This leftover amount of dark matter is called its “relic density”, and it amounts to about 27% of the matter-energy content of the universe.

    However, none of the observations or simulations involving dark matter give a clear indication of what dark matter is made of. We only know that if dark matter is a particle[2], then it must have mass, since it interacts with other matter through the force of gravity. We can hope to understand its nature by observing rare dark matter particles and their interactions from space (where we have already seen its effects), and by trying to produce them in controlled laboratory conditions.

    How particle collisions can create dark matter in a lab

    Experiments at particle accelerators have revealed much about the nature of visible (ordinary) matter, starting from the first prototypes that aided the discovery of the proton and the antiproton to the recent discovery of the Higgs boson. All of the particles observed so far are part of the Standard Model of Particle Physics, describing the fundamental components of matter and their non-gravitational interactions.

    Standard Model of Particle Physics


    Standard Model of Particle Physics from Symmetry Magazine

    The most powerful accelerator ever built is the Large Hadron Collider (LHC) at CERN in Geneva, accelerating protons and colliding them with a total energy of 13 TeV. According to Einstein’s most famous equation, E=mc2, the more energy (E) the more massive particles (with a mass m) one can create (13 TeV corresponds to roughly 14 thousand times the rest mass of a proton). The hope is that at the LHC we can create massive dark matter particles by colliding known particles, in the same way we create the Higgs boson in proton-proton collisions.

    Particles are regularly accelerated to very high energies in the universe in “natural” particle accelerators, such as supernovae explosions, and then collide with other particles in our atmosphere. Cosmic rays, for example, are particles that are generated in outer space and make it to Earth. However, the advantage of laboratory particle accelerators such as the LHC is that there we know the initial conditions of the collisions – namely the type and energy of the particles being collided. We can also create a large (and known) number of collisions and observe them in a controlled environment. These are essential features for detecting dark matter particles at experiments like ATLAS.

    Characteristics of dark matter and consequences for detector signatures

    Since dark matter is dark, it will not interact significantly with instruments made of ordinary matter. For this reason, the underlying signature of dark matter production at the LHC, used by all ATLAS searches, is the presence of invisible particles in proton-proton collisions.

    One might reasonably ask how invisible particles can be observed, since they are by definition undetectable! We solve this problem with a little ingenuity. Before each collision, the protons travel along the direction of the LHC beams, and not in directions perpendicular to the beams. This means that their momenta in these perpendicular directions – their “transverse momentum” – is zero. A fundamental principle of physics is that momentum is conserved and so, after the collision, the sum of the transverse momenta of the products of the collision should still be zero. Therefore, if we add up the transverse momenta of all the visible particles produced in the collision and find it not to be zero, then this could be because we have missed the momentum carried away by invisible particles.

    3
    Figure 3: Diagram showing how missing transverse momentum (ETmiss) is determined in the transverse cross-section of a LHC detector. The LHC beams are entering/exiting through the plane. (Image: C. Doglioni, L.T. Wang & E. Ward/ATLAS Collaboration)

    This happens routinely in ATLAS, in the case of physics processes involving neutrinos. We refer to this missed transverse momentum as “ETmiss”. LHC searches for dark matter look for collisions with large values of ETmiss, where the dark matter is produced in association with other, visible particles from the Standard Model, such as photons, quarks or gluons (forming “jets” of particles), or electrons, muons or tau leptons. While ETmiss can be difficult to measure because it relies on accurate measurements of all the other particles in the collision, it is a powerful tool for observing dark matter.

    A further requirement for the identification of dark matter particles in collisions is that the invisible particles should not decay as they travel through the ATLAS detector. In order for an invisible particle to be a candidate for the “relic” dark matter produced in the Big Bang, it should have a lifetime of at least the age of the universe – of the order of 14 billion years. Particles created in LHC collisions take about 40 nanoseconds to cross the ATLAS detector, so requiring that their lifetime be longer than this is not enough, on its own, to prove they constitute the dark matter. Complementary information from astroparticle experiments searching for relic dark matter would be required. However, it is a very good start!

    It is worth noting that other particles that are connected to dark matter might also be detected at the LHC, for example new short-lived particles that can decay both into dark matter and into known matter. Observing those would be an important complement to an observation of dark matter particles from space, as it would allow us to better understand the landscape of dark matter interactions.

    What could dark matter be? Theoretical hypotheses

    Experimentally, there are very few indications of what dark matter might be. We can, however, make theoretical hypotheses on the nature of dark matter, which are useful to experimentalists. The theorist and experimentalist communities often collaborate, for example within the LHC Dark Matter Working Group[3]. Theoretical models of dark matter can tell us more about how the interaction of dark matter with ordinary matter may take place. From that, we can predict what to expect in our detectors if that model were realised in nature. This is relevant for designing detectors sensitive to dark matter, and for deciding how to analyse the products of the collisions once they have been recorded. It is also useful to know what to look for, as we have to decide in real-time which collisions to save data from (this is done using the ATLAS trigger system). A solid theoretical framework for dark matter is also necessary to put LHC searches into context and to compare them with dark matter searches from other instruments.

    Searches for dark matter at the LHC are commonly guided by theoretical models that would allow us to explain the relic density of dark matter with one or a few kinds of particles. A class of models that satisfies these requirements includes a dark matter particle that only interacts weakly with ordinary particles and has a mass within the energy range that can be probed at the LHC – a Weakly Interacting Massive Particle (WIMP).

    Using WIMP models as our starting point for LHC searches doesn’t mean that we are bound to the idea that dark matter should be described with a single particle and a single interaction! This is especially important when you consider that the content of dark matter in the universe is five times the content of ordinary matter, and ordinary matter is described by a variety of different particles and interactions. At the LHC, we have begun our tour into possible theoretical models of dark matter[4] hoping that the few most prominent components and interactions of dark matter will be detected first, just as the electron, proton and electromagnetic interaction were discovered before all other particles of the Standard Model.

    4
    Figure 4: Key particle discoveries from 1898 to today! (Image: E. Ward/ATLAS Collaboration)

    The simplest models one can build in terms of particle content are those where the dark matter particle is added to the Standard Model. In these models, the interaction between visible and dark matter must proceed through existing particles, such as the Z or Higgs boson. This means that the Z or Higgs boson could decay into two dark matter particles[5], in addition to their ordinary decay modes involving Standard Model particles.

    These models are called “portal” models of dark matter, as known particles act as the portal between what we know (ordinary matter) and what we don’t know (dark matter). While models with a Z boson portal are fairly constrained by precision measurements, including those done at the LEP collider at CERN during the 1990s, now is the first time in the history of particles that we can study the properties of the Higgs boson in detail. We could discover whether one or more of those properties lead to a connection to dark matter.

    In addition to dark matter, one can also conceive of another particle not included in the Standard Model that acts as a portal particle. These are called “mediator” particles, since they mediate a new interaction between ordinary matter and dark matter. In the simplest versions of these models, the mediator is an unstable heavy particle that is produced directly from the interaction of Standard Model particles, such as quarks at the LHC. Therefore, it must also be able to decay into those same particles, or into a pair of dark matter particles. If a model of this kind occurs in nature, we have a chance to directly discover this mediator particle at the LHC, as we would be able to detect its Standard Model decay products. Other simple models don’t have a mediator that can also decay to Standard Model particles, but instead foresee the production of dark matter particles in association with Standard Model particles that can aid the detection of the process over known backgrounds.

    While these models are commonly used to interpret the results of many LHC searches in terms of dark matter, they are too simple to represent the full complexity of a dark matter theory. However, they are still useful as building blocks for more complete theories with more ingredients.

    The most popular example of a more complete theory that includes a dark matter candidate is supersymmetry (SUSY). SUSY was one of the first dark matter models to be studied extensively at the LHC. An appealing feature of supersymmetry is that it also solves a stability problem of the relatively low mass of the Higgs boson and other electroweak particles of the Standard Model (around 100 GeV) compared to the Planck scale (10^19 GeV), at which gravity is expected to become strong and the Standard Model must break down. Quantum field theories like the Standard Model naturally prevent such large differences in energy scale from developing, so a physical mechanism is required to generate them. SUSY models provide such a mechanism and, in many cases, predict the existence of a new stable, invisible particle – the lightest supersymmetric particle (LSP) – which has exactly the right properties to be a WIMP dark matter particle. The search for particles predicted by SUSY is a major focus of the ATLAS physics programme. If produced in LHC collisions, these particles could decay to produce a variety of Standard Model particles that can be observed in the ATLAS detector, together with two escaping LSP dark matter particles that generate the characteristic ETmiss signature discussed above.

    Many other theories, of various degrees of completeness and complexity, contain dark matter particle candidates. Some of them predict new particles similar to the Higgs boson that can decay into dark matter, while others go beyond the WIMP paradigm and include mediators with extremely feeble interactions with known particles that only decay after traveling significant distances inside (or outside!) the detector, or more complex sectors of particles mirroring the Standard Model[6]. It is important for LHC searches to cover all this ground, while also preparing for unexpected, not-yet-theorised discoveries. No stone must be left unturned!

    Experimental techniques and results

    ATLAS already measures many processes involving invisible particles, namely neutrinos from the Standard Model. Fig. 5 shows the results of the measurement of the number of Z bosons decaying into a pair of neutrinos (about one fifth of all Z boson decays). As shown in the diagram in Fig. 6, we use a visible object (in this case a photon) to detect the presence of invisible particles and measure their missing transverse energy, as explained in the previous section.

    6
    Figure 6: Diagram of a Z boson decaying into a neutrino-antineutrino pair where the Z boson is produced in association with a photon. (Image: ATLAS Collaboration/CERN)

    7
    Figure 7: Diagram of a new mediator particle decaying into a pair of dark matter particles, produced in association with a photon. (Image: ATLAS Collaboration/CERN)

    A very similar technique can be used for detecting the presence of dark matter particles. If we take the process in Fig. 6, replace the neutrinos with dark matter particles, replace the Z boson with a generic mediator between ordinary matter and dark matter, then we have the diagram in Fig. 7.

    The detector signature of the processes shown in Fig. 6 and Fig. 7 is identical (and is shown in the event display in Fig. 8). Since we cannot distinguish the processes on a collision-by-collision basis, we have to take a different approach. We start by collecting a large number of events that have a large amount of missing transverse momentum and a highly energetic object. Then, we estimate precisely the number of expected events from Standard Model processes (called “backgrounds”), and look for an excess of additional events that could be due to dark matter processes. This kind of search is called “ETmiss+X”, where X stands for what the dark matter recoils against[7].

    So far, we have not found any excess with respect to backgrounds in this kind of search,as shown in Fig. 12 where the data agrees with the Standard Model-only prediction. Still, the journey of ETmiss+X searches at the LHC is far from over. Adding data and improving the experimental precision of future searches will enable us to search for even weaker dark matter interactions yielding processes that are still rarer than those to which we are already sensitive.

    8
    Figure 8: A visualisation of a photon and ETmiss event recorded in 2016, is shown in the ATLAS detector. A photon with transverse momentum of 265 GeV (yellow bar) is balanced by a ETmiss of 268 GeV (red dashed line in the opposite side of the detector). (Image: ATLAS Collaboration/CERN)

    The advantage of this kind of search is that it makes no specific assumption about the nature of the invisible particles, other than that they are produced in association with a Standard Model particle. It is therefore well-suited to cast a wide net on a variety of dark matter models, as long as the model’s signature includes invisible particles and includes dark matter–Standard Model interactions. Conversely, the very large Standard Model backgrounds in ETmiss+X searches can be reduced by giving up some of their generality, for example by requiring distinctive particles (e.g. top quarks, the Higgs boson or related particles) to be produced in association with the dark matter.

    The mediator particle can also decay to visible particles, leading to a peak or “resonance” in the total mass of those particles. Searches for new particles using resonances in the total mass of visible particles have led to numerous discoveries at colliders, including, most recently, the Higgs boson at the LHC. Given that the LHC is the highest-energy laboratory particle collider, the most obvious goal is to search for extremely massive particles that could not have been produced before.

    Still, dark matter mediators could also appear at lower masses, escaping detection because of very low couplings to protons. This is a region where it has been increasingly difficult to perform searches due to the overwhelming Standard Model backgrounds that exceed the experiment’s data capacity if recorded in their entirety. Since background events are indistinguishable from events coming from decays of dark matter mediators, there is a risk of discarding both. Being able to detect this kind of process has provided motivation for overcoming technical limitations. All the main LHC experiments now employ data-taking techniques that allow them to retain a smaller amount of information for some events, so that more events can be recorded[8]. These searches have not yet yielded any new particles, but improvements to the data selection and data acquisition system may bring surprises for the next LHC run.

    The results of searches for invisible and visible dark matter-mediator decays bring complementary information on different parameters of dark matter models. Together, they could help to characterise the nature of a discovery. We must keep in mind, though, that these searches are interpreted in terms of the processes shown in Fig. 7, which stem from a very simple theoretical model. In this model, the only two new particles are the dark matter and the mediator of the interaction, and that may not describe the full complexity of the unknown matter in the universe.

    This is why ATLAS searches target many other experimental signatures in addition to MET-X and resonance searches. For example, models including putative new Higgs bosons yield an assortment of detector signals that can be targeted by different searches. These results can be compared to see whether there are regions in the model parameter space where we haven’t yet looked and, in some cases, they can be combined to strengthen the discovery potential or constraints on dark matter models. A comprehensive summary of these kinds of searches for dark matter, as well as their connection to astrophysical searches (described in the next section), can be found in a new ATLAS paper published today (arXiv: 1903.01400).

    Compared with ETmiss+X searches, detector signatures from SUSY scenarios offer the possibility to make use of some additional tricks to identify a dark matter signal from the Standard Model background.

    In many models, SUSY particles are produced in pairs due to a requirement to conserve a quantity called “R-parity”[9] (sometimes also denoted “matter-parity”). Whenever a SUSY particle decays, the resulting decay products must include exactly one lighter SUSY particle. The decay chain ends when the lightest SUSY particle, which is a candidate dark matter particle, is produced.

    In contrast to many non-SUSY dark matter models, SUSY particle decays can generate many visible Standard Model particles of high energy. Hence, events containing SUSY particles can be identified by requiring these particles as well as missing transverse momentum. A further trick is to make use of constraints on the momenta of the visible particles produced in the SUSY decays coming from the high masses of their SUSY particle parents. In particular, when two visible particles are produced from two identical decay chains in a SUSY event, we can measure properties of the event which can take on much larger values than those expected in Standard Model background events. An example is shown in Fig. 10.

    9
    Figure 9: Missing transverse momentum distribution in data after selecting events with an energetic photon and ETmiss, compared to the Standard Model predictions. The different background processes are shown in different colours. The expected spectra of an example WIMP dark matter scenario is illustrated with red dashed lines. (Image: ATLAS Collaboration/CERN)

    10
    Figure 10: Distribution in data of a quantity sensitive to the production of pairs of SUSY particles whose decays include dark matter particles, after selecting events with two electrons or muons and ETmiss, compared to the Standard Model predictions. The different background processes are shown in different colours. The expected spectra of example SUSY dark matter scenarios are illustrated with blue and green dashed lines. (Image: ATLAS Collaboration/CERN)

    With the help of these tools, SUSY searches are able to set tight requirements for events with a given set of characteristics, targeting specific models. This makes them less general than ETmiss+X searches, but also less impacted by large numbers of background events.

    ATLAS has not yet found evidence of SUSY LSPs, and has strongly constrained many of the models that would simultaneously solve the dark matter puzzle and provide an explanation for the low mass of the Higgs boson. Nevertheless, many SUSY variants remain interesting and the search isn’t over, as described in the dedicated feature article.

    Many other searches for particles from more complex dark matter theories, e.g. those in footnote 7, are also performed in ATLAS even though we don’t cover them in detail in this article. Some of the characteristics of these particles make them behave very differently compared with the particles the LHC was built to observe. Therefore, searching for these (still well-motivated) variants of dark matter is generally more challenging and requires dedicated techniques to identify and reconstruct candidate particles that would hint at the presence of dark matter. These searches are now at the forefront of the ATLAS and LHC quest for dark matter, and have gathered at least as much interest as searches for WIMPs and their associated particles.

    Connecting collider searches to astrophysical searches

    Searches for dark matter at the LHC are typically searches for the production, rather than the interaction or annihilation, of potential dark matter particles. As such, data from ATLAS would not provide proof that a new particle constitutes the dark matter – the sensitivity to dark matter lifetimes is just too short (see above). Nevertheless, ATLAS data could establish consistency with the predictions of dark matter models, and within those models ATLAS can provide complementary information to the broad range of astroparticle searches for the interaction of relic dark matter particles being carried out around the world. This complementarity can be illustrated taking, for example, the simple dark matter-mediator model.

    11
    Figure 11. Diagram showing dark matter (DM) interactions and their corresponding experimental detection techniques, with time going from left to right. (a) shows DM annihilation to Standard Model (SM) particles, as sought by Indirect Detection (ID) experiments. (b) shows DM -> SM particle scattering, targeted by Direct Detection (DD) experiments. (c) shows the production of DM particles from the annihilation of SM particles at colliders. (d) again shows the pair production of DM at colliders, but in this case the interaction occurs through a mediator particle between DM and SM particles. (Image: C. Doglioni & A. Boveia/ATLAS Collaboration)

    Within this model, in order for dark matter particles to be produced in pairs at the LHC, two strongly interacting quarks or gluons from the colliding protons must interact to produce the two dark matter particles (Fig. 11(b)). These same interactions could enable relic dark matter particles trapped in the Milky Way galaxy to scatter off atomic nuclei on Earth, generating the nuclear recoil signature exploited by “direct” astroparticle searches for dark matter such as XENON in Europe, LUX in North America and PANDA-X in China. Constraints from ATLAS searches can therefore be translated, albeit with assumptions on the mediator–proton and mediator–dark matter interaction, into constraints on the possible signals in those experiments (Fig. 12).

    12
    Figure 12: A comparison of the inferred limits from ATLAS data, including those from both ETmiss+X and mediator resonance searches, to the constraints from direct detection experiments on the WIMP-proton scattering cross section in the context of a model with a new vector particle mediating the Standard Model-dark matter interaction, fixing the given mediator / quarks (gq) and mediator / dark matter (gDM) couplings to the value in the plot. (Image: ATLAS Collaboration/CERN)

    Furthermore, the same interactions also enable relic dark matter particles produced in the early universe to annihilate and create Standard Model particles (Fig. 11(a)). This leads to the signatures for dark matter sought by “indirect” dark matter search experiments – typically high-energy photons (observed by telescopes such as HESS, MAGIC and VERITAS), neutrinos (observed by neutrino telescopes such as IceCube) or anti-particles (detected by space experiments such as AMS on the International Space Station). Results from collider searches can therefore also be compared with results from those experiments.

    The complementarity between recent ATLAS searches and astroparticle searches for dark matter is illustrated by Fig. 12, for the case of the simple dark matter-mediator model.

    When interpreting and combining ATLAS results and those from astroparticle dark matter searches, we need to consider whether the dark matter model being tested is consistent with the observed density of relic dark matter particles. This has been measured with a precision better than 1% through observations of the cosmic microwave background [CMB] by satellites like Planck. When considering a particular dark matter model, this only sets an upper limit on the amount of dark matter the model should produce. This is because, in principle, the dark matter could consist of multiple types of particles, with any one type only contributing a fraction of the amount measured by Planck.

    The relic dark matter density constraint is particularly important for SUSY dark matter models, where the models can often predict more dark matter than the Planck satellite observed. Special characteristics of the model, such as closely-spaced SUSY particle masses or increased dark matter interactions, can reduce this density to values consistent with Planck observations, and searches for models with these characteristics are a high priority for ATLAS.

    The relic dark matter density constraint is particularly important for SUSY dark matter models, where the models can often predict more dark matter than the Planck satellite observed. Special characteristics of the model, such as closely-spaced SUSY particle masses or increased dark matter interactions, can reduce this density to values consistent with Planck observations, and searches for models with these characteristics are a high priority for ATLAS.

    Outlook: where do we go from here?

    ATLAS is searching for dark matter at the LHC in synergy with other experimental collaborations, such as CMS and LHCb. LHC experiments have not yet discovered dark matter candidates from Run 1/2 data, but there is a large number of proton-proton collisions ahead. The upcoming LHC data-taking period (2021-2023, known as Run 3) is expected to more than double the current dataset, and the high-luminosity period beginning 2026 will deliver at least another factor of 10 more data. The experiments will be able to probe dark matter processes that are rarer and more challenging to reconstruct than the ones studied today. In view of the upcoming data-taking, experiments are also making use of more advanced data-collection and data-analysis techniques, such as machine learning[10].
    Direct and indirect searches for signals of the existing dark matter in our galactic neighbourhood are important complementary strategies to LHC searches, since astrophysical experiments are able to detect relic dark matter and they are necessary to confirm that a new invisible particle discovered at the LHC could make up dark matter. We will continue the dialogue with these experiments, exchanging scientific results and perspectives, share theoretical models, and extend the discussion to the broader astrophysics community.
    Other experiments can probe dark matter models to which the LHC experiments are not sensitive, for example models where the interactions between dark matter and ordinary matter are too feeble for dark matter to be produced in collisions of known particles. These experiments are being discussed in the Physics Beyond Colliders effort that recently started at CERN.

    As one of the main outstanding questions in fundamental physics, the identification of the nature of dark matter is a key scientific driver for the future of particle physics. For this reason dark matter searches are a main focus of the discussions, including both experimentalists and theorists, which have taken place in recent initiatives to draw up roadmaps for the future of the field. While the nature of dark matter is currently still unknown, it is clear that the quest to better understand it will be a highlight of humanity’s study of the fundamental constituents of the universe for many years to come.

    [1] For an exhaustive overview of the history of dark matter, with ideas on dark matter that date even further back in time, see Bertone and Hooper’s “A History of Dark Matter” (arXiv: 1605.04909), or Bertone, de Swart and van Dongen’s “How dark matter came to matter” (arXiv: 1703.00013).

    [2] This piece will not discuss the possibility that scientists haven’t understood all of the details of the structure of space-time, including how gravity acts. That hypothesis is discussed in more detail in this article and its references: “Shaking the dark matter paradigm” (Symmetry magazine, 2017).

    [3] For this reason, the community of theorists and experimentalists looking for dark matter at the LHC has joined forces, forming first the Dark Matter Forum and then the Dark Matter Working Group. The goal and results of those group are described here.

    [4] This article does not contain an exhaustive list of models. For a graduate-level lecture series on models of dark matter see, for example, the TASI “Lectures on Dark Matter Physics” by M. Lisanti (arXiv: 1603.03797).

    [5] If the dark matter mass is less than half of that of the Z or the Higgs boson.

    [6] For an introduction to these kind of models see, for example, “If You Can’t Find Dark Matter, Look First for a Dark Force” (Nautilus article, 2017), “Hunting for Dark Matter’s ‘Hidden Valley’” (BNL feature story, 2016), “Voyage into the dark sector” (Symmetry magazine, 2018) and “Long-lived physics” (CERN article, 2018).

    [7] For more information on the missing transverse momentum+jet search, see the 2017 ATLAS Physics Briefing “Chasing the Invisible”.

    [8] For more information on this kind of searches, see the 2018 ATLAS Physics Briefing “A new data-collection method for ATLAS aids in the hunt for new physics”.

    [9] R-parity ensures that in SUSY models protons, and hence all of the atoms in the universe, are unable to decay to other particles quickly by exchanging SUSY particles. In models without R-parity conservation, this can also be prevented. However introducing R-Parity is the simplest possibility.

    [10] For more information on ongoing efforts on Machine Learning, see the DarkMachines research collective. For general perspectives on data acquisition and collection see the HEP Software Foundation.

    See the full article here .


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  • richardmitnick 12:00 pm on February 28, 2019 Permalink | Reply
    Tags: "First ATLAS result with full Run 2 dataset: a search for new heavy particles", , CERN ATLAS, , , , ,   

    From CERN ATLAS: “First ATLAS result with full Run 2 dataset: a search for new heavy particles” 

    CERN/ATLAS detector

    CERN ATLAS Higgs Event

    CERN/ATLAS

    27th February 2019
    ATLAS Collaboration

    1
    Figure 1: Measured dielectron mass distribution for the data (black points), together with the total background fit result is shown (red continuous line), with various possible Z’ signal distributions overlaid (dashed red line). The sub-panel shows the significance of the deviation between the observed data and the background prediction in each bin of the distribution. (Image: ATLAS Collaboration/CERN).

    Could a Grand Unified Theory resolve the remaining mysteries of the Standard Model?

    Standard Model of Particle Physics


    Standard Model of Particle Physics from Symmetry Magazine

    If verified, it would provide an elegant description of the unification of Standard Model forces at very high energies, and might even explain the existence of dark matter and neutrino masses. ATLAS physicists are searching for evidence of new heavy particles predicted by such theories, including a neutral Z’ gauge boson.

    The ATLAS collaboration has today released its very first result utilising its entire LHC Run 2 dataset, collected between 2015 and 2018. This analysis searches for new heavy particles decaying into dilepton final states, where the leptons are either two electrons or two muons. This is one of the most sensitive decays to search for new physics, thanks to the ATLAS detector’s excellent energy and momentum resolution for leptons and the strong signal-to-background differentiation as a result of the simple two-lepton signature.

    The new ATLAS result also employs a novel data-driven approach for estimating the Standard Model background. While the previous analysis predominantly used simulations for the background prediction and was carried out with a fraction of the data, this new analysis takes advantage of the vast Run 2 dataset by fitting the observed data with a functional form motivated by and validated with our understanding of the Standard Model processes contributing to these events. If present, the new particles would appear as bumps on top of a smoothly falling background shape, making them straightforward to identify (see Figure 1). This is similar to one of the ways that the Higgs boson was discovered in 2012, through its decay to two photons.

    In addition to probing unexplored territory in the search for new physics, a great deal of work in this analysis has gone into understanding the ATLAS detector and collaborating with the various detector performance groups to improve the identification of very high-energy electrons and muons. This included accounting for the multiplicity of tracks in the inner part of the detector, as it continuously increased due to the rising average number of proton-proton collisions per bunch crossing during Run 2.

    No significant sign of new physics has been observed thus far. The result sets stringent constraints on the production rate of various types of hypothetical Z’ particles. As well as setting exclusion limits on specific theoretical models, the result has also been provided in a generic format that allows physicists to re-interpret the data under different theoretical assumptions. This study has deepened the exploration of physics at the energy frontier; ATLAS physicists are excited about further analysing the large Run 2 dataset.

    See the full article here .


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  • richardmitnick 2:53 pm on December 20, 2018 Permalink | Reply
    Tags: , CERN ATLAS, , , , , Preparing ATLAS for the future   

    From CERN ATLAS: “Preparing ATLAS for the future” 

    CERN ATLAS Higgs Event

    CERN/ATLAS
    From CERN ATLAS

    20th December 2018
    Katarina Anthony

    Long Shutdown 2 (LS2) of the Large Hadron Collider commenced last week, as the accelerator powered down and the entry to the ATLAS cavern opened wide. Over the next two years, teams from across the ATLAS Collaboration will be upgrading and consolidating their experiment. On the agenda: the refurbishments of key electronics, the maintenance of various detector components and – critically – the installation of new detectors.

    “We are at an important moment for the ATLAS experiment,” says Karl Jakobs, Spokesperson of the ATLAS collaboration. “On the one hand, we continue to maintain and consolidate detector elements that have served us well since the beginning of data taking in 2009. On the other, we are installing new electronics, trigger and detector components as a first step to prepare ATLAS for the High-Luminosity LHC (HL-LHC) in 2026.”

    The HL-LHC will collide beams at up to seven times the luminosity for which the ATLAS detector was designed, resulting in about 200 simultaneous collisions per beam crossing. This will greatly increase ATLAS’ potential to spot new or rare physics processes – but necessitates the development and installation of new detectors with radiation-hard elements, finer granularity and faster readout. In preparation of this, ATLAS has published six Technical Design Reports and one Technical Proposal over 2017-2018, describing the new designs and technologies needed to handle HL-LHC data. While most of these new systems will be installed during the next long shutdown (LS3, scheduled for 2024) – some will already see service in Run 3!

    Key among these is the installation of ATLAS’ 10-metre diametre New Small Wheels (NSW) – the largest and most critical project to be carried out during LS2, and the first major HL-LHC addition to the detector. Teams are currently finalising the construction of the new wheels, the first of which will be installed in 2020.

    3
    The mechanical structure of the New Small Wheel (Image: CERN)

    Meet Jamie, a Mechanical Engineering Technician at CERN who’s working on the ATLAS experiment.

    “The New Small Wheels employ two detector technologies: small-strip Thin Gap Chambers (sTGC) and Micromegas. Both are able to withstand the higher flux of neutrons and photons expected in future LHC interactions, which will produce counting rates as high as 20,000 per second per square centimetre in the inner part of the NSW,” says Ludovico Pontecorvo, ATLAS Technical Coordinator. “Furthermore, these new technologies will greatly improve the ATLAS muon trigger capabilities, allowing for refined event selection.”

    Additional improvements to ATLAS’ muon system include 16 new chambers featuring Small Monitored Drift Tubes (sMDT) and Resistive Plate Chambers (RPCs) to be installed in the barrel of the experiment, thus improving the overall trigger coverage of the detector. The smaller diameter tubes of the sMDTs provide an order of magnitude higher rate capability.

    LS2 will also see the enhancement of the ATLAS Liquid Argon (LAr) calorimeter with new front-end electronics and optical-fibre cabling. This will greatly improve the resolution of the detector at trigger level, providing four-times higher granularity to allow “jets” of particles to be better differentiated from electrons and photons, thus refining the first decision level where collision events are accepted for offline storage or dismissed. ATLAS’ trigger and data-acquisition systems will also be upgraded during LS2 with new electronics boards, further improving the overall resolution of the experiment, and preparing for the HL-LHC.

    “The organisation of LS2 activities is particularly complex, as the maintenance needs of the detectors have to be combined with tight installation schedules,” says Pontecorvo. On the long list of planned maintenance tasks: the complete overhaul of the Tile Calorimeter’s cooling connectors; repairs to the Transition Radiation Tracker (TRT), LAr and RPC detectors; the completion of the ATLAS Forward Proton (AFP) detector; and preparations for a new, all-silicon inner tracker scheduled for installation during LS3.

    “Each improvement we make to the experiment aims to maximise its performance, taking advantage of the ever-improving operation of the machine,” concludes Jakobs. “While these works are ongoing, we will continue to analyse the wealth of data collected during Run 2 – it will be a very busy time for the entire collaboration!”

    See the full article here .


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  • richardmitnick 8:08 am on November 2, 2018 Permalink | Reply
    Tags: , Antimatter particles, “Majorana” particles: particles that are indistinguishable from their antimatter counterparts, , CERN ATLAS, , , , ,   

    From CERN: “Chasing a particle that is its own antiparticle” 

    Cern New Bloc

    Cern New Particle Event

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    From CERN

    1 Nov 2018
    Ana Lopes

    1
    The ATLAS experiment at CERN. (Image: Maximilien Brice/CERN)

    Neutrinos weigh almost nothing: you need at least 250 000 of them to outweigh a single electron. But what if their lightness could be explained by a mechanism that needs neutrinos to be their own antiparticles? The ATLAS collaboration at CERN is looking into this, using data from high-energy proton collisions collected at the Large Hadron Collider (LHC).

    One way to explain neutrinos’ extreme lightness is the so-called seesaw mechanism, a popular extension of the Standard Model of particle physics.

    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

    This mechanism involves pairing up the known light neutrinos with hypothetical heavy neutrinos. The heavier neutrino plays the part of a larger child on a seesaw, lifting the lighter neutrino to give it a small mass. But for this mechanism to work, both neutrinos need to be “Majorana” particles: particles that are indistinguishable from their antimatter counterparts.

    Antimatter particles have the same mass as their corresponding matter particles but have the opposite electric charge. So, for example, an electron has a negative electric charge and its antiparticle, the positron, is positive. But neutrinos have no electric charge, opening up the possibility that they could be their own antiparticles. Finding heavy Majorana neutrinos could not only help explain neutrino mass, it could also lead to a better understanding of why matter is much more abundant in the universe than antimatter.

    In an extended form of the seesaw model, these heavy Majorana neutrinos could potentially be light enough to be detected in LHC data. In a new paper, the ATLAS collaboration describes the results of its latest search for hints of these particles.

    ATLAS looked for instances in which both a heavy Majorana neutrino and a “right-handed” W boson, another hypothetical particle, could appear. They used LHC data from collision events that produce two “jets” of particles plus a pair of energetic electrons or a pair of their heavier cousins, muons.

    The researchers compared the observed number of such events with the number predicted by the Standard Model. They found no significant excess of events over the Standard Model expectation, indicating that no right-handed W bosons and heavy Majorana neutrinos took part in these collisions.

    However, the researchers were able to use their observations to excludepossible masses for these two particles. They excluded heavy Majorana neutrino masses up to about 3 TeV, for a right-handed W boson with a mass of 4.3 TeV. In addition, they explored for the first time the hypothesis that the Majorana neutrino is heavier than the right-handed W boson, placing a lower limit of 1.5 TeV on the mass of Majorana neutrinos. Further studies should be able to put tighter limits on the mass of heavy Majorana neutrinos in the hope of finding them – if, indeed, they exist.

    See the full article here.


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  • richardmitnick 4:07 pm on September 25, 2018 Permalink | Reply
    Tags: , , CERN ATLAS, , New ATLAS result of ultra-rare B-meson decay to muon pair, , ,   

    From CERN ATLAS: “New ATLAS result of ultra-rare B-meson decay to muon pair” 

    CERN ATLAS Higgs Event

    CERN/ATLAS
    From CERN ATLAS

    25th September 2018
    ATLAS Collaboration

    1
    Figure 1: Measured dimuon mass distributions in the selection channel with highest expected signal purity. Superimposed is the result of a fit to the data. The total fit result is shown (black continuous line), with the observed signal component (dashed red line), b→μμ X background (dashed blue), and continuum background (dashed green). Signal components are grouped in one single curve, including the B0s → μ+μ- and B0 → μ+μ- components. The peaking B0(s) → hh′ background (brown dashed line) is, for all BDT bins, very close to the x-axis. (Image: ATLAS Collaboration/CERN)

    The study of hadrons – particles that combine together quarks to form mesons or baryons – is a vital part of the ATLAS physics programme. Their analysis has not only perfected our understanding of the Standard Model, it has also provided excellent opportunities for discovery.

    Among the variety of hadrons available in nature and produced in LHC proton-proton collisions, B-mesons play a fundamental role. They are bound-states of two quarks – one a bottom quark, the other one of the lighter quarks (up, down, strange or charm) – that decay through the weak interaction to lighter hadrons and/or leptons. Over the past decades, physicists have examined rare and precisely predicted phenomena involving neutral B-mesons (i.e. B0 or Bs mesons), searching for slight discrepancies from theory predictions that could be a signal of new physics.

    On 20 September 2018, at the International Workshop on the CKM Unitarity Triangle (CKM 2018), ATLAS revealed the most stringent experimental constraint of the very rare decay of the B0 meson into two muons (μ). The result is a new milestone that complements analyses [Physical Review Letters] previously published by LHC experiments dedicated to the study of B-mesons in a quest spanning almost three decades.

    ATLAS revealed the most stringent experimental constraint of the very rare decay of the B0 meson into two muons.

    2
    Figure 2: Likelihood contours for the combination of the Run 1 and 2015-2016 Run 2 results (in black). The solid, dashed and dashed-dotted contours delimit the one, two and three standard deviation regions, respectively. The contours for the Run 2 2015-2016 result alone are overlaid in blue. The Standard Model prediction and its uncertainties are shown by the solid cross. (Image: ATLAS Collaboration/CERN)

    The rareness of this decay is due to the coincidence of two factors: first, the decay requires quantum loops with several weak interaction vertices, some of which have a low probability to occur; second, angular momentum conservation constrains the decay products of the scalar B0 or Bs meson into a highly unlikely configuration.

    According to the Standard Model, the probability of generating this decay is about 1.1 in 10 billion. The new ATLAS result gets very close, with the tightest available upper limit of 2.1 occurrences in ten billion at the 95% confidence level. The result was obtained using data collected in 2015 and 2016 combined with an analogous analysis of Run 1 data. The result also provided a 4.6 standard deviations evidence for the Bs→ μμ decay, whose branching fraction is measured to be 2.8 +0.8–0.7 x10–9. It confirms previous measurements from the LHCb and CMS collaborations.

    This new ATLAS result is the first milestone towards a more precise measurement that will be obtained with the full Run 2 dataset, which is expected to improve the current precision by about 30%. Further projections towards the high-luminosity LHC (HL-LHC) era predict that ATLAS will be able to further improve the precision of this result by about a factor 3.

    Related journal articles
    _________________________________________________
    See the full article for further references with links.

    See the full article here .


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