Tagged: CERN ATLAS Toggle Comment Threads | Keyboard Shortcuts

  • richardmitnick 11:11 am on May 24, 2017 Permalink | Reply
    Tags: , CERN ATLAS, , , , , Our failure in resolve,   

    From FNAL: “Fermilab scientists set upper limit for Higgs boson mass” 

    FNAL II photo

    FNAL Art Image
    FNAL Art Image by Angela Gonzales

    Fermilab is an enduring source of strength for the US contribution to scientific research world wide.

    In 1977, theoretical physicists at Fermilab — Ben Lee and Chris Quigg, along with Hank Thacker — published a paper setting an upper limit for the mass of the Higgs boson. This calculation helped guide the design of the Large Hadron Collider by setting the energy scale necessary for it to discover the particle. The Large Hadron Collider turned on in 2008, and in 2012, the LHC’s ATLAS and CMS discovered the long-sought Higgs boson — 35 years after the seminal paper.

    1

    CERN CMS Higgs Event


    CERN/CMS Detector


    CERN ATLAS Higgs Event


    CERN/ATLAS detector

    Where it all started:

    FNAL Tevatron

    FNAL/Tevatron map


    FNAL/Tevatron DZero detector


    FNAL/Tevatron CDF detector

    Where we failed and handed it to Europe:

    3
    Sight of the planned Superconducting Super Collider, in the vicinity of Waxahachie, Texas. Cancelled by our idiot Congress under Bill Clinton in 1993. We could have had it all.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    FNAL Icon
    Fermilab Campus

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

     
  • richardmitnick 10:29 am on May 24, 2017 Permalink | Reply
    Tags: , ATLAS kicks off a new year at 13 TeV, CERN ATLAS, , ,   

    From ATLAS: “ATLAS kicks off a new year at 13 TeV” 

    CERN ATLAS Higgs Event

    CERN/ATLAS
    ATLAS

    23rd May 2017
    Katarina Anthony

    1
    One of the early collision events with stable beams recorded by ATLAS on 23 May 2017, with a reconstructed muon candidate. The upper panes show transverse views of the detector and the muon spectrometer, while the lower panes show ATLAS in longitudinal cross-section and an eta-phi view of the energy deposits in the cells of the ATLAS calorimeters. (Image: ATLAS Collaboration/CERN)

    A new season of record-breaking kicked off today, as the ATLAS Experiment began recording first data for physics of 2017. This will be the LHC’s third year colliding beams at an energy of 13 tera electron volts (TeV), allowing the ATLAS Experiment to continue to push the limits of physics.

    “The ATLAS Experiment is ready to enter this new round of data-taking and we are looking forward to another exciting year of LHC physics,” says Karl Jakobs, ATLAS Spokesperson. “We will continue to explore the 13 TeV energy frontier in great depth, to address rarer processes and to increase the precision of many measurements.”

    2017 should be another excellent year for both ATLAS and the LHC, with records in luminosity set to be broken. The higher the luminosity, the more data can be gathered – and the greater the chance of observing rare processes. “The benefits of ATLAS’ fantastic performance in 2016 is clearly seen in our many new results, both in the search for new physics and in measurements of Standard Model processes,” says Dan Tovey, ATLAS Physics Coordinator. “This bodes very well for the rest of Run 2, when the threefold increase in data should give us sensitivity to the most subtle effects of ‘new physics’ in dedicated searches, and will enable us to take measurements with exquisite precision.”

    See the full article here .

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

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

     
  • richardmitnick 1:25 pm on May 16, 2017 Permalink | Reply
    Tags: , , , CERN ATLAS, , Mykaela Reilly, , , ,   

    From BNL: Women in STEM – “Patchogue-Medford High School Student Builds a Remote Sensing System for ATLAS Detector Components” Mykaela Reilly 

    Brookhaven Lab

    1
    Patchogue-Medford High School student Mykaela Reilly (seated) with members of the ATLAS silicon tracker upgrade group in the Physics Department: (from left) Russell Burns, Alessandro Tricoli, Phil Kuczewski, Stefania Stucci, David Lynn, and Gerrit Van Nieuwenhuizen. No image credit.

    May 12, 2017
    Jane Koropsak
    jane@bnl.gov

    When Patchogue-Medford High School student Mykaela Reilly came to the U.S. Department of Energy’s Brookhaven National Laboratory as part of the High School Research Program last summer, she thought she was coming to work for one summer. She never expected that her achievements would result in her being offered to continue at the lab another year. From soldering to building prototypes to computer programming, Reilly says that during the course of the year she learned a lot about how research projects come together and form the foundations of scientific discovery.

    Reilly was tasked with learning LabView, a software system and design program that helps scientists with data acquisition and instrument control. She also programmed micro-controllers used to monitor nitrogen levels to keep humidity low, limit condensation, and maintain steady temperatures inside an experimental area. It took weeks to build the experimental components and test the software that would remotely control that equipment. But, with guidance from her mentor, Lab physicist Alessandro Tricoli of the ATLAS silicon tracker upgrade group in the Physics Department, and research team members Phil Kuczewski and Stefania Stucci, Reilly worked out the “bugs” until she built a sensing system and computer program that her mentors say works seamlessly.

    2

    Reilly’s success may help advance one of the most ambitious scientific projects in the world—the ATLAS detector at the Large Hadron Collider (LHC) near Geneva, Switzerland. Brookhaven scientists have played multiple roles in constructing, operating, and upgrading this particle detector, which is the size of a seven-story building and has opened up new frontiers in the human pursuit of knowledge about elementary particles and their interactions. Reilly conducted experiments using her remote monitoring program to see how electronic components, such as readout chips that could be incorporated in an upgrade at ATLAS, respond to tough environmental conditions—particularly the high level of radiation at the LHC. Radiation-resilient silicon readout chips would reduce power consumption and simplify the design of the entire tracker system at ATLAS.

    “Mykaela’s work will shed light on how we can make the readout chips more resistant to the radiation at the LHC, and how we can keep the radiation effects under control,” said Tricoli. “I applaud her success. With her talent, I hope she decides to pursue a career in science or engineering.”

    What’s next?

    Just before the posting of this story, Reilly announced her plans to attend Stony Brook University to pursue a degree in electrical engineering. “That is wonderful news,” said Tricoli. “I hope to see her back at the Lab soon.”

    When she isn’t busy soldering, programming, or building sensing systems, you can find Reilly on the ice competing on a synchronized figure skating team with her sisters. “I found that synchronized figure skating is a lot like research,” she said. “It’s about hard work, precision, and collaboration.”

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition
    BNL Campus

    One of ten national laboratories overseen and primarily funded by the Office of Science of the U.S. Department of Energy (DOE), Brookhaven National Laboratory conducts research in the physical, biomedical, and environmental sciences, as well as in energy technologies and national security. Brookhaven Lab also builds and operates major scientific facilities available to university, industry and government researchers. The Laboratory’s almost 3,000 scientists, engineers, and support staff are joined each year by more than 5,000 visiting researchers from around the world. Brookhaven is operated and managed for DOE’s Office of Science by Brookhaven Science Associates, a limited-liability company founded by Stony Brook University, the largest academic user of Laboratory facilities, and Battelle, a nonprofit, applied science and technology organization.
    i1

     
  • richardmitnick 2:57 pm on May 15, 2017 Permalink | Reply
    Tags: , CERN ATLAS, , , ,   

    From ATLAS: “New ATLAS precision measurements of the Higgs Boson in the ‘golden channel'” 

    CERN ATLAS Higgs Event

    CERN/ATLAS
    ATLAS

    15th May 2017
    ATLAS Collaboration

    1
    Figure 1: Distribution of the invariant mass of the four leptons selected in the ATLAS measurement of H→ZZ*→4l using the full 2015+2016 data set. The Higgs boson corresponds to the excess of events with respect to the non resonant ZZ* background observed at 125 GeV. (ATLAS Collaboration/CERN)

    The discovery of a Higgs boson in 2012 by the ATLAS and CMS experiments marked a milestone in the history of particle physics. It confirmed a long-standing prediction of the Standard Model, the theory that comprises our present understanding of elementary particles and their interactions.

    With the huge amount of proton–proton collisions delivered by the LHC in 2015 and 2016 at the increased collision energy of 13 TeV, ATLAS has entered a new era of Higgs boson property measurements. The new data allowed ATLAS to perform measurements of inclusive and differential cross sections using the “golden” H→ZZ*→4ℓ decay.

    The four-lepton channel, albeit rare (0.012% branching fraction into final states with electrons or muons), has the clearest and cleanest signature of all the possible Higgs boson decay modes. This is due to the channel’s small background contamination. Figure 1 shows a narrow resonant peak at 125 GeV in the reconstructed invariant mass on top of a locally relatively flat background distribution dominated by (non-resonant) qq→ZZ* production.

    2
    Figure 2: Differential cross section for the transverse momentum (pT4l) of the Higgs boson. The measured cross section is compared to different ggF SM predictions. The error bars on the data points show the total uncertainties, while the systematic uncertainties are indicated by the boxes. (ATLAS Collaboration/CERN)

    The Higgs boson’s transverse momentum can be used to probe different Higgs production mechanisms and possible deviations from the Standard Model interactions. Figure 2 shows the measured differential cross section of the four-lepton transverse momentum (pT4l) compared to various Standard Model predictions.

    By studying the number of jets produced in these events, as well as the transverse momentum of the leading jet, ATLAS can probe and help improve the theoretical modelling of Higgs boson production via gluon fusion (ggF). The measured and predicted differential cross sections as a function of the jet multiplicity are shown in Figure 3.

    Several differential cross sections have been measured for observables sensitive to Higgs boson production and decay, including kinematic distributions of the jets produced in association with the Higgs boson. Good agreement is found between the data and Standard Model predictions. The measurements are used to constrain anomalous Higgs boson interactions (see Figure 4).

    3
    Figure 3: Differential cross section for jet multiplicity associated to the Higgs boson. The measured cross section is compared to different ggF SM predictions. The error bars on the data points show the total uncertainties, while the systematic uncertainties are indicated by the boxes. (ATLAS Collaboration/CERN)

    4
    Figure 4: Limits on modified Higgs-boson decays within the framework of pseudo-observables. The limits are extracted in the plane of εL and εR, which modify the contact terms between the Higgs boson and left- and right-handed leptons, assuming lepton-flavour universality. (ATLAS Collaboration/CERN)

    Links:

    Measurement of inclusive and differential fiducial cross sections in the H→ZZ*→4ℓ decay channel at 13 TeV with the ATLAS detector: link coming soon
    Presentation at LHCP conference by Eleni Mountricha: Higgs measurements in high resolution channels with ATLAS
    See also the full lists of ATLAS Conference Notes and ATLAS Physics Papers

    See the full article here .

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

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

     
  • richardmitnick 2:35 pm on May 9, 2017 Permalink | Reply
    Tags: , CERN ATLAS, , , ,   

    From ATLAS: “New insight into the Standard Model” 

    CERN ATLAS Higgs Event

    CERN/ATLAS
    ATLAS

    9th May 2017
    ATLAS Collaboration

    ATLAS releases the first study of a pair of neutral bosons produced in association with a high-mass dijet system.

    1
    Figure 1: Distribution of (a) the centrality of the Z boson-photon (Zγ) system and (b) the transverse energy of the photon. These studies show data collected by ATLAS in 2012 (black points) compared to Standard Model predictions (coloured histograms). The signal that is looked for is displayed as the dark red histogram and the main background is shown as the light blue one. The bottom panels show the ratio of the data to the sum of all the predictions. The error band (blue) shows the total uncertainty on these predictions. A sign of new physics could appear as an enhancement at large momentum, as shown by the dotted blue line in (b). (Image: ATLAS Collaboration/CERN)

    2
    Figure 2: Feynman diagram of the signal process, the Electroweak production of a Z boson, photon (γ) and two high-energy jets. (Image: ATLAS Collaboration/CERN)

    Ever since the LHC collided its first protons in 2009, the ATLAS Collaboration has been persistently studying their interactions with increasing precision. To this day, it has always observed them to be as expected by the Standard Model.

    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.

    Though it remains unrefuted, physicists are convinced that a better theory must exist to explain certain fundamental questions: What is the nature of the dark matter? Why is the gravitational force so weak compared to the other forces?

    Answers may be found by looking at a very rare process that had previously never been studied by ATLAS: the interaction of four bosons, whose signature is the presence of a Z boson, a photon and two high-energy jets. This is an excellent probe of the electroweak sector of the Standard Model and is very sensitive to new physics models. However, this process is very difficult to detect, given its rarity and the large number of different processes that can mimic its signature (known as “background”). The main background comes from the production of a Z boson and a photon accompanied by two jets, which, unlike the electroweak process we are interested in, is produced via strong interactions.

    This leads to differences in the kinematics of the observed jets, which are described in a recently-submitted paper to the Journal of High Energy Physics [no link found], where ATLAS presents a search for such events using 8 TeV data. Utilizing the knowledge that the recoiling quarks (see Figure 2) will produce jets that have a very large invariant mass and are widely separated in the detector, ATLAS has been able to reduce the background and mitigate the large experimental uncertainties in order to extract the signal.

    The background is suppressed by selecting events where the two jets have an invariant mass larger than 500 GeV. The signal and main background are further separated by quantifying the centrality of the Z-photon system with respect to the two jets. Events with low centrality are more likely to be produced via the electroweak signal process while those with high centrality are more likely to come from strong interactions. This is illustrated in Figure 1(a), where a small excess of events above the predicted background is observed, with a statistical significance of 2σ.

    The centrality is used to measure the event rate (cross section) of the signal alone, and of the sum of the signal and the major background. Both were found to be in agreement with Standard Model predictions within the large statistical uncertainty. Anomalies on the coupling of four bosons have also been searched for, by looking at the tails of the photon transverse energy spectrum that may be enhanced by new physics contributions (blue dotted line in Figure 1(b)). No deviation from the Standard Model has been seen and stringent limits are set on the presence of new physics in this region.

    The Standard Model will continue to keep its secrets… until the next set of results!

    Links:

    Studies of Zγ electroweak production in association with a high-mass dijet system in pp collisions at 8 TeV with the ATLAS detector(arXiv: 1705.01966, see figures)
    See also the full lists of ATLAS Conference Notes and ATLAS Physics Papers.

    See the full article here .

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

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

     
  • richardmitnick 7:45 am on May 2, 2017 Permalink | Reply
    Tags: , , Beams return to the ATLAS Experiment, CERN ATLAS, , ,   

    From ATLAS at CERN: “Beams return to the ATLAS Experiment” 

    CERN ATLAS Higgs Event

    CERN/ATLAS
    ATLAS

    29th April 2017
    Katarina Anthony

    1
    Event display of a “beam splash” seen by the ATLAS Experiment on Saturday 29 April, 2017. The collimator position is 140m in front of the ATLAS interaction point. Left figure shows an axial view of the various components of the ATLAS detector. Right figure shows the energy deposits in the cells of the ATLAS calorimeter. Lower panel shows longitudinal cross-section of ATLAS; the spray of particles enters from the left-hand side. (Image: ATLAS Collaboration/CERN)

    With the year’s first proton beams now circulating in the Large Hadron Collider, physicists have today recorded “beam splashes” in the ATLAS Experiment.

    Like a giant wave striking the shore, beam splashes are generated when circulating protons hit collimators in the beam pipe, leaving a spray of particles to wash across the ATLAS detector. They provide signals that illuminate the various sub-detectors, allowing physicists to synchronise the ATLAS detector elements to the LHC’s clock. This is one of the final actions before protons once again collide in the heart of ATLAS.

    The arrival of beam splashes marks the start of a new year of exploration, which should see records set for instantaneous luminosity. “To study the high-energy frontier, we need to keep our detector at its best performance,” says Masaya Ishino, ATLAS Deputy Run Coordinator. “This requires tremendous effort, not only in the control room, but also from our physicists all over the world. We are ready to cope with the high instantaneous luminosity expected, the highest the ATLAS Experiment has ever seen.”

    “ATLAS teams have worked very hard over the last months to prepare the experiment for the luminosity challenge,” confirms Alexander Oh, ATLAS Run Coordinator. “We have made various improvements during the year-end-technical-stop and are eager to see protons colliding again. Our goal is to take collision data with the highest possible efficiency and quality to optimally explore the physics at 13 TeV energy.”

    With collisions scheduled for the end of May, ATLAS physicists will use the coming weeks to prepare: fine-tuning the subdetectors, testing the trigger and data acquisition systems, and monitoring the circulating beams.

    See the full article here .

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

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

     
  • richardmitnick 11:05 am on April 26, 2017 Permalink | Reply
    Tags: , CERN ATLAS, Charged-particle reconstruction at the energy frontier   

    From ATLAS at CERN: “Charged-particle reconstruction at the energy frontier” 

    CERN ATLAS Higgs Event

    CERN/ATLAS
    ATLAS

    26th April 2017
    ATLAS Collaboration

    1
    Figure 1: Illustration of isolated measurements (left) in the ATLAS pixel detector and merged measurements (right) due to very collimated tracks. Merged measurements are more common in higher energetic jets and are harder to distinguish. The ATLAS event reconstruction software was optimized for Run 2 and is now better able to resolve merged measurements. Different colors represent energy deposits from different charged particles traversing the sensor and the particles trajectories are shown as arrows. (Image: ATLAS Collaboration/CERN)

    A new age of exploration dawned at the start of Run 2 of the Large Hadron Collider, as protons began colliding at the unprecedented centre-of-mass energy of 13 TeV. The ATLAS experiment now frequently observes highly collimated bundles of particles (known as jets) with energies of up to multiple TeV, as well as tau-leptons and b-hadrons that pass through the innermost detector layers before decaying. These energetic collisions are prime hunting grounds for signs of new physics, including massive, hypothetical new particles that would decay to much lighter – and therefore highly boosted – bosons.

    In these very energetic jets, the average separation of charged particles is comparable to the size of individual inner detector elements. This easily creates confusion within the algorithms responsible for reconstructing charged particle trajectories (tracks). Therefore, without careful consideration, this can limit the track reconstruction efficiency in these dense environments. This would result in poor identification of long-lived b-hadrons and hadronic tau decays, and difficulties in calibrating the energy and mass of jets.

    2
    Figure 2: Efficiency to reconstruct a track of a charged particle from decays of a tau-lepton, rho-meson and B0-hadron as a function of these particles initial transverse momentum. At higher transverse momentum, merged measurements are more abundant and therefore the efficiency drops. This effect is exacerbated by a higher charged-particle multiplicity in the decay, as clearly visible for the tau-lepton’s decay into five charged particles (green circles). (Image: ATLAS Collaboration/CERN)

    Similar to increasing the magnification of a microscope, in preparation for Run 2, the ATLAS event reconstruction software was optimized to better resolve these close-by particles. As a result, at angular separations between a jet and a charged particle below 0.02, the reconstruction efficiency for a charged particle track is still around 80% for jets with a transverse momentum of 1400 to 1600 GeV in simulated dijet events. This has maximised the potential for discovery, allowing for more detailed measurements of the newly opened kinematic regime.

    Recently published results give a general overview of the new track reconstruction algorithm, highlighting the ATLAS detector’s excellent performance in reconstructing charged particles in dense environments. The results also present, for the first time, a novel method for determining in situ (i.e. from data) the efficiency of reconstructing tracks in such an environment. The study uses the ionization energy loss (dE/dx), measured with the ATLAS pixel detector, to deduce the probability of failing to reconstruct a track. The obtained results confirm the excellent performance expected from studies on simulated data.

    3
    Figure 3: The ionization energy loss (dE/dx) of charged particles in the ATLAS pixel detector. Three distinct distributions were created to extract the track reconstruction performance in the core of jets: isolated measurements (blue); merged measurements (green); and the data (black circles) which, due to specific selections, should resemble isolated measurements. A possible inefficiency of the track reconstruction is determined by fitting the green and blue distributions to the data (the result is shown as a red line). The fitted contribution of the green distribution to the data corresponds to an inefficiency of the track reconstruction. (Image: ATLAS Collaboration/CERN)

    Links:

    Performance of the ATLAS Track Reconstruction Algorithms in Dense Environments in LHC Run 2: arXiv link to come.
    See also the full lists of ATLAS Conference Notes and ATLAS Physics Papers.

    See the full article here .

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

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

     
  • richardmitnick 11:32 am on April 6, 2017 Permalink | Reply
    Tags: , CERN ATLAS, , Improving our understanding of photon pairs, ,   

    From CERN ATLAS: “Improving our understanding of photon pairs” 

    CERN ATLAS Higgs Event

    CERN/ATLAS
    ATLAS

    5th April 2017
    ATLAS Collaboration

    1
    Figure 1: The measured differential cross section as a function of the invariant mass of the photon pair is compared to predictions from four theoretical computations. The invariant mass is often the most scrutinized distribution when searching for new physics. (Image: ATLAS Collaboration/CERN)

    High-energy photon pairs at the LHC are famous for two things. First, as a clean decay channel of the Higgs boson. Second, for triggering some lively discussions in the scientific community in late 2015, when a modest excess above Standard Model predictions was observed by the ATLAS and CMS collaborations. When the much larger 2016 dataset was analysed, however, no excess was observed.

    Yet most photon pairs produced at the LHC do not originate from the decay of a Higgs boson (or a new, undiscovered particle). Instead, more than 99% are from rather simple interactions between the proton constituents, such as quark-antiquark annihilation. ATLAS physicists have put significant effort into improving our understanding of these Standard Model processes.

    ATLAS has released a new measurement of the inclusive di-photon cross section based on the full 2012 proton-proton collision dataset recorded at a centre-of-mass energy of 8 TeV. The precision is increased by a factor of two compared to the previous ATLAS measurement (based on the smaller 2011 data sample recorded at 7 TeV), such that the total experimental uncertainty is now typically 5%.

    According to the theory of strong interactions, the production rate of such Standard Model processes is sensitive to both high-order perturbative terms (more complex particle interactions involving quantum fluctuations) and the dynamics of additional low-energy particles emitted during the scattering process. Theoretical predictions are thus currently precise only at the 10% level. Calculations based on a fixed number of perturbative terms in the series expansion (next-to-leading order and next-to-next to leading order in the strong coupling strength) underestimate the data beyond the projected theoretical uncertainties.

    2
    Figure 2: The measured differential cross section as a function of the φ* variable is compared to predictions from four theoretical computations. The low φ* region is most sensitive to the dynamics of additional low-energy particles emitted during the scattering process. (Image: ATLAS Collaboration/CERN)

    In the new ATLAS result, the distortion in the photon pair production rate originating from the emission of low-energy particles has been probed very precisely thanks to the study of two new observables. By accurately modelling the additional emission, the predictions are found to agree with the data in the sensitive regions.

    These results provide crucial information for both experimentalists and theorists on the dynamics of the strong interaction at the LHC, and should lead to improved Standard Model predictions of di-photon processes.

    Links:

    Measurements of integrated and differential cross sections for isolated photon pair production in pp collisions at 8TeV with the ATLAS detector.

    See the full article here .

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

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

     
  • richardmitnick 10:25 am on April 2, 2017 Permalink | Reply
    Tags: , , CERN ATLAS, , ,   

    From CERN ATLAS: “ATLAS highlights from Moriond” 

    CERN ATLAS Higgs Event

    CERN/ATLAS
    ATLAS

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

    At this year’s Rencontres de Moriond, the ATLAS collaboration presented the first results examining the combined 2015/2016 LHC data at 13 TeV proton–proton collision energy. Thanks to outstanding performance of the CERN accelerator complex last year, this new dataset is almost three times larger than that available at ICHEP, the last major particle physics conference held in August 2016.

    The significant increase in data volume has greatly improved ATLAS’ sensitivity to possible new particles predicted by theories beyond the Standard Model. At the same time, it has also allowed ATLAS physicists to perform precise measurements of the properties of known Standard Model particles.

    A selection of Moriond 2017 highlights are explored below; find the full list of ATLAS public results here, with recent Run 2 results here.

    The search for supersymmetry

    Supersymmetry (SUSY) has long been considered a front-runner for solving a number of mysteries left unexplained by the Standard Model, including the magnitude of the mass of the Higgs boson and the nature of the dark matter. Among the key new results presented at Moriond were the first searches for SUSY particles using the new dataset. These new ATLAS results, along with those from the CMS experiment, provide the most challenging tests of the SUSY theory carried out so far.

    Searches for “squark” and “gluino” particles decaying to Standard Model particles revealed no evidence for their existence, and have set limits on the masses of these particles which extend, for the first time, as high as 2 TeV. Searches for “top squark” particles, the existence of which is crucial if SUSY is to explain the mass of the Higgs boson, also found no deviations from expected Standard Model processes.

    A new search for long-lived “chargino” particles was also presented. This search utilizes the Insertable B-Layer (IBL) detector installed during the 2014 LHC shutdown. The IBL is a new piece of ATLAS charged particle detection hardware as close as 3.3 cm to the LHC beam pipe. The new search looks for ‘disappearing’ tracks created by charginos traversing the IBL before decaying into invisible dark matter. No evidence for such tracks was found, significantly constraining a large class of SUSY models. An alternative search for new long-lived particles decaying to charged particles via the signature of displaced decay vertices also found the data to be consistent with Standard Model expectations.

    Exotic explorations

    In addition to searches for SUSY particles, ATLAS reported a number of new results in the search for “exotic” forms of beyond the Standard Model physics. Searches for new heavy particles that decay into pairs of jets (thus sensitive to a possible quark substructure) or to a Higgs boson and a W or Z boson set constraints on the masses of these exotic new particles as high as 6 TeV.

    Searches for the production of dark matter particles were also reported. These look at events in which Standard Model particles, such as photons or Higgs bosons, recoil against the invisible dark matter particles to generate an eve­­nt property called missing transverse energy. Again, the data were consistent with expectations from Standard Model processes.

    In addition, a search for a heavy partner of the W boson (a W’ boson), predicted by many Standard Model extensions, was carried out with the new dataset. In the absence of evidence of a signal, the search has set new limits on the W’ mass up to 5.1 TeV.

    Rare Higgs decays

    Following the discovery of the Higgs boson in 2012, a major component of the ATLAS physics programme has been devoted to measuring its properties and searching for rare processes by which it may decay. These analyses are crucial to establish whether the Higgs boson observed by ATLAS is that predicted by the Standard Model, or if it is instead the first evidence of new physics.

    The ATLAS collaboration presented a new search for a rare process where the Higgs boson decays to muon pairs. Observation of this process above the rate predicted by the Standard Model could provide evidence for new physics. No evidence was seen however, allowing limits to be set on the decay probability of 2.7 times the Standard Model expectation. That limit probes (and proves) the fundamental Standard Model prediction of different Higgs boson-to-lepton couplings for different lepton generations.

    Standard Model measurements

    Analysing data taken in 2012, the ATLAS Collaboration presented a number of measurements of the production and properties of known Standard Model particles. Among these was a major milestone result for the LHC programme: the first measurement of the W boson mass by the ATLAS experiment. Measured with a precision of 19 MeV, the result rivals the best previous result from a single experiment. The measurement provides an excellent test of the Standard Model via so-called virtual corrections through the interplay between the W boson, top-quark and Higgs boson masses, all precisely measured by ATLAS.

    Another key new result was a measurement of the decay properties of Bd mesons decaying to a K* meson and two muons. The LHCb and Belle collaborations had previously reported evidence of an excess above Standard Model expectations in one particular decay parameter, P5’. The new ATLAS measurement also provides evidence of a modest excess, albeit with significant statistical uncertainties. Analysis of the new dataset should enable a clearer picture of this process to be obtained.

    In addition, ATLAS presented precise new measurements of the production and properties of photon pairs in 8 TeV collisions. This result represents an important addition to our understanding of quantum chromodynamics (QCD), the Standard Model theory of the strong force.

    The search continues

    While no evidence for new physics has yet been found, these new results have provided crucial input to our theoretical models and has greatly improved our understanding of the Standard Model. We can look forward more results using the new dataset in the coming months. What is more, with the LHC set to continue its excellent performance in 2017, ATLAS can expect even greater sensitivity in results to come.

    See the full article here .

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

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

     
  • richardmitnick 11:59 am on March 22, 2017 Permalink | Reply
    Tags: , , CERN ATLAS, , , Quest for the lost arc   

    From ATLAS: “Quest for the lost arc” 

    CERN ATLAS Higgs Event

    CERN/ATLAS
    ATLAS

    21st March 2017
    ATLAS Collaboration

    1
    Figure 1: ATLAS simulation showing a hypothetical new charged particle (χ1+) traversing the four layers of the pixel system and decaying to an invisible neutral particle (χ10) and an un-detected pion (π+). The red squares represent the particle interactions with the detector. (Image: ATLAS Collaboration/CERN)

    Nature has surprised physicists many times in history and certainly will do so again. Therefore, physicists have to keep an open mind when searching for phenomena beyond the Standard Model.

    ..
    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.

    Some theories predict the existence of new particles that live for a very short time. These particles would decay to known particles that interact with the sophisticated “eyes” of the ATLAS detector. However, this may not be the case. An increasingly popular alternative is that some of these new particles may have masses very close to each other, and would thus travel some distance before decaying. This allows for the intriguing possibility of directly observing a new type of particle with the ATLAS experiment, rather than reconstructing it via its decay products as physicists do for example for the Higgs boson.

    2
    Figure 2: The number of reconstructed short tracks (tracklets) as a function of their transverse momentum (pT). ATLAS data (black points) are compared with the expected contribution from background sources (gray solid line shows the total) . A new particle would appear as an additional contribution at large pT, as shown for example by the dashed red line. The bottom panel shows the ratio of the data and the background predictions. The error band shows the uncertainty of the background expectation including both statistical and systematic uncertainties. (Image: ATLAS Collaboration/CERN)

    An attractive scenario predicts the existence of a new electrically charged particle, a chargino (χ1±), that may live long enough to travel a few tens of centimetres before decaying to an invisible neutral weakly interacting particle, a neutralino (χ10). A charged pion would also be produced in the decay but, due to the very similar mass of the chargino and the neutralino, its energy would not be enough for it to be detected. As shown in Figure 1, simulations predict a quite spectacular signature of a charged particle “disappearing” due to the undetected decay products.

    ATLAS physicists have developed dedicated algorithms to directly observe charged particles travelling as little as 12 centimetres from their origin. Thanks to the new Insertable B-Layer, these algorithms show improved performance reconstructing such charged particles that do not live long enough to interact with other ATLAS detector systems. So far, the abundance and properties of the observed particles are in agreement with what is expected from known background processes.

    New results presented at the Moriond Electroweak conference set very stringent limits on what mass such particles may have, if they exist. These limits severely constrain one important type of Supersymmetry dark matter. Although no new particle has been observed, ATLAS physicists continue the search for this “lost arc”. Stay tuned!

    See the full article here .

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

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

     
c
Compose new post
j
Next post/Next comment
k
Previous post/Previous comment
r
Reply
e
Edit
o
Show/Hide comments
t
Go to top
l
Go to login
h
Show/Hide help
shift + esc
Cancel
%d bloggers like this: