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  • richardmitnick 6:10 pm on February 13, 2018 Permalink | Reply
    Tags: , CERN ATLAS, , , , , , Supersymmetry (SUSY)   

    From CERN Courier: “ATLAS extends searches for natural supersymmetry” 


    CERN Courier

    Jan 15, 2018

    1
    Exclusion limits

    Despite many negative searches during the last decade and more, supersymmetry (SUSY) remains a popular extension of the Standard Model (SM). Not only can SUSY accommodate dark matter and gauge–force unification at high energy, it offers a natural explanation for why the Higgs boson is so light compared to the Planck scale. In the SM, the Higgs boson mass can be decomposed into a “bare” mass and a modification due to quantum corrections. Without SUSY, but in the presence of a high-energy new physics scale, these two numbers are extremely large and thus must almost exactly oppose one another – a peculiar coincidence called the hierarchy problem. SUSY introduces a set of new particles that each balances the mass correction of its SM partner, providing a “natural” explanation for the Higgs boson mass.

    Thanks to searches at the LHC and previous colliders, we know that SUSY particles must be heavier than their SM counterparts. But if this difference in mass becomes too large, particularly for the particles that produce the largest corrections to the Higgs boson mass, SUSY would not provide a natural solution of the hierarchy problem.

    New SUSY searches from ATLAS using data recorded at an energy of 13 TeV in 2015 and 2016 (some of which were shown for the first time at SUSY 2017 in Mumbai from 11–15 December) have extended existing bounds on the masses of the top squark and higgsinos, the SUSY partners of the top quark and Higgs bosons, respectively, that are critical for natural SUSY. For SUSY to remain natural, the mass of the top squark should be below around 1 TeV and that of the higgsinos below a few hundred GeV.

    ATLAS has now completed a set of searches for the top squark that push the mass limits up to 1 TeV. With no sign of SUSY yet, these searches have begun to focus on more difficult to detect scenarios in which SUSY could hide amongst the SM background. Sophisticated techniques including machine learning are employed to ensure no signal is missed.

    First ATLAS results have also been released for higgsino searches. If the lightest SUSY particles are higgsino-like, their masses will often be close together and such “compressed” scenarios lead to the production of low-momentum particles. One new search at ATLAS targets scenarios with leptons reconstructed at the lowest momenta still detectable. If the SUSY mass spectrum is extremely compressed, the lightest charged SUSY particle will have an extended lifetime, decay invisibly, and leave an unusual detector signature known as a “disappearing track”.

    Such a scenario is targeted by another new ATLAS analysis. These searches extend for the first time the limits on the lightest higgsino set by the Large Electron Positron (LEP) collider 15 years ago. The search for higgsinos remains among the most challenging and important for natural SUSY. With more data and new ideas, it may well be possible to discover, or exclude, natural SUSY in the coming years.

    See the full article here .

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  • richardmitnick 9:54 am on February 12, 2018 Permalink | Reply
    Tags: , , CERN ATLAS, First high-precision measurement of the mass of the W boson at the LHC, , , , ,   

    From CERN ATLAS : “First high-precision measurement of the mass of the W boson at the LHC” 

    CERN ATLAS Higgs Event

    CERN/ATLAS
    ATLAS

    12th February 2018

    1
    Display of a candidate event for a W boson decaying into one muon and one neutrino from proton-proton collisions recorded by ATLAS with LHC stable beams at a collision energy of 7 TeV. (Image: ATLAS Collaboration/CERN).

    In a paper published today in the European Physical Journal C, the ATLAS Collaboration reports the first high-precision measurement at the Large Hadron Collider (LHC) of the mass of the W boson. This is one of two elementary particles that mediate the weak interaction – one of the forces that govern the behaviour of matter in our universe. The reported result gives a value of 80370±19 MeV for the W mass, which is consistent with the expectation from the Standard Model of Particle Physics, the theory that describes known particles and their interactions.

    The measurement is based on around 14 million W bosons recorded in a single year (2011), when the LHC was running at the energy of 7 TeV. It matches previous measurements obtained at Large Electron-Positron Collider[LEP] , the ancestor of the LHC at CERN, and at the Tevatron , a former accelerator at Fermilab [FNAL] in the United States, whose data made it possible to continuously refine this measurement over the last 20 years.

    2
    CERN LEP

    3
    FNAL Tevatron

    FNAL/Tevatron

    The W boson is one of the heaviest known particles in the universe. Its discovery in 1983 crowned the success of CERN’s Super Proton Synchrotron , leading to the Nobel Prize in physics in 1984. Although the properties of the W boson have been studied for more than 30 years, measuring its mass to high precision remains a major challenge.

    4
    Super Proton Synchrotron

    “Achieving such a precise measurement despite the demanding conditions present in a hadron collider such as the LHC is a great challenge,” said the physics coordinator of the ATLAS Collaboration, Tancredi Carli. “Reaching similar precision, as previously obtained at other colliders, with only one year of Run 1 data is remarkable. It is an extremely promising indication of our ability to improve our knowledge of the Standard Model and look for signs of new physics through highly accurate measurements.”

    The Standard Model is very powerful in predicting the behaviour and certain characteristics of the elementary particles and makes it possible to deduce certain parameters from other well-known quantities. The masses of the W boson, the top quark and the Higgs boson for example, are linked by quantum physics relations. It is therefore very important to improve the precision of the W boson mass measurements to better understand the Higgs boson, refine the Standard Model and test its overall consistency.

    Remarkably, the mass of the W boson can be predicted today with a precision exceeding that of direct measurements. This is why it is a key ingredient in the search for new physics, as any deviation of the measured mass from the prediction could reveal new phenomena conflicting with the Standard Model.

    The measurement relies on a thorough calibration of the detector and of the theoretical modelling of the W boson production. These were achieved through the study of Z boson events and several other ancillary measurements. The complexity of the analysis meant it took almost five years for the ATLAS team to achieve this new result. Further analysis with the huge sample of now-available LHC data, will allow even greater accuracy in the near future.

    See the full article here .

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  • richardmitnick 12:16 pm on January 18, 2018 Permalink | Reply
    Tags: , , CERN ATLAS, , Measurements of weak top quark processes gain strength, ,   

    From ATLAS at CERN: “Measurements of weak top quark processes gain strength” 

    This post is dedicated to L.Z. from H.P. and Rutgers Physics. I hope that he sees it.

    CERN ATLAS Higgs Event

    CERN/ATLAS
    ATLAS

    18th January 2018
    ATLAS Collaboration

    1
    Normalised differential cross-sections as a function of the mass of the two charged leptons and the b-jet unfolded from data, compared with selected Monte Carlo models. (Image: ATLAS Collaboration/CERN)

    The production of top quarks in association with vector bosons is a hot topic at the LHC. ATLAS first reported strong evidence for the production of a top quark in association with a Z boson at the EPS 2017 conference. In a paper submitted to the Journal of High-Energy Physics, the ATLAS experiment describes the measurement of top-quark production in association with a W boson in 13 TeV collisions.

    The new ATLAS result using the full 2015 and 2016 dataset extracts differential cross-sections for the production of a top quark in association with a W boson for the first time. This is particularly complex as top quarks almost always decay into a b quark and a W boson, and thus there are two W bosons in final state that decay very quickly. Events are selected that contain two charged leptons (electrons or muons), a jet that is identified as containing a hadron with a b quark, and missing transverse momentum due to the presence of neutrinos.

    Multivariate techniques are used to suppress large background contributions, especially from the production of a top quark with a top antiquark that occurs with much larger rate. They achieve a signal to background ratio of about 1:2, which allows the signal cross-section to be extracted as a function of kinematic observables. The measured background-subtracted distributions are corrected to remove the effects of experimental resolution so that they can be directly compared with theoretical predictions.

    Differential cross-sections as a function of several variables related to both the event and top quark or W boson kinematic properties have been measured and compared to theory predictions, implemented in different Monte Carlo programmes. The figure shows one out of the six extracted cross-sections.

    The uncertainty on the measurements is at the 20­-50% level, dominated by statistical effects. While this does not allow to draw firm conclusions, the data tend to have more events with high-momentum final-state objects than predicted. This effect can be seen in the figure. A quantitative analysis reveals, however, that the tested Monte Carlo models are all statistically compatible with the data. As ATLAS continues to study this channel, the increased size of the data sample and improvements in the predictions should make such comparisons more significant.

    Links:

    Measurement of differential cross-sections of a single top quark produced in association with a W boson at 13 TeV with ATLAS (arXiv: 1712.01602, see figures ).
    Measurement of the cross-section for producing a W boson in association with a single top quark in pp collisions at 13 TeV with ATLAS (arXiv: 1612.07231).
    Measurement of the production cross-section of a single top quark in association with a Z boson in proton-proton collisions at 13 TeV with the ATLAS detector (ATLAS-CONF-2017-052).
    See also the full lists of ATLAS Conference Notes and ATLAS Physics Papers.

    See the full article here .

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

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

    CERN ATLAS Higgs Event

    CERN/ATLAS
    ATLAS

    18th December 2017
    ATLAS Collaboration

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

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

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

    Standard model of Supersymmetry DESY

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

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

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

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

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

    Links:
    See the full article for further references with links.

    See the full article here .

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  • richardmitnick 3:36 pm on September 22, 2017 Permalink | Reply
    Tags: , ATLAS hunts for new physics with dibosons, CERN ATLAS, , , ,   

    From CERN Courier: “ATLAS hunts for new physics with dibosons” 


    CERN Courier

    Sep 22, 2017

    1
    WZ data

    Beyond the Standard Model of particle physics (SM), crucial open questions remain such as the nature of dark matter, the overabundance of matter compared to antimatter in the universe, and also the mass scale of the scalar sector (what makes the Higgs boson so light?). Theorists have extended the SM with new symmetries or forces that address these questions, and many such extensions predict new resonances that can decay into a pair of bosons (diboson), for example: VV, Vh, Vγ and γγ, where V stands for a weak boson (W and Z), h for the Higgs boson, and γ is a photon.

    The ATLAS collaboration has a broad search programme for diboson resonances, and the most recent results using 36 fb–1 of proton–proton collision data at the LHC taken at a centre-of-mass energy of 13 TeV in 2015 and 2016 have now been released. Six different final states characterised by different boson decay modes were considered in searches for a VV resonance: 4ℓ, ℓℓνν, ℓℓqq, ℓνqq, ννqq and qqqq, where ℓ, ν and q stand for charged leptons (electrons and muons), neutrinos and quarks, respectively. For the Vh resonance search, the dominant Higgs boson decay into a pair of b-quarks (branching fraction of 58%) was exploited together with four different V decays leading to ℓℓbb, ℓνbb, ννbb and qqbb final states. A Zγ resonance was sought in final states with two leptons and a photon.

    A new resonance would appear as an excess (bump) over the smoothly distributed SM background in the invariant mass distribution reconstructed from the final-state particles. The left figure shows the observed WZ mass distribution in the qqqq channel together with simulations of some example signals. An important key to probe very high-mass signals is to identify high-momentum hadronically decaying V and h bosons. ATLAS developed a new technique to reconstruct the invariant mass of such bosons combining information from the calorimeters and the central tracking detectors. The resulting improved mass resolution for reconstructed V and h bosons increased the sensitivity to very heavy signals.

    No evidence for a new resonance was observed in these searches, allowing ATLAS to set stringent exclusion limits. For example, a graviton signal predicted in a model with extra spatial dimensions was excluded up to masses of 4 TeV, while heavy weak-boson-like resonances (as predicted in composite Higgs boson models) decaying to WZ bosons are excluded for masses up to 3.3 TeV. Heavier Higgs partners can be excluded up to masses of about 350 GeV, assuming specific model parameters.

    See the full article here .

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    CERN ATLAS New

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  • richardmitnick 6:15 pm on September 18, 2017 Permalink | Reply
    Tags: , , , , CERN ATLAS, , , , ,   

    From BNL: “Three Brookhaven Lab Scientists Selected to Receive Early Career Research Program Funding” 

    Brookhaven Lab

    August 15, 2017 [Just caught up with this via social media.]
    Karen McNulty Walsh,
    kmcnulty@bnl.gov
    (631) 344-8350
    Peter Genzer,
    genzer@bnl.gov
    (631) 344-3174

    Three scientists at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory have been selected by DOE’s Office of Science to receive significant research funding through its Early Career Research Program.

    The program, now in its eighth year, is designed to bolster the nation’s scientific workforce by providing support to exceptional researchers during the crucial early career years, when many scientists do their most formative work. The three Brookhaven Lab recipients are among a total of 59 recipients selected this year after a competitive review of about 700 proposals.

    The scientists are each expected to receive grants of up to $2.5 million over five years to cover their salary plus research expenses. A list of the 59 awardees, their institutions, and titles of research projects is available on the Early Career Research Program webpage.

    This year’s Brookhaven Lab awardees include:

    1
    Sanjaya Senanayake

    Brookhaven Lab chemist Sanjaya D. Senanayake was selected by DOE’s Office of Basic Energy Sciences to receive funding for “Unraveling Catalytic Pathways for

    Low Temperature Oxidative Methanol Synthesis from Methane.” His overarching goal is to study and improve catalysts that enable the conversion of methane (CH4), the primary component of natural gas, directly into methanol (CH3OH), a valuable chemical intermediate and potential renewable fuel.

    This research builds on the recent discovery of a single step catalytic process for this reaction that proceeds at low temperatures and pressures using inexpensive earth abundant catalysts. The reaction promises to be more efficient than current multi-step processes, which are energy-intensive, and a significant improvement over other attempts at one-step reactions where higher temperatures convert most of the useful hydrocarbon building blocks into carbon monoxide and carbon dioxide rather than methanol. With Early Career funding, Senanayake’s team will explore the nature of the reaction, and build on ways to further improve catalytic performance and specificity.

    The project will exploit unique capabilities of facilities at Brookhaven Lab, particularly at the National Synchrotron Light Source II (NSLS-II), that make it possible to study catalysts in real-world reaction environments (in situ) using x-ray spectroscopy, electron imaging, and other in situ methods.

    BNL NSLS-II


    BNL NSLS II

    Experiments using well defined model surfaces and powders will reveal atomic level catalytic structures and reaction dynamics. When combined with theoretical modeling, these studies will help the scientists identify the essential interactions that take place on the surface of the catalyst. Of particular interest are the key features that activate stable methane molecules through “soft” oxidative activation of C-H bonds so methane can be converted to methanol using oxygen (O2) and water (H2O) as co-reactants.

    This work will establish and experimentally validate principles that can be used to design improved catalysts for synthesizing fuel and other industrially relevant chemicals from abundant natural gas.

    “I am grateful for this funding and the opportunity to pursue this promising research,” Senanayake said. “These fundamental studies are an essential step toward overcoming key challenges for the complex conversion of methane into valued chemicals, and for transforming the current model catalysts into practical versions that are inexpensive, durable, selective, and efficient for commercial applications.”

    Sanjaya Senanayake earned his undergraduate degree in material science and Ph.D. in chemistry from the University of Auckland in New Zealand in 2001 and 2006, respectively. He worked as a research associate at Oak Ridge National Laboratory from 2005-2008, and served as a local scientific contact at beamline U12a at the National Synchrotron Light Source (NSLS) at Brookhaven Lab from 2005 to 2009. He joined the Brookhaven staff as a research associate in 2008, was promoted to assistant chemist and associate chemist in 2014, while serving as the spokesperson for NSLS Beamline X7B. He has co-authored over 100 peer reviewed publications in the fields of surface science and catalysis, and has expertise in the synthesis, characterization, reactivity of catalysts and reactions essential for energy conversion. He is an active member of the American Chemical Society, North American Catalysis Society, the American Association for the Advancement of Science, and the New York Academy of Science.

    3
    Alessandro Tricoli

    Brookhaven Lab physicist Alessandro Tricoli will receive Early Career Award funding from DOE’s Office of High Energy Physics for a project titled “Unveiling the Electroweak Symmetry Breaking Mechanism at ATLAS and at Future Experiments with Novel Silicon Detectors.”

    CERN/ATLAS detector

    His work aims to improve, through precision measurements, the search for exciting new physics beyond what is currently described by the Standard Model [SM], the reigning theory 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.

    The discovery of the Higgs boson at the Large Hadron Collider (LHC) at the European Organization for Nuclear Research (CERN) in Switzerland confirmed how the quantum field associated with this particle generates the masses of other fundamental particles, providing key insights into electroweak symmetry breaking—the mass-generating “Higgs mechanism.”

    CERN ATLAS Higgs Event

    But at the same time, despite direct searches for “new physics” signals that cannot be explained by the SM, scientists have yet to observe any evidence for such phenomena at the LHC—even though they know the SM is incomplete (for example it does not include an explanation for gravity).

    Tricoli’s research aims to make precision measurements to test fundamental predictions of the SM to identify anomalies that may lead to such discoveries. He focuses on the analysis of data from the LHC’s ATLAS experiment to comprehensively study electroweak interactions between the Higgs and particles called W and Z bosons. Any discovery of anomalies in such interactions could signal new physics at very high energies, not directly accessible by the LHC.

    This method of probing physics beyond the SM will become even more stringent once the high-luminosity upgrade of ATLAS, currently underway, is completed for longer-term LHC operations planned to begin in 2026.

    Tricoli’s work will play an important role in the upgrade of ATLAS’s silicon detectors, using novel state-of-the art technology capable of precision particle tracking and timing so that the detector will be better able to identify primary particle interactions and tease out signals from the background events. Designing these next-generation detector components could also have a profound impact on the development of future instruments that can operate in high radiation environments, such as in future colliders or in space.

    “This award will help me build a strong team around a research program I feel passionate about at ATLAS and the LHC, and for future experiments,” Tricoli said.

    “I am delighted and humbled by the challenge given to me with this award to take a step forward in science.”

    Alessandro Tricoli received his undergraduate degree in physics from the University of Bologna, Italy, in 2001, and his Ph.D. in particle physics from Oxford University in 2007. He worked as a research associate at Rutherford Appleton Laboratory in the UK from 2006 to 2009, and as a research fellow and then staff member at CERN from 2009 to 2015, receiving commendations on his excellent performance from both institutions. He joined Brookhaven Lab as an assistant physicist in 2016. A co-author on multiple publications, he has expertise in silicon tracker and detector design and development, as well as the analysis of physics and detector performance data at high-energy physics experiments. He has extensive experience tutoring and mentoring students, as well as coordinating large groups of physicists involved in research at ATLAS.

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    Chao Zhang

    Brookhaven Lab physicist Chao Zhang was selected by DOE’s Office of High Energy Physics to receive funding for a project titled, “Optimization of Liquid Argon TPCs for Nucleon Decay and Neutrino Physics.” Liquid Argon TPCs (for Time Projection Chambers) form the heart of many large-scale particle detectors designed to explore fundamental mysteries in particle physics.

    Among the most compelling is the question of why there’s a predominance of matter over antimatter in our universe. Though scientists believe matter and antimatter were created in equal amounts during the Big Bang, equal amounts would have annihilated one another, leaving only light. The fact that we now have a universe made almost entirely of matter means something must have tipped the balance.

    A US-hosted international experiment scheduled to start collecting data in the mid-2020s, called the Deep Underground Neutrino Experiment (DUNE), aims to explore this mystery through the search for two rare but necessary conditions for the imbalance: 1) evidence that some processes produce an excess of matter over antimatter, and 2) a sizeable difference in the way matter and antimatter behave.

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


    FNAL DUNE Argon tank at SURF


    Surf-Dune/LBNF Caverns at Sanford



    SURF building in Lead SD USA

    The DUNE experiment will look for signs of these conditions by studying how protons (one of the two “nucleons” that make up atomic nuclei) decay as well as how elusive particles called neutrinos oscillate, or switch identities, among three known types.

    The DUNE experiment will make use of four massive 10-kiloton detector modules, each with a Liquid Argon Time Projection Chamber (LArTPC) at its core. Chao’s aim is to optimize the performance of the LArTPCs to fully realize their potential to track and identify particles in three dimensions, with a particular focus on making them sensitive to the rare proton decays. His team at Brookhaven Lab will establish a hardware calibration system to ensure their ability to extract subtle signals using specially designed cold electronics that will sit within the detector. They will also develop software to reconstruct the three-dimensional details of complex events, and analyze data collected at a prototype experiment (ProtoDUNE, located at Europe’s CERN laboratory) to verify that these methods are working before incorporating any needed adjustments into the design of the detectors for DUNE.

    “I am honored and thrilled to receive this distinguished award,” said Chao. “With this support, my colleagues and I will be able to develop many new techniques to enhance the performance of LArTPCs, and we are excited to be involved in the search for answers to one of the most intriguing mysteries in science, the matter-antimatter asymmetry in the universe.”

    Chao Zhang received his B.S. in physics from the University of Science and Technology of China in 2002 and his Ph.D. in physics from the California Institute of Technology in 2010, continuing as a postdoctoral scholar there until joining Brookhaven Lab as a research associate in 2011. He was promoted to physics associate III in 2015. He has actively worked on many high-energy neutrino physics experiments, including DUNE, MicroBooNE, Daya Bay, PROSPECT, JUNO, and KamLAND, co-authoring more than 40 peer reviewed publications with a total of over 5000 citations. He has expertise in the field of neutrino oscillations, reactor neutrinos, nucleon decays, liquid scintillator and water-based liquid scintillator detectors, and liquid argon time projection chambers. He is an active member of the American Physical Society.

    See the full article here .

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    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 11:32 am on August 14, 2017 Permalink | Reply
    Tags: , ATLAS sees first direct evidence of light-by-light scattering at high energy, , CERN ATLAS, , ,   

    From ATLAS at CERN: “ATLAS sees first direct evidence of light-by-light scattering at high energy” 

    CERN ATLAS Higgs Event

    CERN/ATLAS
    ATLAS

    14th August 2017
    Katarina Anthony

    1
    A light-by-light scattering candidate event measured in the ATLAS detector. (Image: ATLAS Collaboration/CERN).

    Physicists from the ATLAS experiment at CERN have found the first direct evidence of high energy light-by-light scattering, a very rare process in which two photons – particles of light – interact and change direction. The result, published today in Nature Physics , confirms one of the oldest predictions of quantum electrodynamics (QED).

    “This is a milestone result: the first direct evidence of light interacting with itself at high energy,” says Dan Tovey (University of Sheffield), ATLAS Physics Coordinator. “This phenomenon is impossible in classical theories of electromagnetism; hence this result provides a sensitive test of our understanding of QED, the quantum theory of electromagnetism.”

    Direct evidence for light-by-light scattering at high energy had proven elusive for decades – until the Large Hadron Collider’s second run began in 2015. As the accelerator collided lead ions at unprecedented collision rates, obtaining evidence for light-by-light scattering became a real possibility. “This measurement has been of great interest to the heavy-ion and high-energy physics communities for several years, as calculations from several groups showed that we might achieve a significant signal by studying lead-ion collisions in Run 2,” says Peter Steinberg (Brookhaven National Laboratory), ATLAS Heavy Ion Physics Group Convener.

    Heavy-ion collisions provide a uniquely clean environment to study light-by-light scattering. As bunches of lead ions are accelerated, an enormous flux of surrounding photons is generated. When ions meet at the centre of the ATLAS detector, very few collide, yet their surrounding photons can interact and scatter off one another. These interactions are known as ‘ultra-peripheral collisions’.

    Studying more than 4 billion events taken in 2015, the ATLAS collaboration found 13 candidates for light-by-light scattering. This result has a significance of 4.4 standard deviations, allowing the ATLAS collaboration to report the first direct evidence of this phenomenon at high energy.

    “Finding evidence of this rare signature required the development of a sensitive new ‘trigger’ for the ATLAS detector,” says Steinberg. “The resulting signature — two photons in an otherwise empty detector — is almost the diametric opposite of the tremendously complicated events typically expected from lead nuclei collisions. The new trigger’s success in selecting these events demonstrates the power and flexibility of the system, as well as the skill and expertise of the analysis and trigger groups who designed and developed it.”

    ATLAS physicists will continue to study light-by-light scattering during the upcoming LHC heavy-ion run, scheduled for 2018. More data will further improve the precision of the result and may open a new window to studies of new physics. In addition, the study of ultra-peripheral collisions should play a greater role in the LHC heavy-ion programme, as collision rates further increase in Run 3 and beyond.

    See the full article here .

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  • richardmitnick 8:25 am on August 3, 2017 Permalink | Reply
    Tags: 5 fundamental parameters from top quark decay, , CERN ATLAS, , ,   

    From ATLAS: “5 fundamental parameters from top quark decay” 

    CERN ATLAS Higgs Event

    CERN/ATLAS
    ATLAS

    3rd August 2017
    ATLAS Collaboration

    Number 4 may shock you!

    1
    Figure 1: The radiation of decay products from a polarized top quark follows patterns like those shown here. An analysis of the radiation is used to measure top quark properties in a new analysis. (Image: ATLAS Collaboration/CERN)

    For many physicists, discovering “new physics” means bringing to light a new particle. Another path to discovery lies in carefully measuring the properties of known particles and the interactions between them. The ATLAS experiment has now released new results on the top quark’s interaction with the charged intermediate vector boson.

    While the Higgs boson, which escaped observation until as recently as 2012, is certainly the most intriguing elementary particle, the top quark is arguably second. Heavier than even the Higgs boson, the top quark packs as much mass as a gold nucleus into a single point like constituent. Precise measurements of the top quark have taken a great leap forward at the Large Hadron Collider (LHC), where the production rate is high and the backgrounds are low.

    In the recent paper [JHEP], the ATLAS collaboration presents results obtained from decays of polarized top quarks. The polarization occurs when top quarks are produced singly through the parity-violating weak interaction, rather than in pairs through the parity-conserving strong interaction. Singly-produced top quarks have only recently become an important tool for discovery.

    2
    Figure 2: The plot shows the strength of each of the decay patterns such as those shown in Figure 1. The red solid line is the Standard Model expectation. The data points are the measurement. The measurement is in very good agreement with the Standard Model. (Image: ATLAS Collaboration/CERN)

    Nine distinct decay patterns, examples of which are shown in Figure 1, can be discerned in these decays, similar to antennae patterns. These patterns depend upon five fundamental constants (called f1, f1+,f0+,δ-, and P) governing the interaction between the top, its partner the bottom quark, and the charged weak boson (W±). Current understanding of physics holds that the top quark couplings should be “left-handed” like those of the other quarks, and that they should be identical between the top quarks and its antiparticle, the top antiquark. The new measurements put that understanding to the test.

    By studying the full multidimensional decay patterns, ATLAS measures many properties of the top-bottom-W interaction at the same time and without some of the assumptions that have been made in the past. The analysis is known for both its complexity and its power. Results are shown in Figure 2. Five fundamental constants are determined from the decay. The fourth one, δ–, quantifies the matter-antimatter asymmetry in top quark decays. It is consistent with zero and consistent with the current understanding of fundamental physics. Shocking? Maybe – but physicists are slowly getting used to the idea that the Standard Model is an excellent description of nature.

    The analysis uses proton–antiproton collisions at 8 TeV, data collected at the LHC in 2012. Investigators are now applying even more refined techniques to collisions at a higher centre-of-mass energy of 13 TeV now being collected at CERN. Future measurements at higher precision may finally observe deviations from the Standard Model.

    Links:

    Analysis of the Wtb vertex from the measurement of triple-differential angular decay rates of single top quarks produced in the t-channel at s√ = 8 TeV with the ATLAS detector (arXiv:1707.05393) [Above].
    EPS 2017 presentation by Susana Cabrera Urban: Anomalous couplings in single top and searches for rare top quark couplings with the ATLAS detector
    See also the full lists of ATLAS Conference Notes and ATLAS Physics Papers.

    See the full article here .

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  • richardmitnick 12:01 pm on July 9, 2017 Permalink | Reply
    Tags: , , CERN ATLAS, , , , Probing physics beyond the Standard Model with heavy vector bosons   

    From ATLAS: “Probing physics beyond the Standard Model with heavy vector bosons” 

    CERN ATLAS Higgs Event

    CERN/ATLAS
    ATLAS

    8th July 2017
    ATLAS Collaboration

    1
    Figure 1: The reconstructed mass of the selected candidate events decaying to WW or ZZ bosons, with the qqqq final state. The black markers represent the data. The blue and green curves represent the hypothesized signal for two different masses. The red curve represents the Standard Model processes. (Image: ATLAS Collaboration/CERN)

    Although the discovery of the Higgs boson by the ATLAS and CMS Collaborations in 2012 completed the Standard Model, many mysteries remain unexplained. For instance, why is the mass of the Higgs boson so much lighter than one would expect and why is gravity so weak?

    CERN CMS Higgs Event


    CERN ATLAS Higgs Event

    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.

    Numerous models beyond the Standard Model attempt to explain these mysteries. Some explain the apparent weakness of gravity by introducing additional dimensions of space in which gravity propagates. One model goes beyond that, and considers the real world as a higher-dimensional universe described by warped geometry, which leads to strongly interacting massive graviton states. Other models propose, for example, additional types of Higgs bosons.

    All these models predict the existence of new heavy particles that can decay into pairs of massive weak bosons (WW, WZ or ZZ). The search for such particles has benefited greatly from the increase in the proton–proton collision energy during Run 2 of the Large Hadron Collider (LHC).

    _______________________________________________________________________
    The search for new heavy particles has benefited greatly from the LHC’s increase in proton–proton collision energy.
    _______________________________________________________________________

    2
    Figure 2: The limit on the cross-section times branching ratio of hypothetical particle described by one of the models for the different final states. (Image: ATLAS Collaboration/CERN)

    The W and Z bosons are carrier particles that mediate the weak force. They decay into other Standard Model particles, like charged leptons (l), neutrinos (ν) and quarks (q). These particles are reconstructed differently in the detector. Quarks, for instance, are reconstructed as localized sprays of hadrons, denoted jets. The two bosons could yield several combinations of these particles in the final states. The ATLAS Collaboration has released results on searches involving all relevant decays of the boson pair: ννqq, llqq, lνqq and qqqq (where the lepton is an electron or muon).

    What do these searches have in common? In each of these, at least one of the bosons decays into a pair of quarks. When the sought-after particle is very massive, the two bosons from its decay are ejected with such large momenta that their respective decay products are collimated and the pair of quarks merge into a single large jet. This phenomenon provides a powerful means to distinguish the new physics signal from strong-interaction Standard Model processes. As some exemplary results of the searches, Figure 1 shows the distributions of the reconstructed mass of the candidate particle. Figure 2 shows the limit on the cross-section times branching ratio of a hypothetical particle described by one of the models.

    So far, no evidence of a new particle has been observed. The search continues with increased sensitivity as ATLAS collects more data.

    Links:
    See the full article for further references with links.

    See the full article here .

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  • richardmitnick 7:00 am on July 7, 2017 Permalink | Reply
    Tags: , CERN ATLAS, , , ,   

    From ATLAS: “Why should there be only one? Searching for additional Higgs Bosons beyond the Standard Model” 

    CERN ATLAS Higgs Event

    CERN/ATLAS
    ATLAS

    6th July 2017
    ATLAS Collaboration

    1
    Figure 1: Feynman diagram for leading order production of a neutral MSSM Higgs boson in association with b-quarks. (Image: ATLAS Collaboration/CERN)

    CERN CMS Higgs Event

    Since the discovery of the elusive Higgs boson in 2012, researchers have been looking beyond the Standard Model to answer many outstanding questions. An attractive extension to the Standard Model is Supersymmetry (SUSY), which introduces a plethora of new particles, some of which may be candidates for Dark Matter.

    Standard model of Supersymmetry DESY

    One of the most popular SUSY models – the Minimal Supersymmetric Standard Model (MSSM) – predicts the existence of five Higgs bosons. In this model, the recently discovered Higgs boson (h) would be considered to be the lightest of the set. Two charged Higgs (H+, H–) and two neutral Higgs (A/H) would complete the set, and could exist within a wide range of masses above that of the discovered Higgs boson. The LHC experiments are poised to search for these additional bosons using techniques similar to those used in the initial Higgs searches.

    In July 2017, the ATLAS collaboration presented a new result on the search for neutral (A/H) Higgs bosons decaying to two tau leptons. Taus are particularly interesting to the search as there is a stronger coupling between A/H and down-type fermions (e, μ, τ, d, s, b) for certain values of the MSSM parameter-space. This will enhance the probability of decays to tau leptons, as well as the production of A/H in association with b-quarks (Figure 1), providing a larger cross-section. Like with the Standard Model Higgs boson, gluon-fusion production of A/H remains an important production process in the MSSM to varying degrees (depending on the chosen model parameters). Thus, by classifying events by their probability of containing b-flavoured jets, the ATLAS search has been optimised for both b-associated and gluon-fusion production of A/H, respectively.

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    Figure 2 (left): The observed and expected 95% CL upper limits on the production cross section times di-tau branching fraction for a scalar boson produced via b-associated production. Figure 3 (right): The observed and expected 95% CL limits on tanβ as a function of the mass of the A boson in the hMSSM scenario. The area above the black curve has been excluded. The exclusion arising from the Standard Model Higgs boson coupling measurements and the exclusion limit from the ATLAS 2015 H/A→ ττ search are shown. (Images: ATLAS Collaboration/CERN)

    See the full article here .

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