13,159 research outputs found

    LHC Coverage of RPV MSSM with Light Stops

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    We examine the sensitivity of recent LHC searches to signatures of supersymmetry with R-parity violation (RPV). Motivated by naturalness of the Higgs potential, which would favor light third-generation squarks, and the stringent LHC bounds on spectra in which the gluino or first and second generation squarks are light, we focus on scenarios dominated by the pair production of light stops. We consider the various possible direct and cascade decays of the stop that involve the trilinear RPV operators. We find that in many cases, the existing searches exclude stops in the natural mass range and beyond. However, typically there is little or no sensitivity to cases dominated by UDD operators or LQD operators involving taus. We propose several ideas for searches which could address the existing gaps in experimental coverage of these signals.Comment: 41 pages, 12 figures; v2: included new searches (see footnote 10), minor corrections and improvement

    Standard Model Higgs boson searches with the ATLAS detector at the Large Hadron Collider

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    The investigation of the mechanism responsible for electroweak symmetry breaking is one of the most important tasks of the scientific program of the Large Hadron Collider. The experimental results on the search of the Standard Model Higgs boson with 1 to 2 fb^-1 of proton proton collision data at sqrt s=7 TeV recorded by the ATLAS detector are presented and discussed. No significant excess of events is found with respect to the expectations from Standard Model processes, and the production of a Higgs boson is excluded at 95% Confidence Level for the mass regions 144-232, 256-282 and 296-466 GeV.Comment: Proceedings of the Lepton Photon 2011 Conference, to appear in "Pramana - journal of phsyics". 11 pages, 13 figure

    Recent Heavy Ion Results with the ATLAS Detector at the LHC

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    Results are presented from the ATLAS collaboration from the 2010 LHC heavy ion run, during which nearly 10 inverse microbarns of luminosity were delivered. Soft physics results include charged particle multiplicities and collective flow. The charged particle multiplicity, which tracks initial state entropy production, increases by a factor of two relative to the top RHIC energy, with a centrality dependence very similar to that already measured at RHIC. Measurements of elliptic flow out to large transverse momentum also show similar results to what was measured at RHIC, but no significant pseudorapidity dependence. Extensions of these measurements to higher harmonics have also been made, and can be used to explain structures in the two-particle correlation functions that had long been attributed to jet-medium interactions. New hard probe measurements include single muons, jets and high pTp_T hadrons. Single muons at high momentum are used to extract the yield of W±W^{\pm} bosons and are found to be consistent within statistical uncertainties with binary collision scaling. Conversely, jets are found to be suppressed in central events by a factor of two relative to peripheral events, with no significant dependence on the jet energy. Fragmentation functions are also found to be the same in central and peripheral events. Finally, charged hadrons have been measured out to 30 GeV, and their centrality dependence relative to peripheral events is similar to that found for jets.Comment: 9 pages, 9 figures, proceedings for Quark Matter 2011, Annecy, France, May 23-28, 201

    Properties of jets measured from tracks in proton-proton collisions at center-of-mass energy sqrt(s) = 7 TeV with the ATLAS detector

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    Jets are identified and their properties studied in center-of-mass energy sqrt(s) = 7 TeV proton-proton collisions at the Large Hadron Collider using charged particles measured by the ATLAS inner detector. Events are selected using a minimum bias trigger, allowing jets at very low transverse momentum to be observed and their characteristics in the transition to high-momentum fully perturbative jets to be studied. Jets are reconstructed using the anti-kt algorithm applied to charged particles with two radius parameter choices, 0.4 and 0.6. An inclusive charged jet transverse momentum cross section measurement from 4 GeV to 100 GeV is shown for four ranges in rapidity extending to 1.9 and corrected to charged particle-level truth jets. The transverse momenta and longitudinal momentum fractions of charged particles within jets are measured, along with the charged particle multiplicity and the particle density as a function of radial distance from the jet axis. Comparison of the data with the theoretical models implemented in existing tunings of Monte Carlo event generators indicates reasonable overall agreement between data and Monte Carlo. These comparisons are sensitive to Monte Carlo parton showering, hadronization, and soft physics models.Comment: 15 pages plus author list (27 pages total), 6 figures, 3 tables, final version published in Physical Review

    Search for pair production of Higgs bosons in the bbˉbbˉb\bar{b}b\bar{b} final state using proton--proton collisions at s=13\sqrt{s} = 13 TeV with the ATLAS detector

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    A search for Higgs-boson pair production in the bbˉbbˉb\bar{b}b\bar{b} final state is carried out with 3.2 fb1^{-1} of proton--proton collision data collected at s=13\sqrt{s} = 13 TeV with the ATLAS detector. The data are consistent with the estimated background and are used to set upper limits on the production cross section of Higgs-boson pairs times branching ratio to bbˉbbˉb\bar{b}b\bar{b} for both nonresonant and resonant production. In the case of resonant production of Kaluza--Klein gravitons within the Randall--Sundrum model, upper limits in the 24 to 91 fb range are obtained for masses between 600 and 3000 GeV, at the 95% confidence level. The production cross section times branching ratio for nonresonant Higgs-boson pairs is also constrained to be less than 1.22 pb, at the 95% confidence level.Comment: 25 pages plus author list + cover pages (43 pages total), 10 figures, 7 tables, published in Phys. Rev. D, All figures including auxiliary figures are available at https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/EXOT-2015-1

    Measurement of long-range pseudorapidity correlations and azimuthal harmonics in sNN\sqrt{s_{\mathrm{NN}}} = 5.02 TeV proton-lead collisions with the ATLAS detector

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    Measurements of two-particle correlation functions and the first five azimuthal harmonics, v1v_1 to v5v_5, are presented, using 28 nb1\mathrm{nb}^{-1} of pp+Pb collisions at a nucleon-nucleon center-of-mass energy of sNN=5.02\sqrt{s_{\mathrm{NN}}}=5.02 TeV measured with the ATLAS detector at the LHC. Significant long-range ``ridge-like'' correlations are observed for pairs with small relative azimuthal angle (Δϕ<π/3|\Delta\phi|<\pi/3) and back-to-back pairs (Δϕ>2π/3|\Delta\phi|>2\pi/3) over the transverse momentum range 0.4<pT<120.4<p_{\rm T}<12 GeV and in different intervals of event activity. The event activity is defined by either the number of reconstructed tracks or the total transverse energy on the Pb-fragmentation side. The azimuthal structure of such long-range correlations is Fourier decomposed to obtain the harmonics vnv_n as a function of pTp_{\rm T} and event activity. The extracted vnv_n values for n=2n=2 to 55 decrease with nn. The v2v_2 and v3v_3 values are found to be positive in the measured pTp_{\rm T} range. The v1v_1 is also measured as a function of pTp_{\rm T} and is observed to change sign around pT1.52.0p_{\rm T}\approx 1.5-2.0 GeV and then increase to about 0.1 for pT>4p_{\rm T}>4 GeV. The v2(pT)v_2(p_{\rm T}), v3(pT)v_3(p_{\rm T}) and v4(pT)v_4(p_{\rm T}) are compared to the vnv_n coefficients in Pb+Pb collisions at sNN=2.76\sqrt{s_{\mathrm{NN}}} =2.76 TeV with similar event multiplicities. Reasonable agreement is observed after accounting for the difference in the average pTp_{\rm T} of particles produced in the two collision systems.Comment: 21 pages plus author list + cover pages (34 pages total), 16 figures, submitted to Phys. Rev. C., All figures including auxiliary figures can be found at https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HION-2013-04

    Search for the Standard Model Higgs boson in the diphoton decay channel with 4.9 fb-1 of pp collisions at sqrt(s) = 7 TeV with ATLAS

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    A search for the Standard Model Higgs boson is performed in the diphoton decay channel. The data used corresponds to an integrated luminosity of 4.9 fb-1 collected with the ATLAS detector at the Large Hadron Collider in proton-proton collisions at a center-of-mass energy of sqrt(s) = 7 TeV. In the diphoton mass range 110-150 GeV, the largest excess with respect to the background-only hypothesis is observed at 126.5 GeV, with a local significance of 2.8 standard deviations. Taking the look-elsewhere effect into account in the range 110-150 GeV, this significance becomes 1.5 standard deviations. The Standard Model Higgs boson is excluded at 95% confidence level in the mass ranges of 113-115 GeV and 134.5-136 GeV.Comment: 6 pages plus author list (19 pages total), 4 figures, 3 tables, matches published version in Physical Review Letter
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