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CERN-LHC. A search for Supersymmetry involving the pair production of gluinos decaying via third-generation squarks to the lightest neutralino is reported.
CERN-LHC. Study of events with a final state comprising two photons and missing transverse energy in proton-proton interactions at a centre-of-mass energy of 7 TeV using a data sample, collected in 2011, with an integrated luminosity of 4.8 fb-1. The analysis is focussed on a search for photons originating in the decay of neutral long-lived particles, exploiting the electromagnetic calorimeters ability to make measurements of the flight direction of the photons as well as excellent time resolution. Results are compared with Standard Model (SM ) expections as well as the predictions from GMSB models.
CERN-LHC. Measurements of the cross sections for Wgamma and Zgamma production in proton proton collisions at a centre-of mass energy of 7 TeV. The integrated and differential fiducial cross sections for the production of a W or Z boson in association with a high-energy photon are measured at sqrt(s) = 7 TeV. The analyses use a data sample with an integrated luminosity of 4.6 fb^{-1} collected by the ATLAS detector during the 2011 LHC data-taking period. Events are selected using leptonic decays of the W and Z bosons (W(e nu, mu nu) and Z(e+ e-, mu+ mu-, nu nubar)) with the requirement of an associated isolated photon. The data are used to test the electroweak sector of the Standard Model and search for evidence for new phenomena. The measurements are used to probe the anomalous WWgamma, ZZgamma and Zgammagamma triple-gauge-boson couplings and to search for the production of vector resonances decaying to Zgamma and Wgamma.
CERN-LHC. Double-differential dijet cross sections measured in pp collisions at the LHC with a 7 TeV centre-of-mass energy are presented as functions of dijet mass and rapidity separation of the two highest-pT jets. These measurements are obtained using data corresponding to an integrated luminosity of 4.5 fb^-1, recorded by the ATLAS detector in 2011. The data are corrected for detector effects so that cross sections are presented at the particle level. Cross sections are measured up to 5 TeV dijet mass using jets reconstructed with the anti-kt algorithm for values of the jet radius parameter of 0.4 and 0.6.
CERN-LHC. Measurements are presented of differential cross sections for top quark pair production in pp collisions at sqrt(s) = 7 TeV relative to the total inclusive top quark pair production cross section. A data sample of 2.05 fb^-1 recorded by the ATLAS detector between March and August 2011 at the Large Hadron Collider is used. Relative differential cross sections are derived as a function of the invariant mass, the transverse momentum and the rapidity of the top quark pair system. Events are selected in the lepton (electron or muon) + jets channel. The background-subtracted differential distributions are corrected for detector effects and normalized to the total inclusive top quark pair production cross section. The measurement uncertainties range typically between 10% and 20% and are generally dominated by systematic effects.
CERN-LHC. A model-independent search is made for the production of new resonances in photon + jet events using 20 fb^-1 of proton-proton LHC data recorded with the ATLAS detector in 2012 at a centre-of-mass energy of sqrt(s) = 8 TeV. The photon + jet mass distribution is compared to a background model fit from data; no significant deviation from the background-only hypothesis is found. Limits are set at 95% credibility level on generic Gaussian-shaped signals and two benchmark phenomena beyond the Standard Model: non-thermal quantum black holes and excited quarks. Non-thermal quantum black holes are excluded below masses of 4.6 TeV and excited quarks are excluded below masses of 3.5 TeV.
CERN-LHC. Measurement of the production cross section of events with two isolated photons in the final state, in proton-proton collisions at sqrt(s) = 7 TeV, by the ATLAS experiment at the LHC. The full data set collected in 2011, corresponding to an integrated luminosity of 4.9 fb^-1, is used. The amount of background, from hadronic jets and isolated electrons, is estimated with data-driven techniques and subtracted. The total cross section, for two isolated photons with transverse energies above 25 GeV and 22 GeV respectively, in the acceptance of the electromagnetic calorimeter (|eta| < 1.37 and 1.52 < |eta| < 2.37) and with an angular separation Delta(R) > 0.4, is 44.0 +3.2,-4.2 pb. The differential cross sections as a function of the di-photon invariant mass, transverse momentum, azimuthal separation, and cosine of the polar angle of the largest transverse energy photon in the Collins-Soper di-photon rest frame are also measured.
CERN-LHC. A search is presented for dark matter pair production in association with a W or Z boson in pp collisions representing 20.3 fb^-1 of integrated luminosity at sqrt(s) = 8 TeV using data recorded with the ATLAS detector in 2012 at the Large Hadron Collider. Events with a hadronic jet with the jet-mass consistent with a W or Z boson, and with large missing transverse momentum are analyzed. The data are consistent with the Standard Model expectations, and limits are set on the mass scale in effective field theories that describe the interaction of dark matter and Standard Model particles.
CERN-LHC. The results of a search for pair production of supersymmetric partners of the Standard Model third-generation quarks are reported. This search uses 20.1 fb^-1 of pp collisions at sqrt(s) = 8 TeV collected during the period from March to December 2012 by the ATLAS experiment at the Large Hadron Collider. The lightest bottom and top squarks (b1 and t1 respectively) are searched for in a final state with large missing transverse momentum and two jets identified as originating from b-quarks. No excess of events above the expected level of Standard Model background is found. The results are used to set upper limits on the visible cross section for processes beyond the Standard Model. Exclusion limits at the 95% confidence level on the masses of the third-generation squarks are derived in phenomenological supersymmetric R-parity-conserving models in which either the bottom or the top squark is the lightest squark. The b1 is assumed to decay via b1 --> b chi1^0 (where chi1^0 is the lightest neutralino) and the t1 via t1 --> b chi1^{+/-} (where chi1^{+/-} is the lightest chargino), with undetectable products of the subsequent decay of the chi1^{+/-} due to the small mass splitting between the chi1^{+/-} and the chi1^0.
CERN-LHC. Search for quantum black-hole production using 20.3 inverse fb of data collected in 2012 with the ATLAS detector in pp collisions at the LHC at sqrt(s) = 8 TeV. The quantum black holes are assumed to decay into a lepton (electron or muon) and a jet. In either channel, no event with a lepton-jet invariant mass of 3.5 TeV or more is observed, consistent with the expected background. Limits are set on the product of cross sections and branching fractions for the lepton+jet final states of quantum black holes produced in a search region for invariant masses above 1 TeV. The combined 95% confidence level upper limit on this product for quantum black holes with threshold mass above 3.5 TeV is 0.18 fb. This limit constrains the threshold quantum black-hole mass to be above 5.3 TeV in the model considered.
CERN-LHC. The process P P --> W J/PSI X provides a powerful probe of the production mechanism of charmonium in hadronic collisions, and is also sensitive to multiple parton interactions in the colliding protons. Using the 2011 ATLAS dataset of 4.5 fb^-1 of sqrt(s) = 7 TeV pp collisions at the LHC, the first observation is made of the production of W + prompt J/psi events in hadronic collisions, using W --> MU NU and J/PSI --> MU MU. A yield of 27.4 +7.5,-6.5 W + prompt J/psi events is observed, with a statistical significance of 5.1 sigma. The production rate as a ratio to the inclusive W boson production rate is measured, and the double parton scattering contribution to the cross section is estimated.
CERN-LHC. Measurements of the cross section for ZZ production using the 4l and 2l2nu decay channels in proton-proton collisions at a centre-of-mass energy of 7 TeV with 4.6 fb^-1 of data collected in 2011. The final states used are 4 electrons, 4 muons, 2 electrons and 2 muons, 2 electrons and missing transverse momentum, and 2 muons and missing transverse momentum (MET).
CERN-LHC. Measurement of the jet shapes in top-quark pair events from proton-proton collisions at a centre-of-mass energy of 7 TeV. The analysis uses a data sample of integrated luminosity 1.8 fb-1. Jets are reconstructed using the anti-kT algorithm with jet radius R=0.4. Jet shapes are presented as differential and integrated defined as: Differential (RHO) - scalar sum of jet PT in an annulus of thickness deltaR around radius r / scalar sum of jet PT in the cone of radius R. Integrated (PSI) - scalar sum of jet PT in cone radius r / scalar sum of jet PT in cone radius R. Data are shown separated into b-jets and light-jet events.
CERN-LHC. A search is presented for direct chargino production based on a disappearing-track signature using 20.3 fb^-1 of proton-proton collisions at sqrt(s) = 8 TeV collected in 2012 with the ATLAS experiment at the LHC. In anomaly-mediated supersymmetry breaking (AMSB) models, the lightest chargino is nearly mass-degenerate with the lightest neutralino and its lifetime is long enough to be detected in the tracking detectors by identifying decays that result in tracks with no associated hits in the outer region of the tracking system. Some models with supersymmetry also predict charginos with a significant lifetime. This analysis attains sensitivity for charginos with a lifetime between 0.1 ns and 10 ns, and significantly surpasses the reach of the LEP experiments. No significant excess above the background expectation is observed for candidate tracks with large transverse momentum, and constraints on chargino properties are obtained. In the AMSB scenarios, a chargino mass below 270 GeV is excluded at 95% confidence level.
CERN-LHC. The distributions of event-by-event harmonic flow coefficients vn for n = 2-4 are measured in sqrt(sNN) = 2.76 TeV Pb+Pb collisions using the ATLAS detector at the LHC. The measurements are performed using charged particles with transverse momentum pT > 0.5 GeV and in the pseudorapidity range |eta| < 2.5 in a dataset of approximately 7 mub^-1 recorded in 2010. The shapes of the vn distributions suggest that the associated flow vectors are described by a two-dimensional Gaussian function in central collisions for v2 and over most of the measured centrality range for v3 and v4. Significant deviations from this function are observed for v2 in mid-central and peripheral collisions, and a small deviation is observed for v3 in mid-central collisions. In order to be sensitive to these deviations, it is shown that the commonly used multi-particle cumulants, involving four particles or more, need to be measured with a precision better than a few percent. The vn distributions are also measured independently for charged particles with 0.5 < pT < 1 GeV and pT > 1 GeV. When these distributions are rescaled to the same mean values, the adjusted shapes are found to be nearly the same for these two pT ranges. The vn distributions are compared with the eccentricity distributions from two models for the initial collision geometry: a Glauber model and a model that includes corrections to the initial geometry due to gluon saturation effects. Both models fail to describe the experimental data consistently over most of the measured centrality range.
CERN-LHC. Two-particle correlations in relative azimuthal angle, Delta(phi), and pseudorapidity, Delta(eta), are measured in sqrt(sNN) = 5.02 TeV p+Pb collisions recorded in September 2012 using the ATLAS detector at the LHC. The measurements are performed using approximately 1 mub^1 of data as a function of transverse momentum (pT) and the transverse energy summed over 3.1 < eta < 4.9 in the direction of the Pb beam. The correlation function, constructed from charged particles, exhibits a long-range (2 < |Delta(eta)| < 5) near-side (Delta(phi) ~ 0) correlation that grows rapidly with increasing transverse energy. A long-range away-side (Delta(phi) ~ pi) correlation, obtained by subtracting the expected contributions from recoiling dijets and other sources estimated using events with small transverse energy, is found to match the near-side correlation in magnitude, shape (in Delta(eta) and Delta(phi)) and transverse energy dependence. The resultant Delta(phi) correlation is approximately symmetric about pi/2, and is consistent with a dominant cos(2*Delta(phi)) modulation for all transverse energy ranges and particle pT.
CERN-LHC. A search for the direct production of charginos and neutralinos in final states with three leptons and missing transverse momentum is presented. The analysis is based on 20.3 fb^-1 of sqrt(s) = 8 TeV proton--proton collision data delivered by the Large Hadron Collider and recorded in 2012 with the ATLAS detector. Observations are consistent with the Standard Model expectations and limits are set in R-parity-conserving phenomenological Minimal Supersymmetric Standard Models and in simplified supersymmetric models, significantly extending previous results. For simplified supersymmetric models of direct chargino (C1) and next-to-lightest neutralino (N2) production with decays to lightest neutralino (N1) via either all three generations of sleptons, staus only, gauge bosons, or Higgs bosons, C1 and N2 masses are excluded up to 700 GeV, 380 GeV, 345 GeV, or 148 GeV respectively, for a massless N1.
CERN-LHC. The production of a W boson in association with a single charm quark is studied using 4.6 fb^-1 of pp collision data at sqrt(s) = 7 TeV collected in 2011 with the ATLAS detector at the Large Hadron Collider. In events in which a W boson decays to an electron or muon, the charm quark is tagged either by its semileptonic decay to a muon or by the presence of a charmed meson. The integrated and differential cross sections as a function of the pseudorapidity of the lepton from the W-boson decay are measured. Results are compared to the predictions of next-to-leading-order QCD calculations obtained from various parton distribution function parameterisations. The ratio of the strange-to-down sea quark distributions is determined to be 0.96 +0.26,-0.30 at Q^2 = 1.9 GeV^2 which supports the hypothesis of an SU(3) symmetric composition of the light quark sea. Additionally, the cross-section ratio sigma(W^+ + cbar)/sigma(W^- + c) is compared to the predictions obtained using parton distribution function parameterisations with different assumptions on the s-sbar asymmetry.