30 research outputs found
Search for high-mass dilepton resonances in pp collisions at root s=8 TeV with the ATLAS detector
No full textSee paper for full list of authors - 14 pages plus author list + cover page (31 pages total), 5 figures, 4 tables, submitted to Physical Review D, All figures including auxiliary figures are available at http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/EXOT-2012-19/International audienceThis article describes a search for high-mass resonances decaying to a pair of photons using a sample of fb of collisions at TeV recorded with the ATLAS detector at the Large Hadron Collider. The data are found to be in agreement with the Standard Model prediction, and limits are reported in the framework of the Randall-Sundrum model. This theory leads to the prediction of graviton states, the lightest of which could be observed at the Large Hadron Collider. A lower limit of () TeV at 95% confidence level is set on the mass of the lightest graviton for couplings of ()
Search for new resonances decaying to a or boson and a Higgs boson in the , , and channels with collisions at TeV with the ATLAS detector
No full textSee paper for full list of authors, 18 pages (plus author list + cover pages: 36 pages total), 13 figures, 1 table. Submitted to PLB. All figures including auxiliary figures are available at https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/EXOT-2015-18/International audienceA search is presented for new resonances decaying to a or boson and a Higgs boson in the , , and channels in collisions at TeV with the ATLAS detector at the Large Hadron Collider using a total integrated luminosity of 3.2 fb. The search is conducted by looking for a localized excess in the / invariant or transverse mass distribution. No significant excess is observed, and the results are interpreted in terms of constraints on a simplified model based on a phenomenological Lagrangian of heavy vector triplets
Search for massive, long-lived particles using multitrack displaced vertices or displaced lepton pairs in pp collisions at = 8 TeV with the ATLAS detector
No full textSee paper for full list of authors - 25 pages plus author list + cover pages (38 pages total), 20 figures, 3 tables, submitted to Phys. Rev. D, All figures including auxiliary figures are available at http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/SUSY-2014-02/International audienceMany extensions of the Standard Model posit the existence of heavy particles with long lifetimes. This article presents the results of a search for events containing at least one long-lived particle that decays at a significant distance from its production point into two leptons or into five or more charged particles. This analysis uses a data sample of proton-proton collisions at = 8 TeV corresponding to an integrated luminosity of 20.3 fb collected in 2012 by the ATLAS detector operating at the Large Hadron Collider. No events are observed in any of the signal regions, and limits are set on model parameters within supersymmetric scenarios involving R-parity violation, split supersymmetry, and gauge mediation. In some of the search channels, the trigger and search strategy are based only on the decay products of individual long-lived particles, irrespective of the rest of the event. In these cases, the provided limits can easily be reinterpreted in different scenarios
Measurements of the Total and Differential Higgs Boson Production Cross Sections Combining the H→γγ and H→ZZ∗→4ℓ Decay Channels at s√=8 TeV with the ATLAS Detector
No full text6 pages plus author list + cover page + supplemental material (26 pages total), 7 figures, 23 tables, submitted to Physical Review Letters. All figures including auxiliary figures are available at this http URL - See paper for full list of authorsInternational audienceMeasurements of the total and differential cross sections of Higgs boson production are performed using 20.3 fb−1 of pp collisions produced by the Large Hadron Collider at a center-of-mass energy of s√=8 TeV and recorded by the ATLAS detector. Cross sections are obtained from measured H→γγ and H→ZZ∗→4ℓ event yields, which are combined accounting for detector efficiencies, fiducial acceptances and branching fractions. Differential cross sections are reported as a function of Higgs boson transverse momentum, Higgs boson rapidity, number of jets in the event, and transverse momentum of the leading jet. The total production cross section is determined to be σpp→H=33.0±5.3(stat)±1.6(sys)pb. The measurements are compared to state-of-the-art predictions
A measurement of material in the ATLAS tracker using secondary hadronic interactions in 7 TeV pp collisions
No full textSee paper for full list of authors, 28 pages plus author list + cover pages (45 pages total),14 figures, 8 tables, submitted to Journal of Instrum., All figures including auxiliary figures are available at https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/PERF-2015-06/International audienceKnowledge of the material in the ATLAS inner tracking detector is crucial in understanding the reconstruction of charged-particle tracks, the performance of algorithms that identify jets containing b-hadrons and is also essential to reduce background in searches for exotic particles that can decay within the inner detector volume. Interactions of primary hadrons produced in pp collisions with the material in the inner detector are used to map the location and amount of this material. The hadronic interactions of primary particles may result in secondary vertices, which in this analysis are reconstructed by an inclusive vertex-finding algorithm. Data were collected using minimum-bias triggers by the ATLAS detector operating at the LHC during 2010 at centre-of-mass energy = 7 TeV, and correspond to an integrated luminosity of nb. Kinematic properties of these secondary vertices are used to study the validity of the modelling of hadronic interactions in simulation. Secondary-vertex yields are compared between data and simulation over a volume of about 0.7 m around the interaction point, and agreement is found within overall uncertainties
Beam-induced and cosmic-ray backgrounds observed in the ATLAS detector during the LHC 2012 proton-proton running period
No full textSee paper for full list of authors - 65 pages plus author list (82 pages total), 60 figures, 4 tables, submitted to JINST, all figures are available at https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/DAPR-2014-01/International audienceThis paper discusses various observations on beam-induced and cosmic-ray backgrounds in the ATLAS detector during the LHC 2012 proton-proton run. Building on published results based on 2011 data, the correlations between background and residual pressure of the beam vacuum are revisited. Ghost charge evolution over 2012 and its role for backgrounds are evaluated. New methods to monitor ghost charge with beam-gas rates are presented and observations of LHC abort gap population by ghost charge are discussed in detail. Fake jets from colliding bunches and from ghost charge are analysed with improved methods, showing that ghost charge in individual radio-frequency buckets of the LHC can be resolved. Some results of two short periods of dedicated cosmic-ray background data-taking are shown; in particular cosmic-ray muon induced fake jet rates are compared to Monte Carlo simulations and to the fake jet rates from beam background. A thorough analysis of a particular LHC fill, where abnormally high background was observed, is presented. Correlations between backgrounds and beam intensity losses in special fills with very high are studied
Performance of the ATLAS Transition Radiation Tracker in Run 1 of the LHC: tracker properties
No full textSee paper for full list of authors - 45 pages in total, author list starting page 29, 23 figures, 0 tables, submitted to JINST, All figures are available at https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/IDET-2015-01International audienceThe tracking performance parameters of the ATLAS Transition Radiation Tracker (TRT) as part of the ATLAS inner detector are described in this paper for different data-taking conditions in proton-proton, proton-lead and lead-lead collisions at the Large Hadron Collider (LHC). The performance is studied using data collected for different data-taking conditions in proton-proton, proton-lead and lead-lead collisions at the Large Hadron Collider (LHC). The performance is studied using data collected during the first period of LHC operation (Run 1) and is compared with Monte Carlo simulations. The performance of the TRT, operating with two different gas mixtures (xenon-based and argon-based) and its dependence on the TRT occupancy is presented. These studies show that the tracking performance of the TRT is similar for the two gas mixtures and that a significant contribution to the particle momentum resolution is made by the TRT up to high particle densities
Search for top squarks in final states with one isolated lepton, jets, and missing transverse momentum in TeV collisions with the ATLAS detector
No full textSee paper for full list of authors - 30 pages plus author list + cover pages (48 pages total), 9 figures, 4 tables, submitted to PRD, All figures including auxiliary figures are available at http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/SUSY-2015-02/International audienceThe results of a search for the stop, the supersymmetric partner of the top quark, in final states with one isolated electron or muon, jets, and missing transverse momentum are reported. The search uses the 2015 LHC collision data at a center-of-mass energy of TeV recorded by the ATLAS detector and corresponding to an integrated luminosity of 3.2 fb. The analysis targets two types of signal models: gluino-mediated pair production of stops with a nearly mass-degenerate stop and neutralino; and direct pair production of stops, decaying to the top quark and the lightest neutralino. The experimental signature in both signal scenarios is similar to that of a top quark pair produced in association with large missing transverse momentum. No significant excess over the Standard Model background prediction is observed, and exclusion limits on gluino and stop masses are set at 95% confidence level. The results extend the LHC Run-1 exclusion limit on the gluino mass up to 1460 GeV in the gluino-mediated scenario in the high gluino and low stop mass region, and add an excluded stop mass region from 745 to 780 GeV for the direct stop model with a massless lightest neutralino. The results are also reinterpreted to set exclusion limits in a model of vector-like top quarks
Measurement of the correlation between flow harmonics of different order in lead-lead collisions at =2.76 TeV with the ATLAS detector
No full textSee paper for full list of authors – 22 pages plus author list + cover pages (35 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-2014-03/International audienceCorrelations between the elliptic or triangular flow coefficients (=2 or 3) and other flow harmonics (=2 to 5) are measured using TeV Pb+Pb collision data collected in 2010 by the ATLAS experiment at the LHC, corresponding to an integrated lumonisity of 7 b. The - correlations are measured in midrapidity as a function of centrality, and, for events within the same centrality interval, as a function of event ellipticity or triangularity defined in a forward rapidity region. For events within the same centrality interval, is found to be anticorrelated with and this anticorrelation is consistent with similar anticorrelations between the corresponding eccentricities and . On the other hand, it is observed that increases strongly with , and increases strongly with both and . The trend and strength of the - correlations for =4 and 5 are found to disagree with - correlations predicted by initial-geometry models. Instead, these correlations are found to be consistent with the combined effects of a linear contribution to and a nonlinear term that is a function of or of , as predicted by hydrodynamic models. A simple two-component fit is used to separate these two contributions. The extracted linear and nonlinear contributions to and are found to be consistent with previously measured event-plane correlations
Study of the material of the ATLAS inner detector for Run 2 of the LHC
No full textInternational audienceThe ATLAS inner detector comprises three different sub-detectors: the pixel detector, the silicon strip tracker, and the transition-radiation drift-tube tracker. The Insertable B-Layer, a new innermost pixel layer, was installed during the shutdown period in 2014, together with modifications to the layout of the cables and support structures of the existing pixel detector. The material in the inner detector is studied with several methods, using a low-luminosity √s=13 TeV pp collision sample corresponding to around 2.0 nb−1 collected in 2015 with the ATLAS experiment at the LHC. In this paper, the material within the innermost barrel region is studied using reconstructed hadronic interaction and photon conversion vertices. For the forward rapidity region, the material is probed by a measurement of the efficiency with which single tracks reconstructed from pixel detector hits alone can be extended with hits on the track in the strip layers. The results of these studies have been taken into account in an improved description of the material in the ATLAS inner detector simulation, resulting in a reduction in the uncertainties associated with the charged-particle reconstruction efficiency determined from simulation
