998 resultados para ATLAS, particle physics, SM, ZZ, aTGC
Resumo:
The ATLAS detector at the Large Hadron Collider is used to search for high-mass resonances decaying to dielectron or dimuon final states. Results are presented from an analysis of proton-proton (pp ) collisions at a center-of-mass energy of 8 TeV corresponding to an integrated luminosity of 20.3fb 1 in the dimuon channel. A narrow resonance with Standard Model Z couplings to fermions is excluded at 95% confidence level for masses less than 2.79 TeV in the dielectron channel, 2.53 TeV in the dimuon channel, and 2.90 TeV in the two channels combined. Limits on other model interpretations are also presented, including a grand-unification model based on the E 6 gauge group, Z bosons, minimal Z' models, a spin-2 graviton excitation from Randall-Sundrum models, quantum black holes, and a minimal walking technicolor model with a composite Higgs boson.
Resumo:
The integrated elliptic flow of charged particles produced in Pb+Pb collisions at sNN = 2.76 TeV has been measured with the ATLAS detector using data collected at the Large Hadron Collider. The anisotropy parameter, v2, was measured in the pseudorapidity range || 2.5 with the event-plane method. In order to include tracks with very low transverse momentum pT, thus reducing the uncertainty in v2 integrated over pT, a 1 b1 data sample recorded without a magnetic field in the tracking detectors is used. The centrality dependence of the integrated v2 is compared to other measurements obtained with higher pT thresholds. The integrated elliptic flow is weakly decreasing with ||. The integrated v2 transformed to the rest frame of one of the colliding nuclei is compared to the lower-energy RHIC data.
Resumo:
The liquid argon calorimeter is a key component of the ATLAS detector installed at the CERN Large Hadron Collider. The primary purpose of this calorimeter is the measurement of electron and photon kinematic properties. It also provides a crucial input for measuring jets and missing transverse momentum. An advanced data monitoring procedure was designed to quickly identify issues that would affect detector performance and ensure that only the best quality data are used for physics analysis. This article presents the validation procedure developed during the 2011 and 2012 LHC data-taking periods, in which more than 98% of the proton-proton luminosity recorded by ATLAS at a centre-of-mass energy of 78 TeV had calorimeter data quality suitable for physics analysis.
Resumo:
The prompt and non-prompt production cross-sections for the c1 and c2 charmonium states are measured in pp collisions at s = 7TeV with the ATLAS detector at the LHC using 4.5 fb1 of integrated luminosity. The c states are reconstructed through the radiative decay c J/ (with J/ +) where photons are reconstructed from e+e conversions. The production rate of the c2 state relative to the c1 state is measured for prompt and non-prompt c as a function of J/ transverse momentum. The prompt c cross-sections are combined with existing measurements of prompt J/ production to derive the fraction of prompt J/ produced in feed-down from c decays. The fractions of c1 and c2 produced in b-hadron decays are also measured.
Resumo:
This paper presents a study of the performance of the muon reconstruction in the analysis of protonproton collisions at s = 7TeV at theLHC, recorded by the ATLAS detector in 2010. This performance is described in terms of reconstruction and isolation efficiencies and momentum resolutions for different classes of reconstructed muons. The results are obtained from an analysis of J/ meson and Z boson decays to dimuons, reconstructed from a data sample corresponding to an integrated luminosity of 40 pb1. The measured performance is compared to Monte Carlo predictions and deviations from the predicted performance are discussed.
Resumo:
A search for strongly produced supersymmetric particles is conducted using signatures involving multiple energetic jets and either two isolated leptons (e or ) with the same electric charge, or at least three isolated leptons. The search also utilises jets originating from b-quarks, missing transverse momentum and other observables to extend its sensitivity. The analysis uses a data sample corresponding to a total integrated luminosity of 20.3 fb1 of ps = 8TeV proton-proton collisions recorded with the ATLAS detector at the Large Hadron Collider in 2012. No deviation from the Standard Model expectation is observed. New or significantly improved exclusion limits are set on a wide variety of supersymmetric models in which the lightest squark can be of the first, second or third generations, and in which R-parity can be conserved or violated.
Resumo:
Many of the interesting physics processes to be measured at the LHC have a signature involving one or more isolated electrons. The electron reconstruction and identification efficiencies of the ATLAS detector at the LHC have been evaluated using protonproton collision data collected in 2011 at s = 7 TeV and corresponding to an integrated luminosity of 4.7 fb1. Tag-and-probe methods using events with leptonic decays of W and Z bosons and J/ mesons are employed to benchmark these performance parameters. The combination of all measurements results in identification efficiencies determined with an accuracy at the few per mil level for electron transverse energy greater than 30 GeV.
Resumo:
The differential cross section for the process Z/ ( = e, ) as a function of dilepton invariant mass is measured in pp collisions at ps = 7TeV at the LHC using the ATLAS detector. The measurement is performed in the e and channels for invariant masses between 26 GeV and 66 GeV using an integrated luminosity of 1.6 fb1 collected in 2011 and these measurements are combined. The analysis is extended to invariant masses as low as 12 GeV in the muon channel using 35 pb1 of data collected in 2010. The cross sections are determined within fiducial acceptance regions and corrections to extrapolate the measurements to the full kinematic range are provided. Next-to-next-to-leading-order QCD predictions provide a significantly better description of the results than next-to-leadingorder QCD calculations, unless the latter are matched to a parton shower calculation.
Resumo:
A measurement of the parity-violating decay asymmetry parameter, b , and the helicity amplitudes for the decay 0 b J/( + ) 0 (p ) is reported. The analysis is based on 1400 0 b and 0 b baryons selected in 4.6fb 1 of protonproton collision data with a center-of-mass energy of 7 TeV recorded by the ATLAS experiment at the LHC. By combining the 0 b and 0 b samples under the assumption of CP conservation, the value of b is measured to be 0.300.16(stat)0.06(syst) . This measurement provides a test of theoretical models based on perturbative QCD or heavy-quark effective theory.
Resumo:
A search is performed for flavour-changing neutral currents in the decay of a top quark to an up-type (c, u) quark and a Higgs boson, where the Higgs boson decays to two photons. The proton-proton collision data set used corresponds to 4.7 fb1 at s = 7TeV and 20.3 fb1 at s = 8TeV collected by the ATLAS experiment at the LHC. Top quark pair events are searched for in which one top quark decays to qH and the other decays to bW. Both the hadronic and the leptonic decay modes of the W boson are used. No significant signal is observed and an upper limit is set on the t qH branching ratio of 0.79% at the 95% confidence level. The corresponding limit on the tqH coupling combination q2t cH + 2t uH is 0.17.
Resumo:
Measurements of four-lepton (4 , =e, ) production cross sections at the Z resonance in pp collisions at the LHC with the ATLAS detector are presented. For dilepton and four-lepton invariant mass regions m + >5GeV and 80<m 4 <100GeV , the measured cross sections are 7618(stat)4(syst)1.4(lumi)fb and 1079(stat)4(syst)3.0(lumi)fb at s =7 and 8 TeV, respectively. By subtracting the nonresonant 4 production contributions and normalizing with Z + events, the branching fraction for the Z boson decay to 4 is determined to be (3.200.25(stat)0.13(syst))10 6 , consistent with the standard model prediction.
Resumo:
Searches for the electroweak production of charginos, neutralinos and sleptons in final states characterized by the presence of two leptons (electrons and muons) and missing transverse momentum are performed using 20.3 fb1 of proton-proton collision data at ps = 8TeV recorded with the ATLAS experiment at the Large Hadron Collider. No significant excess beyond Standard Model expectations is observed. Limits are set on the masses of the lightest chargino, next-to-lightest neutralino and sleptons for different lightest-neutralino mass hypotheses in simplified models. Results are also interpreted in various scenarios of the phenomenological Minimal Supersymmetric Standard Model.
Resumo:
A search is presented for direct top-squark pair production in final states with two leptons (electrons or muons) of opposite charge using 20.3 fb1 of pp collision data at ps = 8TeV, collected by the ATLAS experiment at the Large Hadron Collider in 2012. No excess over the Standard Model expectation is found. The results are interpreted under the separate assumptions (i) that the top squark decays to a b-quark in addition to an on-shell chargino whose decay occurs via a real or virtual W boson, or (ii) that the top squark decays to a t-quark and the lightest neutralino. A top squark with a mass between 150 GeV and 445 GeV decaying to a b-quark and an on-shell chargino is excluded at 95% confidence level for a top squark mass equal to the chargino mass plus 10 GeV, in the case of a 1 GeV lightest neutralino. Top squarks with masses between 215 (90) GeV and 530 (170) GeV decaying to an on-shell (off-shell) t-quark and a neutralino are excluded at 95% confidence level for a 1 GeV neutralino.
Resumo:
A measurement of event-plane correlations involving two or three event planes of different order is presented as a function of centrality for 7 b 1 Pb+Pb collision data at s NN =2.76 TeV, recorded by the ATLAS experiment at the Large Hadron Collider. Fourteen correlators are measured using a standard event-plane method and a scalar-product method, and the latter method is found to give a systematically larger correlation signal. Several different trends in the centrality dependence of these correlators are observed. These trends are not reproduced by predictions based on the Glauber model, which includes only the correlations from the collision geometry in the initial state. Calculations that include the final-state collective dynamics are able to describe qualitatively, and in some cases also quantitatively, the centrality dependence of the measured correlators. These observations suggest that both the fluctuations in the initial geometry and the nonlinear mixing between different harmonics in the final state are important for creating these correlations in momentum space.
