998 resultados para ATLAS, particle physics, SM, ZZ, aTGC
Resumo:
This paper presents a measurement of the cross-section for high transverse momentum W and Z bosons produced in pp collisions and decaying to allhadronic final states. The data used in the analysis were recorded by the ATLAS detector at the CERN Large Hadron Collider at a centre-of-mass energy of s = 7 TeV and correspond to an integrated luminosity of 4.6 fb1. The measurement is performed by reconstructing the boosted W or Z bosons in single jets. The reconstructed jet mass is used to identify the W and Z bosons, and a jet substructure method based on energy cluster information in the jet centre-of mass frame is used to suppress the large multi-jet background. The cross-section for events with a hadronically decaying W or Z boson, with transverse momentum pT > 320 GeV and pseudorapidity || < 1.9, is measured to be + = W Z 8.5 1.7 pb and is compared to next-to-leading-order calculations. The selected events are further used to study jet grooming techniques.
Resumo:
Results of a search for supersymmetry via direct production of third-generation squarks are reported, using 20.3fb 1 of proton-proton collision data at s =8TeV recorded by the ATLAS experiment at the LHC in 2012. Two different analysis strategies based on monojetlike and c -tagged event selections are carried out to optimize the sensitivity for direct top squark-pair production in the decay channel to a charm quark and the lightest neutralino (t 1 c+ 0 1 ) across the top squarkneutralino mass parameter space. No excess above the Standard Model background expectation is observed. The results are interpreted in the context of direct pair production of top squarks and presented in terms of exclusion limits in the m t 1, m X0 1 ) parameter space. A top squark of mass up to about 240 GeV is excluded at 95% confidence level for arbitrary neutralino masses, within the kinematic boundaries. Top squark masses up to 270 GeV are excluded for a neutralino mass of 200 GeV. In a scenario where the top squark and the lightest neutralino are nearly degenerate in mass, top squark masses up to 260 GeV are excluded. The results from the monojetlike analysis are also interpreted in terms of compressed scenarios for top squark-pair production in the decay channel t 1 b+ff + 0 1 and sbottom pair production with b 1 b+ 0 1 , leading to a similar exclusion for nearly mass-degenerate third-generation squarks and the lightest neutralino. The results in this paper significantly extend previous results at colliders.
Resumo:
A search for supersymmetry (SUSY) in events with large missing transverse momentum, jets, at least one hadronically decaying tau lepton and zero or one additional light leptons (electron/muon), has been performed using 20.3 fb1 of proton-proton collision data at s = 8TeV recorded with the ATLAS detector at the Large Hadron Collider. No excess above the Standard Model background expectation is observed in the various signal regions and 95% confidence level upper limits on the visible cross section for new phenomena are set. The results of the analysis are interpreted in several SUSY scenarios, significantly extending previous limits obtained in the same final states. In the framework of minimal gauge-mediated SUSY breaking models, values of the SUSY breaking scale below 63TeV are excluded, independently of tan . Exclusion limits are also derived for an mSUGRA/CMSSM model, in both the R-parity-conserving and R-parity-violating case. A further interpretation is presented in a framework of natural gauge mediation, in which the gluino is assumed to be the only light coloured sparticle and gluino masses below 1090GeV are excluded.
Resumo:
This paper reports the results of a search for strong production of supersymmetric particles in 20.1 fb1 of proton-proton collisions at a centre-of-mass energy of 8TeV using the ATLAS detector at the LHC. The search is performed separately in events with either zero or at least one high-pT lepton (electron or muon), large missing transverse momentum, high jet multiplicity and at least three jets identified as originated from the fragmentation of a b-quark. No excess is observed with respect to the Standard Model predictions. The results are interpreted in the context of several supersymmetric models involving gluinos and scalar top and bottom quarks, as well as a mSUGRA/CMSSM model. Gluino masses up to 1340 GeV are excluded, depending on the model, significantly extending the previous ATLAS limits.
Resumo:
This paper describes a measurement of the Z/* boson transverse momentum spectrum using ATLAS proton-proton collision data at a centre-of-mass energy of s = 7 TeV at the LHC. The measurement is performed in the Z/* e+e and Z/* + channels, using data corresponding to an integrated luminosity of 4.7 fb1. Normalized differential cross sections as a function of the Z/* boson transverse momentum are measured for transverse momenta up to 800 GeV. The measurement is performed inclusively for Z/* rapidities up to 2.4, as well as in three rapidity bins. The channel results are combined, compared to perturbative and resummed QCD calculations and used to constrain the parton shower parameters of Monte Carlo generators.
Resumo:
The results of a search for direct pair production of the scalar partner to the top quark using an integrated luminosity of 20.1fb1 of proton-proton collision data at s=8 TeV recorded with the ATLAS detector at the LHC are reported. The top squark is assumed to decay via t~t~01 or t~b~1bW()~01, where ~01 (~1) denotes the lightest neutralino (chargino) in supersymmetric models. The search targets a fully-hadronic final state in events with four or more jets and large missing transverse momentum. No significant excess over the Standard Model background prediction is observed, and exclusion limits are reported in terms of the top squark and neutralino masses and as a function of the branching fraction of t~t~01. For a branching fraction of 100%, top squark masses in the range 270-645 GeV are excluded for ~01 masses below 30 GeV. For a branching fraction of 50% to either t~t~01 or t~b~1, and assuming the ~1 mass to be twice the ~01 mass, top squark masses in the range 250-550 GeV are excluded for ~01 masses below 60 GeV.
Resumo:
A likelihood-based discriminant for the identification of quark- and gluon-initiated jets is built and validated using 4.7 fb1 of protonproton collision data at s = 7 TeV collected with the ATLAS detector at the LHC. Data sampleswith enriched quark or gluon content are used in the construction and validation of templates of jet properties that are the input to the likelihood-based discriminant. The discriminating power of the jet tagger is established in both data and Monte Carlo samples within a systematic uncertainty of 1020 %. In data, light-quark jets can be tagged with an efficiency of 50% while achieving a gluon-jet mis-tag rate of 25% in a pT range between 40 GeV and 360 GeV for jets in the acceptance of the tracker. The rejection of gluon-jets found in the data is significantly below what is attainable using a Pythia 6Monte Carlo simulation, where gluon-jet mis-tag rates of 10% can be reached for a 50% selection efficiency of light-quark jets using the same jet properties.
Resumo:
This Letter presents the first study of W W jj , same-electric-charge diboson production in association with two jets, using 20.3fb 1 of proton-proton collision data at s =8TeV recorded by the ATLAS detector at the Large Hadron Collider. Events with two reconstructed same-charge leptons (e e , e , and ) and two or more jets are analyzed. Production cross sections are measured in two fiducial regions, with different sensitivities to the electroweak and strong production mechanisms. First evidence for W W jj production and electroweak-only W W jj production is observed with a significance of 4.5 and 3.6 standard deviations, respectively. The measured production cross sections are in agreement with standard model predictions. Limits at 95% confidence level are set on anomalous quartic gauge couplings
Resumo:
A search for an excess of events with multiple high transverse momentum objects including charged leptons and jets is presented, using 20.3 fb1 of proton-proton collision data recorded by the ATLAS detector at the Large Hadron Collider in 2012 at a centre-of-mass energy of s = 8TeV. No excess of events beyond Standard Model expectations is observed. Using extra-dimensional models for black hole and string ball production and decay, exclusion contours are determined as a function of the mass threshold for production and the fundamental gravity scale for two, four and six extra dimensions. For six extra dimensions, mass thresholds of 4.86.2TeV are excluded at 95% confidence level, depending on the fundamental gravity scale and model assumptions. Upper limits on the fiducial cross-sections for non-Standard Model production of these final states are set.