18,708 research outputs found
Search for the Standard Model Higgs Boson in the Diphoton Decay Channel with 4.9 fb^(-1) of pp Collision Data at âs = 7 TeV with ATLAS
A search for the standard model Higgs boson is performed in the diphoton decay channel. The data used correspond to an integrated luminosity of 4.9ââfb^(-1) collected with the ATLAS detector at the Large Hadron Collider in proton-proton collisions at a center-of-mass energy of âs = 7ââTeV. In the diphoton mass range 110â150 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 110â150 GeV, this significance becomes 1.5 standard deviations. The standard model Higgs boson is excluded at 95% confidence level in the mass ranges of 113â115 GeV and 134.5â136 GeV
Measurement of the W âÏÎœ_Ï cross section in pp collisions at âs = 7 TeV with the ATLAS experiment
The cross section for the production of W bosons with subsequent decay WâÏÎœ_Ï is measured with the ATLAS detector at the LHC. The analysis is based on a data sample that was recorded in 2010 at a protonâproton center-of-mass energy of âs = 7TeV and corresponds to an integrated luminosity of 34 pb^(â1). The cross section is measured in a region of high detector acceptance and then extrapolated to the full phase space. The product of the total W production cross section and the WâÏÎœ_Ï branching ratio is measured to be Ï^(tot)
_(WâÏÎœÏ) = 11.1±0.3 (stat)±1.7 (syst)±0.4 (lumi) nb
Measurement of the W^±Z production cross section and limits on anomalous triple gauge couplings in protonâproton collisions at âs = 7 TeV with the ATLAS detector
This Letter presents a measurement of W^± Z production in 1.02 fb^(â1) of pp collision data at âs = 7 TeV collected by the ATLAS experiment in 2011. Doubly leptonic decay events are selected with electrons, muons and missing transverse momentum in the final state. In total 71 candidates are observed, with a background expectation of 12.1 ± 1.4(stat.)^(+4.1)_(â2.0)(syst.) events. The total cross section for W^± Z production for Z/Îł^â masses within the range 66 GeV to 116 GeV is determined to be Ï^(tot)_(WZ) = 20.5^(+3.1)_(â2.8)(stat.)^(+1.4)_(â1.3)(syst.)^(+0.9)_(â0.8)(lumi.) pb, which is consistent with the Standard Model expectation of 17.3^(+1.3) _(0.8) pb. Limits on anomalous triple gauge boson couplings are extracted
Measurement of the azimuthal anisotropy for charged particle production in âs_(NN) = 2.76 TeV lead-lead collisions with the ATLAS detector
Differential measurements of charged particle azimuthal anisotropy are presented for lead-lead collisions at âs_(NN)=2.76 TeV with the ATLAS detector at the LHC, based on an integrated luminosity of approximately 8 ÎŒb^(â1). This anisotropy is characterized via a Fourier expansion of the distribution of charged particles in azimuthal angle relative to the reaction plane, with the coefficients v_n denoting the magnitude of the anisotropy. Significant v_2âv_6 values are obtained as a function of transverse momentum (0.52) and one particle with p_T<3 GeV, the v_(2,2)âv_(6,6) values are found to factorize as v_(n,n)(p_T^a,p_T^b)âv_n(p_T^a)v_n(p_T^b) in central and midcentral events. Such factorization suggests that these values of v_(2,2)âv_(6,6) are primarily attributable to the response of the created matter to the fluctuations in the geometry of the initial state. A detailed study shows that the v_(1,1)(p_T^a,p_T^b) data are consistent with the combined contributions from a rapidity-even v_1 and global momentum conservation. A two-component fit is used to extract the v_1 contribution. The extracted v_1 is observed to cross zero at p_Tâ1.0 GeV, reaches a maximum at 4â5 GeV with a value comparable to that for v_3, and decreases at higher p_T
Search for a Light Higgs Boson Decaying to Long-Lived Weakly Interacting Particles in Proton-Proton Collisions at âs = 7 TeV with the ATLAS Detector
A search for the decay of a light Higgs boson (120â140 GeV) to a pair of weakly interacting, long-lived particles in 1.94ââfb^(-1) of proton-proton collisions at âs=7ââTeV recorded in 2011 by the ATLAS detector is presented. The search strategy requires that both long-lived particles decay inside the muon spectrometer. No excess of events is observed above the expected background and limits on the Higgs boson production times branching ratio to weakly interacting, long-lived particles are derived as a function of the particle proper decay length
Search for contact interactions in dilepton events from pp collisions at âs = 7 TeV with the ATLAS detector
This Letter presents a search for contact interactions in the dielectron and dimuon channels using data from protonâproton collisions produced by the LHC at âs=7 TeV and recorded by the ATLAS detector. The data sample, collected in 2011, corresponds to an integrated luminosity of 1.08 and 1.21 fb^(â1) in the e^+e^â and ÎŒ^+ÎŒ^â channels, respectively. No significant deviations from the standard model are observed. Using a Bayesian approach with a prior flat in 1/Î^2, the following 95% CL lower limits are placed on the energy scale of ââqq contact interactions: Îâ>10.1 TeV (Î^+>9.4 TeV) in the electron channel and Î^â>8.0 TeV (Î^+>7.0 TeV) in the muon channel for constructive (destructive) interference in the leftâleft isoscalar contact interaction model. Limits are also provided for a prior flat in 1/Î^4
Search for an excess of events with an identical flavour lepton pair and significant missing transverse momentum in âs =7 TeV protonâproton collisions with the ATLAS detector
Results are presented of a search for particles decaying into final states with significant missing transverse momentum and exactly two identical flavour leptons (e, ÎŒ) of opposite charge in âs = 7 TeV collisions at the Large Hadron Collider. This channel is particularly sensitive to supersymmetric particle cascade decays producing flavour correlated lepton pairs. Flavour uncorrelated backgrounds are subtracted using a sample of opposite flavour lepton pair events. Observation of an excess beyond Standard Model expectations following this subtraction procedure would offer one of the best routes to measuring the masses of supersymmetric particles. In a data sample corresponding to an integrated luminosity of 35 pb^(â1) no such excess is observed. Model-independent limits are set on the contribution to these final states from supersymmetry and are used to exclude regions of a phenomenological supersymmetric parameter space
Electron performance measurements with the ATLAS detector using the 2010 LHC proton-proton collision data
Detailed measurements of the electron performance of the ATLAS detector at the LHC are reported, using decays of the Z, W and J/Ï particles. Data collected in 2010 at s=7~TeV are used, corresponding to an integrated luminosity of almost 40 pb^(â1). The inter-alignment of the inner detector and the electromagnetic calorimeter, the determination of the electron energy scale and resolution, and the performance in terms of response uniformity and linearity are discussed. The electron identification, reconstruction and trigger efficiencies, as well as the charge misidentification probability, are also presented
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