Muon reconstruction efficiency and momentum resolution of the ATLAS experiment in proton-proton collisions at [Formula: see text] TeV in 2010

This paper presents a study of the performance of the muon reconstruction in the analysis of proton-proton collisions at [Formula: see text] TeV at the LHC, 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 [Formula: see text] meson and [Formula: see text] boson decays to dimuons, reconstructed from a data sample corresponding to an integrated luminosity of 40 pb[Formula: see text]. The measured performance is compared to Monte Carlo predictions and deviations from the predicted performance are discussed.

The differential production cross section of the [Formula: see text](1020) meson in [Formula: see text] = 7 TeV [Formula: see text] collisions measured with the ATLAS detector

A measurement is presented of the [Formula: see text] production cross section at [Formula: see text] = 7 TeV using [Formula: see text] collision data corresponding to an integrated luminosity of 383 [Formula: see text], collected with the ATLAS experiment at the LHC. Selection of [Formula: see text](1020) mesons is based on the identification of charged kaons by their energy loss in the pixel detector. The differential cross section is measured as a function of the transverse momentum, [Formula: see text], and rapidity, [Formula: see text], of the [Formula: see text](1020) meson in the fiducial region 500 [Formula: see text] 1200 MeV, [Formula: see text] 0.8, kaon [Formula: see text] 230 MeV and kaon momentum [Formula: see text] 800 MeV. The integrated [Formula: see text]-meson production cross section in this fiducial range is measured to be [Formula: see text] = 570 [Formula: see text] 8 (stat) [Formula: see text] 66 (syst) [Formula: see text] 20 (lumi) [Formula: see text].

Search for quantum black hole production in high-invariant-mass lepton+jet final states using pp collisions at √s=8 TeV and the ATLAS detector

This Letter presents a search for quantum black-hole production using 20.3 fb-1 of data collected with the ATLAS detector in pp collisions at the LHC at √s = 8 TeV. The quantum black holes are assumed to decay into a final state characterized by 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.

Search for dark matter in events with a hadronically decaying W or Z boson and missing transverse momentum in pp collisions at √s=8 TeV with the ATLAS detector

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 √s=8  TeV using data recorded with the ATLAS detector 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. Limits are set on the mass scale in effective field theories that describe the interaction of dark matter and standard model particles, and on the cross section of Higgs production and decay to invisible particles. In addition, cross section limits on the anomalous production of W or Z bosons with large missing transverse momentum are set in two fiducial regions.

Measurement of jet shapes in top-quark pair events at [Formula: see text] using the ATLAS detector

A measurement of jet shapes in top-quark pair events using 1.8 fb-1 of [Formula: see text]pp collision data recorded by the ATLAS detector at the LHC is presented. Samples of top-quark pair events are selected in both the single-lepton and dilepton final states. The differential and integrated shapes of the jets initiated by bottom-quarks from the top-quark decays are compared with those of the jets originated by light-quarks from the hadronic W-boson decays [Formula: see text] in the single-lepton channel. The light-quark jets are found to have a narrower distribution of the momentum flow inside the jet area than b-quark jets.

Measurement of top quark polarization in top-antitop events from proton-proton collisions at √s=7 TeV using the ATLAS detector

This Letter presents measurements of the polarization of the top quark in top-antitop quark pair events, using 4.7  fb(-1) of proton-proton collision data recorded with the ATLAS detector at the Large Hadron Collider at √s=7  TeV. Final states containing one or two isolated leptons (electrons or muons) and jets are considered. Two measurements of α(ℓ)P, the product of the leptonic spin-analyzing power and the top quark polarization, are performed assuming that the polarization is introduced by either a CP conserving or a maximally CP violating production process. The measurements obtained, α(ℓ)P(CPC)=-0.035±0.014(stat)±0.037(syst) and α(ℓ)P(CPV)=0.020±0.016(stat)(-0.017)(+0.013)(syst), are in good agreement with the standard model prediction of negligible top quark polarization.

Measurement of the azimuthal angle dependence of inclusive jet yields in Pb+Pb collisions at √(sNN)=2.76 TeV with the ATLAS detector

Measurements of the variation of inclusive jet suppression as a function of relative azimuthal angle, Δφ, with respect to the elliptic event plane provide insight into the path-length dependence of jet quenching. ATLAS has measured the Δφ dependence of jet yields in 0.14 nb(-1) of √(s(NN))=2.76 TeV Pb+Pb collisions at the LHC for jet transverse momenta p(T)>45 GeV in different collision centrality bins using an underlying event subtraction procedure that accounts for elliptic flow. The variation of the jet yield with Δφ was characterized by the parameter, v(2)(jet), and the ratio of out-of-plane (Δφ~π/2) to in-plane (Δφ~0) yields. Nonzero v(2)(jet) values were measured in all centrality bins for p(T)<160 GeV. The jet yields are observed to vary by as much as 20% between in-plane and out-of-plane directions.

Improved luminosity determination in pp collisions at [Formula: see text] using the ATLAS detector at the LHC

The luminosity calibration for the ATLAS detector at the LHC during pp collisions at [Formula: see text] in 2010 and 2011 is presented. Evaluation of the luminosity scale is performed using several luminosity-sensitive detectors, and comparisons are made of the long-term stability and accuracy of this calibration applied to the pp collisions at [Formula: see text]. A luminosity uncertainty of [Formula: see text] is obtained for the 47 pb-1 of data delivered to ATLAS in 2010, and an uncertainty of [Formula: see text] is obtained for the 5.5 fb-1 delivered in 2011.

Measurement of the inclusive jet cross-section in pp collisions at [Formula: see text] and comparison to the inclusive jet cross-section at [Formula: see text] using the ATLAS detector

The inclusive jet cross-section has been measured in proton-proton collisions at [Formula: see text] in a dataset corresponding to an integrated luminosity of [Formula: see text] collected with the ATLAS detector at the Large Hadron Collider in 2011. Jets are identified using the anti-kt algorithm with two radius parameters of 0.4 and 0.6. The inclusive jet double-differential cross-section is presented as a function of the jet transverse momentum pT and jet rapidity y, covering a range of 20≤pT<430 GeV and |y|<4.4. The ratio of the cross-section to the inclusive jet cross-section measurement at [Formula: see text], published by the ATLAS Collaboration, is calculated as a function of both transverse momentum and the dimensionless quantity [Formula: see text], in bins of jet rapidity. The systematic uncertainties on the ratios are significantly reduced due to the cancellation of correlated uncertainties in the two measurements. Results are compared to the prediction from next-to-leading order perturbative QCD calculations corrected for non-perturbative effects, and next-to-leading order Monte Carlo simulation. Furthermore, the ATLAS jet cross-section measurements at [Formula: see text] and [Formula: see text] are analysed within a framework of next-to-leading order perturbative QCD calculations to determine parton distribution functions of the proton, taking into account the correlations between the measurements.

Search for a light charged Higgs boson in the decay channel [Formula: see text] in [Formula: see text] events using pp collisions at [Formula: see text] with the ATLAS detector

A search for a charged Higgs boson (H+) in [Formula: see text] decays is presented, where one of the top quarks decays via t→H+b, followed by H+→ two jets ([Formula: see text]). The other top quark decays to Wb, where the W boson then decays into a lepton (e/μ) and a neutrino. The data were recorded in pp collisions at [Formula: see text] by the ATLAS detector at the LHC in 2011, and correspond to an integrated luminosity of 4.7 fb-1. With no observation of a signal, 95 % confidence level (CL) upper limits are set on the decay branching ratio of top quarks to charged Higgs bosons varying between 5 % and 1 % for H+ masses between 90 GeV and 150 GeV, assuming [Formula: see text].

Measurement of kT splitting scales in W→ℓν events at [Formula: see text] with the ATLAS detector

A measurement of splitting scales, as defined by the kT clustering algorithm, is presented for final states containing a W boson produced in proton-proton collisions at a centre-of-mass energy of 7 TeV. The measurement is based on the full 2010 data sample corresponding to an integrated luminosity of 36 pb-1 which was collected using the ATLAS detector at the CERN Large Hadron Collider. Cluster splitting scales are measured in events containing W bosons decaying to electrons or muons. The measurement comprises the four hardest splitting scales in a kT cluster sequence of the hadronic activity accompanying the W boson, and ratios of these splitting scales. Backgrounds such as multi-jet and top-quark-pair production are subtracted and the results are corrected for detector effects. Predictions from various Monte Carlo event generators at particle level are compared to the data. Overall, reasonable agreement is found with all generators, but larger deviations between the predictions and the data are evident in the soft regions of the splitting scales.

Observation of associated near-side and away-side long-range correlations in sqrt[s(NN)]=5.02 TeV proton-lead collisions with the ATLAS detector

Two-particle correlations in relative azimuthal angle (Δø) and pseudorapidity (Δη) are measured in sqrt[s(NN)] = 5.02 TeV p+Pb collisions using the ATLAS detector at the LHC. The measurements are performed using approximately 1 μb(-1) of data as a function of transverse momentum (p(T)) and the transverse energy (ΣE(T)(Pb)) summed over 3.1 < η < 4.9 in the direction of the Pb beam. The correlation function, constructed from charged particles, exhibits a long-range (2 < |Δ η | < 5) "near-side" (Δø ~ 0) correlation that grows rapidly with increasing ΣE(T)(Pb). A long-range "away-side" (Δø ~ π) correlation, obtained by subtracting the expected contributions from recoiling dijets and other sources estimated using events with small ΣE(T)(Pb), is found to match the near-side correlation in magnitude, shape (in Δη and Δø) and ΣE(T)(Pb) dependence. The resultant Δø correlation is approximately symmetric about π/2, and is consistent with a dominant cos2Δø modulation for all ΣE(T)(Pb) ranges and particle p(T).

Multi-channel search for squarks and gluinos in [Formula: see text]pp collisions with the ATLAS detector at the LHC

A search for supersymmetric particles in final states with zero, one, and two leptons, with and without jets identified as originating from b-quarks, in 4.7 fb-1 of [Formula: see text]pp collisions produced by the Large Hadron Collider and recorded by the ATLAS detector is presented. The search uses a set of variables carrying information on the event kinematics transverse and parallel to the beam line that are sensitive to several topologies expected in supersymmetry. Mutually exclusive final states are defined, allowing a combination of all channels to increase the search sensitivity. No deviation from the Standard Model expectation is observed. Upper limits at 95 % confidence level on visible cross-sections for the production of new particles are extracted. Results are interpreted in the context of the constrained minimal supersymmetric extension to the Standard Model and in supersymmetry-inspired models with diverse, high-multiplicity final states.

Measurement of the [Formula: see text] production cross section in the tau + jets channel using the ATLAS detector

A measurement of the top quark pair production cross section in the final state with a hadronically decaying tau lepton and jets is presented. The analysis is based on proton-proton collision data recorded by the ATLAS experiment at the LHC, with a centre-of-mass energy of 7 TeV. The data sample corresponds to an integrated luminosity of 1.67 fb-1. The cross section is measured to be [Formula: see text] and is in agreement with other measurements and with the Standard Model prediction.

Jet energy resolution in proton-proton collisions at [Formula: see text] recorded in 2010 with the ATLAS detector

The measurement of the jet energy resolution is presented using data recorded with the ATLAS detector in proton-proton collisions at [Formula: see text]. The sample corresponds to an integrated luminosity of 35 pb-1. Jets are reconstructed from energy deposits measured by the calorimeters and calibrated using different jet calibration schemes. The jet energy resolution is measured with two different in situ methods which are found to be in agreement within uncertainties. The total uncertainties on these measurements range from 20 % to 10 % for jets within |y|<2.8 and with transverse momenta increasing from 30 GeV to 500 GeV. Overall, the Monte Carlo simulation of the jet energy resolution agrees with the data within 10 %.

Measurement of Z boson production in Pb-Pb collisions at sqrt[s(NN)]=2.76 TeV with the ATLAS detector

The ATLAS experiment has observed 1995 Z boson candidates in data corresponding to 0.15  nb(-1) of integrated luminosity obtained in the 2011 LHC Pb+Pb run at sqrt[s(NN)]=2.76  TeV. The Z bosons are reconstructed via dielectron and dimuon decay channels, with a background contamination of less than 3%. Results from the two channels are consistent and are combined. Within the statistical and systematic uncertainties, the per-event Z boson yield is proportional to the number of binary collisions estimated by the Glauber model. The elliptic anisotropy of the azimuthal distribution of the Z boson with respect to the event plane is found to be consistent with zero.

Search for dark matter candidates and large extra dimensions in events with a photon and missing transverse momentum in pp collision data at sqrt[s]=7 TeV with the ATLAS detector

Results of a search for new phenomena in events with an energetic photon and large missing transverse momentum in proton-proton collisions at sqrt[s] = 7 TeV are reported. Data collected by the ATLAS experiment at the LHC corresponding to an integrated luminosity of 4.6 fb(-1) are used. Good agreement is observed between the data and the standard model predictions. The results are translated into exclusion limits on models with large extra spatial dimensions and on pair production of weakly interacting dark matter candidates.

Search for magnetic monopoles in sqrt[s]=7 TeV pp collisions with the ATLAS detector

This Letter presents a search for magnetic monopoles with the ATLAS detector at the CERN Large Hadron Collider using an integrated luminosity of 2.0  fb(-1) of pp collisions recorded at a center-of-mass energy of sqrt[s]=7  TeV. No event is found in the signal region, leading to an upper limit on the production cross section at 95% confidence level of 1.6/ϵ  fb for Dirac magnetic monopoles with the minimum unit magnetic charge and with mass between 200 GeV and 1500 GeV, where ϵ is the monopole reconstruction efficiency. The efficiency ϵ is high and uniform in the fiducial region given by pseudorapidity |η|<1.37 and transverse kinetic energy 600-700<E(kin)sinθ<1400  GeV. The minimum value of 700 GeV is for monopoles of mass 200 GeV, whereas the minimum value of 600 GeV is applicable for higher mass monopoles. Therefore, the upper limit on the production cross section at 95% confidence level is 2 fb in this fiducial region. Assuming the kinematic distributions from Drell-Yan pair production of spin-1/2 Dirac magnetic monopoles, the efficiency is in the range 1%-10%, leading to an upper limit on the cross section at 95% confidence level that varies from 145 fb to 16 fb for monopoles with mass between 200 GeV and 1200 GeV. This limit is weaker than the fiducial limit because most of these monopoles lie outside the fiducial region.

Search for direct top squark pair production in final states with one isolated lepton, jets, and missing transverse momentum in sqrt[s] = 7 TeV pp collisions using 4.7 fb(-10 of ATLAS data

A search is presented for direct top squark pair production in final states with one isolated electron or muon, jets, and missing transverse momentum in proton-proton collisions at sqrt[s] = 7 TeV. The measurement is based on 4.7 fb(-1) of data collected with the ATLAS detector at the LHC. Each top squark is assumed to decay to a top quark and the lightest supersymmetric particle (LSP). The data are found to be consistent with standard model expectations. Top squark masses between 230 GeV and 440 GeV are excluded with 95% confidence for massless LSPs, and top squark masses around 400 GeV are excluded for LSP masses up to 125 GeV.

Search for a supersymmetric partner to the top quark in final states with jets and missing transverse momentum at sqrt[s] = 7 TeV with the ATLAS detector

A search for direct pair production of supersymmetric top squarks (t(1)) is presented, assuming the t(1) decays into a top quark and the lightest supersymmetric particle, χ(1)(0), and that both top quarks decay to purely hadronic final states. A total of 16 (4) events are observed compared to a predicted standard model background of 13.5(-3.6)(+3.7)(4.4(-1.3)(+1.7)) events in two signal regions based on ∫Ldt = 4.7 fb(-1) of pp collision data taken at sqrt[s] = 7 TeV with the ATLAS detector at the LHC. An exclusion region in the t(1) versus χ(1)(0) mass plane is evaluated: 370<m(t)(1)}<465 GeV is excluded for m(χ)(1)(0) ~ 0 GeV while m(t)(1) = 445 GeV is excluded for m(χ)(1)(0) ≤ 50 GeV.

A search for [Formula: see text] resonances with the ATLAS detector in 2.05 fb-1 of proton-proton collisions at [Formula: see text]

A search for top quark pair resonances in final states containing at least one electron or muon has been performed with the ATLAS experiment at the CERN Large Hadron Collider. The search uses a data sample corresponding to an integrated luminosity of 2.05 fb-1, which was recorded in 2011 at a proton-proton centre-of-mass energy of 7 TeV. No evidence for a resonance is found and limits are set on the production cross-section times branching ratio to [Formula: see text] for narrow and wide resonances. For narrow Z' bosons, the observed 95 % Bayesian credibility level limits range from 9.3 pb to 0.95 pb for masses in the range of mZ'=500 GeV to mZ'=1300 GeV. The corresponding excluded mass region for a leptophobic topcolour Z' boson (Kaluza-Klein gluon excitation in the Randall-Sundrum model) is mZ'<880 GeV ([Formula: see text]).