This article presents measurements of the t-channel single top-quark (t) and top-antiquark (t¯) total production cross sections σ(tq) and σ(t¯q), their ratio Rt=σ(tq)/σ(t¯q), and a measurement of th...
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This article presents measurements of the t-channel single top-quark (t) and top-antiquark (t¯) total production cross sections σ(tq) and σ(t¯q), their ratio Rt=σ(tq)/σ(t¯q), and a measurement of the inclusive production cross section σ(tq+t¯q) in proton-proton collisions at s=7 TeV at the LHC. Differential cross sections for the tq and t¯q processes are measured as a function of the transverse momentum and the absolute value of the rapidity of t and t¯, respectively. The analyzed data set was recorded with the ATLAS detector and corresponds to an integrated luminosity of 4.59 fb−1. Selected events contain one charged lepton, large missing transverse momentum, and two or three jets. The cross sections are measured by performing a binned maximum-likelihood fit to the output distributions of neural networks. The resulting measurements are σ(tq)=46±1(stat)±6(syst) pb, σ(t¯q)=23±1(stat)±3(syst) pb, Rt=2.04±0.13(stat)±0.12(syst), and σ(tq+t¯q)=68±2(stat)±8(syst) pb, consistent with the Standard Model expectation. The uncertainty on the measured cross sections is dominated by systematic uncertainties, while the uncertainty on Rt is mainly statistical. Using the ratio of σ(tq+t¯q) to its theoretical prediction, and assuming that the top-quark-related CKM matrix elements obey the relation |Vtb|≫|Vts|,|Vtd|, we determine |Vtb|=1.02±0.07.
Results of a search for supersymmetry via direct production of third-generation squarks are reported, using 20.3 fb−1 of proton-proton collision data at s=8 TeV recorded by the ATLAS experiment at the LHC in 2012. T...
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Results of a search for supersymmetry via direct production of third-generation squarks are reported, using 20.3 fb−1 of proton-proton collision data at s=8 TeV 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+χ˜10) across the top squark–neutralino 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 (mt˜1, mχ˜10) 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′+χ˜10 and sbottom pair production with b˜1→b+χ˜10, 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.
Measurements of normalized differential cross sections for top-quark pair production are presented as a function of the top-quark transverse momentum, and of the mass, transverse momentum, and rapidity of the tt¯...
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Measurements of normalized differential cross sections for top-quark pair production are presented as a function of the top-quark transverse momentum, and of the mass, transverse momentum, and rapidity of the tt¯ system, in proton–proton collisions at a center-of-mass energy of s=7 TeV. The data set corresponds to an integrated luminosity of 4.6 fb−1, recorded in 2011 with the ATLAS detector at the CERN Large Hadron Collider. Events are selected in the lepton+jets channel, requiring exactly one lepton and at least four jets with at least one of the jets tagged as originating from a b-quark. The measured spectra are corrected for detector efficiency and resolution effects and are compared to several Monte Carlo simulations and theory calculations. The results are in fair agreement with the predictions in a wide kinematic range. Nevertheless, data distributions are softer than predicted for higher values of the mass of the tt¯ system and of the top-quark transverse momentum. The measurements can also discriminate among different sets of parton distribution functions.
Results from a search for supersymmetry in events with four or more leptons including electrons, muons and taus are presented. The analysis uses a data sample corresponding to 20.3 fb−1 of proton-proton collisions de...
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Results from a search for supersymmetry in events with four or more leptons including electrons, muons and taus are presented. The analysis uses a data sample corresponding to 20.3 fb−1 of proton-proton collisions delivered by the Large Hadron Collider at s=8 TeV and recorded by the ATLAS detector. Signal regions are designed to target supersymmetric scenarios that can be either enriched in or depleted of events involving the production of a Z boson. No significant deviations are observed in data from standard model predictions and results are used to set upper limits on the event yields from processes beyond the standard model. Exclusion limits at the 95% confidence level on the masses of relevant supersymmetric particles are obtained. In R-parity-violating simplified models with decays of the lightest supersymmetric particle to electrons and muons, limits of 1350 and 750 GeV are placed on gluino and chargino masses, respectively. In R-parity-conserving simplified models with heavy neutralinos decaying to a massless lightest supersymmetric particle, heavy neutralino masses up to 620 GeV are excluded. Limits are also placed on other supersymmetric scenarios.
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 ce...
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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 20.3 fb−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 E6 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.
A measurement of the Bs0→J/ψϕ decay parameters, updated to include flavor tagging is reported using 4.9 fb−1 of integrated luminosity collected by the ATLAS detector from s=7 TeV pp collisions recorded in 2011 at ...
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A measurement of the Bs0→J/ψϕ decay parameters, updated to include flavor tagging is reported using 4.9 fb−1 of integrated luminosity collected by the ATLAS detector from s=7 TeV pp collisions recorded in 2011 at the LHC. The values measured for the physical parameters are ϕs=0.12±0.25(stat)±0.05(syst) radΔΓs=0.053±0.021(stat)±0.010(syst) ps−1Γs=0.677±0.007(stat)±0.004(syst) ps−1|A∥(0)|2=0.220±0.008(stat)±0.009(syst)|A0(0)|2=0.529±0.006(stat)±0.012(syst)δ⊥=3.89±0.47(stat)±0.11(syst) radwhere the parameter ΔΓs is constrained to be positive. The S-wave contribution was measured and found to be compatible with zero. Results for ϕs and ΔΓs are also presented as 68% and 95% likelihood contours, which show agreement with the Standard Model expectations.
Measurements of spin correlation in top quark pair production are presented using data collected with the ATLAS detector at the LHC with proton-proton collisions at a center-of-mass energy of 7 TeV, corresponding to a...
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Measurements of spin correlation in top quark pair production are presented using data collected with the ATLAS detector at the LHC with proton-proton collisions at a center-of-mass energy of 7 TeV, corresponding to an integrated luminosity of 4.6 fb−1. Events are selected in final states with two charged leptons and at least two jets and in final states with one charged lepton and at least four jets. Four different observables sensitive to different properties of the top quark pair production mechanism are used to extract the correlation between the top and antitop quark spins. Some of these observables are measured for the first time. The measurements are in good agreement with the Standard Model prediction at next-to-leading-order accuracy.
A measurement of the parity-violating decay asymmetry parameter, αb, and the helicity amplitudes for the decay Λb0→J/ψ(μ+μ−)Λ0(pπ−) is reported. The analysis is based on 1400 Λb0 and Λ¯b0 baryons select...
A measurement of the parity-violating decay asymmetry parameter, αb, and the helicity amplitudes for the decay Λb0→J/ψ(μ+μ−)Λ0(pπ−) is reported. The analysis is based on 1400 Λb0 and Λ¯b0 baryons selected in 4.6 fb−1 of proton–proton collision data with a center-of-mass energy of 7 TeV recorded by the ATLAS experiment at the LHC. By combining the Λb0 and Λ¯b0 samples under the assumption of CP conservation, the value of αb is measured to be 0.30±0.16(stat)±0.06(syst). This measurement provides a test of theoretical models based on perturbative QCD or heavy-quark effective theory.
A measurement of the cross section for the production of isolated prompt photons in pp collisions at a center-of-mass energy s=7 TeV is presented. The results are based on an integrated luminosity of 4.6 fb−1 collec...
A measurement of the cross section for the production of isolated prompt photons in pp collisions at a center-of-mass energy s=7 TeV is presented. The results are based on an integrated luminosity of 4.6 fb−1 collected with the ATLAS detector at the LHC. The cross section is measured as a function of photon pseudorapidity ηγ and transverse energy ETγ in the kinematic range 100≤ETγ<1000 GeV and in the regions |ηγ|<1.37 and 1.52≤|ηγ|<2.37. The results are compared to leading-order parton-shower Monte Carlo models and next-to-leading-order perturbative QCD calculations. Next-to-leading-order perturbative QCD calculations agree well with the measured cross sections as a function of ETγ and ηγ.
The ATLAS detector at the Large Hadron Collider is used to search for the lepton flavor violating process Z→eμ in pp collisions using 20.3 fb−1 of data collected at s=8 TeV. An enhancement in the eμ invariant mas...
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The ATLAS detector at the Large Hadron Collider is used to search for the lepton flavor violating process Z→eμ in pp collisions using 20.3 fb−1 of data collected at s=8 TeV. An enhancement in the eμ invariant mass spectrum is searched for at the Z-boson mass. The number of Z bosons produced in the data sample is estimated using events of similar topology, Z→ee and μμ, significantly reducing the systematic uncertainty in the measurement. There is no evidence of an enhancement at the Z-boson mass, resulting in an upper limit on the branching fraction, B(Z→eμ)<7.5×10−7 at the 95% confidence level.
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