J. Almond

A search for the Standard Model Higgs boson in proton–proton collisions with the ATLAS detector at the LHC is presented. The datasets used correspond to integrated luminosities of approximately 4.8 fb−1 collected at s=7TeV in 2011 and 5.8 fb−1 at s=8TeV in 2012. Individual searches in the channels ⁎H→ZZ(⁎)→4ℓ, H→γγ and ⁎H→WW(⁎)→eνμν in the 8 TeV data are combined with previously published results of searches for ⁎H→ZZ(⁎), ⁎WW(⁎), bb¯ and τ+τ− in the 7 TeV data and results from improved analyses of the ⁎H→ZZ(⁎)→4ℓ and H→γγ channels in the 7 TeV data. Clear evidence for the production of a neutral boson with a measured mass of 126.0±0.4(stat)±0.4(sys)GeV is presented. This observation, which has a significance of 5.9 standard deviations, corresponding to a background fluctuation probability of 1.7×10−9, is compatible with the production and decay of the Standard Model Higgs boson.

The simulation software for the ATLAS Experiment at the Large Hadron Collider is being used for large-scale production of events on the LHC Computing Grid. This simulation requires many components, from the generators that simulate particle collisions, through packages simulating the response of the various detectors and triggers. All of these components come together under the ATLAS simulation infrastructure. In this paper, that infrastructure is discussed, including that supporting the detector description, interfacing the event generation, and combining the GEANT4 simulation of the response of the individual detectors. Also described are the tools allowing the software validation, performance testing, and the validation of the simulated output against known physics processes.

Using the ATLAS detector, observations have been made of a centrality-dependent dijet asymmetry in the collisions of lead ions at the Large Hadron Collider. In a sample of lead-lead events with a per-nucleon center of mass energy of 2.76 TeV, selected with a minimum bias trigger, jets are reconstructed in fine-grained, longitudinally-segmented electromagnetic and hadronic calorimeters. The underlying event is measured and subtracted event-by-event, giving estimates of jet transverse energy above the ambient background. The transverse energies of dijets in opposite hemispheres is observed to become systematically more unbalanced with increasing event centrality leading to a large number of events which contain highly asymmetric dijets. This is the first observation of an enhancement of events with such large dijet asymmetries, not observed in proton-proton collisions, and which may point to an interpretation in terms of strong jet energy loss in a hot, dense medium.

Two-particle correlations in relative azimuthal angle ($\ensuremath{\Delta}\ensuremath{\phi}$) and pseudorapidity ($\ensuremath{\Delta}\ensuremath{\eta}$) are measured in $\sqrt{{s}_{\mathrm{NN}}}=5.02\text{ }\text{ }\mathrm{TeV}$ $p+\mathrm{Pb}$ collisions using the ATLAS detector at the LHC. The measurements are performed using approximately $1\text{ }\text{ }\ensuremath{\mu}{\mathrm{b}}^{\ensuremath{-}1}$ of data as a function of transverse momentum (${p}_{\mathrm{T}}$) and the transverse energy ($\ensuremath{\Sigma}{E}_{\mathrm{T}}^{\mathrm{Pb}}$) summed over $3.1<\ensuremath{\eta}<4.9$ in the direction of the Pb beam. The correlation function, constructed from charged particles, exhibits a long-range ($2<|\ensuremath{\Delta}\ensuremath{\eta}|<5$) ``near-side'' ($\ensuremath{\Delta}\ensuremath{\phi}\ensuremath{\sim}0$) correlation that grows rapidly with increasing $\ensuremath{\Sigma}{E}_{\mathrm{T}}^{\mathrm{Pb}}$. A long-range ``away-side'' ($\ensuremath{\Delta}\ensuremath{\phi}\ensuremath{\sim}\ensuremath{\pi}$) correlation, obtained by subtracting the expected contributions from recoiling dijets and other sources estimated using events with small $\ensuremath{\Sigma}{E}_{\mathrm{T}}^{\mathrm{Pb}}$, is found to match the near-side correlation in magnitude, shape (in $\ensuremath{\Delta}\ensuremath{\eta}$ and $\ensuremath{\Delta}\ensuremath{\phi}$) and $\ensuremath{\Sigma}{E}_{\mathrm{T}}^{\mathrm{Pb}}$ dependence. The resultant $\ensuremath{\Delta}\ensuremath{\phi}$ correlation is approximately symmetric about $\ensuremath{\pi}/2$, and is consistent with a dominant $\mathrm{cos}2\ensuremath{\Delta}\ensuremath{\phi}$ modulation for all $\ensuremath{\Sigma}{E}_{\mathrm{T}}^{\mathrm{Pb}}$ ranges and particle ${p}_{\mathrm{T}}$.

Studies of the spin and parity quantum numbers of the Higgs boson are presented, based on proton-proton collision data collected by the ATLAS experiment at the LHC. The Standard Model spin-parity JP = 0+ hypothesis is compared with alternative hypotheses using the Higgs boson decays H->gamma gamma, H -> ZZ -> 4 leptons and H->WW -> l nu l nu, as well as the combination of these channels. The analysed dataset corresponds to an integrated luminosity of 20.7 fb-1 collected at a centre-of-mass energy of sqrt(s) = 8 TeV. For the H -> ZZ -> 4-lepton decay mode the dataset corresponding to an integrated luminosity of 4.6 fb-1 collected at sqrt(s) = 7 TeV is added. The data are compatible with the Standard Model JP = 0+ quantum numbers for the Higgs boson, whereas all alternative hypotheses studied in this letter, namely some specific JP = 0-; 1+; 1-; 2+ models, are excluded at confidence levels above 97.8%. This exclusion holds independently of the assumptions on the coupling strengths to the Standard Model particles and in the case of the JP = 2+ model, of the relative fractions of gluon-fusion and quark-antiquark production of the spin-2 particle. The data thus provide evidence for the spin-0 nature of the Higgs boson, with positive parity being strongly preferred.

Measurements are presented from proton–proton collisions at centre-of-mass energies of , 2.36 and 7 TeV recorded with the ATLAS detector at the LHC. Events were collected using a single-arm minimum-bias trigger. The charged-particle multiplicity, its dependence on transverse momentum and pseudorapidity and the relationship between the mean transverse momentum and charged-particle multiplicity are measured. Measurements in different regions of phase space are shown, providing diffraction-reduced measurements as well as more inclusive ones. The observed distributions are corrected to well-defined phase-space regions, using model-independent corrections. The results are compared to each other and to various Monte Carlo (MC) models, including a new AMBT1 pythia6 tune. In all the kinematic regions considered, the particle multiplicities are higher than predicted by the MC models. The central charged-particle multiplicity per event and unit of pseudorapidity, for tracks with pT>100 MeV, is measured to be 3.483±0.009 (stat)±0.106 (syst) at and 5.630±0.003 (stat)±0.169 (syst) at .

Differential measurements of charged particle azimuthal anisotropy are presented for lead-lead collisions at sqrt(s_NN) = 2.76 TeV with the ATLAS detector at the LHC, based on an integrated luminosity of approximately 8 mb^-1. This anisotropy is characterized via a Fourier expansion of the distribution of charged particles in azimuthal angle (phi), with the coefficients v_n denoting the magnitude of the anisotropy. Significant v_2-v_6 values are obtained as a function of transverse momentum (0.5<pT<20 GeV), pseudorapidity (|eta|<2.5) and centrality using an event plane method. The v_n values for n>=3 are found to vary weakly with both eta and centrality, and their pT dependencies are found to follow an approximate scaling relation, v_n^{1/n}(pT) \propto v_2^{1/2}(pT). A Fourier analysis of the charged particle pair distribution in relative azimuthal angle (Dphi=phi_a-phi_b) is performed to extract the coefficients v_{n,n}=<cos (n Dphi)>. For pairs of charged particles with a large pseudorapidity gap (|Deta=eta_a-eta_b|>2) and one particle with pT<3 GeV, the v_{2,2}-v_{6,6} values are found to factorize as v_{n,n}(pT^a,pT^b) ~ v_n(pT^a)v_n(pT^b) in central and mid-central events. Such factorization suggests that these values of v_{2,2}-v_{6,6} are primarily due 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}(pT^a,pT^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 pT\sim1.0 GeV, reaches a maximum at 4-5 GeV with a value comparable to that for v_3, and decreases at higher pT.

A combined search for the Standard Model Higgs boson with the ATLAS experiment at the LHC using datasets corresponding to integrated luminosities from 1.04 fb−1 to 4.9 fb−1 of pp collisions collected at s=7TeV is presented. The Higgs boson mass ranges 112.9–115.5 GeV, 131–238 GeV and 251–466 GeV are excluded at the 95% confidence level (CL), while the range 124–519 GeV is expected to be excluded in the absence of a signal. An excess of events is observed around mH∼126GeV with a local significance of 3.5 standard deviations (σ). The local significances of H→γγ, ⁎H→ZZ(⁎)→ℓ+ℓ−ℓ′+ℓ′− and ⁎H→WW(⁎)→ℓ+νℓ′−ν¯, the three most sensitive channels in this mass range, are 2.8σ, 2.1σ and 1.4σ, respectively. The global probability for the background to produce such a fluctuation anywhere in the explored Higgs boson mass range 110–600 GeV is estimated to be ∼1.4% or, equivalently, 2.2σ.

Measurements are presented of production properties and couplings of the recently discovered Higgs boson using the decays into boson pairs, H→γγ, H→ZZ⁎→4ℓ⁎ and H→WW⁎→ℓνℓν⁎. The results are based on the complete pp collision data sample recorded by the ATLAS experiment at the CERN Large Hadron Collider at centre-of-mass energies of s=7 TeV and s=8 TeV, corresponding to an integrated luminosity of about 25 fb−1. Evidence for Higgs boson production through vector-boson fusion is reported. Results of combined fits probing Higgs boson couplings to fermions and bosons, as well as anomalous contributions to loop-induced production and decay modes, are presented. All measurements are consistent with expectations for the Standard Model Higgs boson.

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.

Detailed measurements of the electron performance of the ATLAS detector at the LHC are reported, using decays of the Z, W and J/ψ particles. Data collected in 2010 at $\sqrt{s}=7\mbox{~TeV}$ are used, corresponding to an integrated luminosity of almost 40 pb−1. 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.

Proton–proton collisions at $\sqrt{s}=7$ TeV and heavy ion collisions at $\sqrt{s_{NN}}=2.76$ TeV were produced by the LHC and recorded using the ATLAS experiment’s trigger system in 2010. The LHC is designed with a maximum bunch crossing rate of 40 MHz and the ATLAS trigger system is designed to record approximately 200 of these per second. The trigger system selects events by rapidly identifying signatures of muon, electron, photon, tau lepton, jet, and B meson candidates, as well as using global event signatures, such as missing transverse energy. An overview of the ATLAS trigger system, the evolution of the system during 2010 and the performance of the trigger system components and selections based on the 2010 collision data are shown. A brief outline of plans for the trigger system in 2011 is presented.

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.3 MoreReceived 16 May 2014DOI:https://doi.org/10.1103/PhysRevD.90.052005This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.© 2014 CERN, for the ATLAS Collaboration

This paper describes the measurement of elliptic flow of charged particles in lead-lead collisions at sqrt(s_NN) = 2.76 TeV using the ATLAS detector at the Large Hadron Collider (LHC). The results are based on an integrated luminosity of approximately 7 ub^-1. Elliptic flow is measured over a wide region in pseudorapidity, |eta| < 2.5, and over a broad range in transverse momentum, 0.5 < p_T < 20 GeV. The elliptic flow parameter v_2 is obtained by correlating individual tracks with the event plane measured using energy deposited in the forward calorimeters. As a function of transverse momentum, v_2(p_T) reaches a maximum at p_T of about 3 GeV, then decreases and becomes weakly dependent on p_T above 7 - 8 GeV. Over the measured pseudorapidity region, v_2 is found to be approximately independent of |eta| for all collision centralities and particle transverse momenta, something not observed in lower energy collisions. The results are discussed in the context of previous measurements at lower collision energies, as well as recent results from the LHC.

The measurement of missing transverse momentum in the ATLAS detector, described in this paper, makes use of the full event reconstruction and a calibration based on reconstructed physics objects. The performance of the missing transverse momentum reconstruction is evaluated using data collected in pp collisions at a centre-of-mass energy of 7 TeV in 2010. Minimum bias events and events with jets of hadrons are used from data samples corresponding to an integrated luminosity of about 0.3 inverse nb and 600 inverse nb, together with events containing a Z boson decaying to two leptons (electrons or muons) or a W boson decaying to a lepton (electron or muon) and a neutrino, from a data sample corresponding to an integrated luminosity of about 36 inverse pb. An estimate of the systematic uncertainty on the missing transverse momentum scale is presented.

An improved measurement of the mass of the Higgs boson is derived from a combined fit to the reconstructed invariant mass spectra of the decay channels H→γγ and H→ZZ∗→4ℓ. The analysis uses the pp collision data sample recorded by the ATLAS experiment at the CERN Large Hadron Collider at center-of-mass energies of 7 TeV and 8 TeV, corresponding to an integrated luminosity of 25 fb−1. The measured value of the Higgs boson mass is mH=125.36±0.37(stat)±0.18(syst) GeV. This result is based on improved energy-scale calibrations for photons, electrons, and muons as well as other analysis improvements, and supersedes the previous result from ATLAS. Upper limits on the total width of the Higgs boson are derived from fits to the invariant mass spectra of the H→γγ and H→ZZ∗→4ℓ decay channels.2 MoreReceived 17 June 2014DOI:https://doi.org/10.1103/PhysRevD.90.052004This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.© 2014 CERN, for the ATLAS Collaboration

Measurements of inclusive jet suppression in heavy ion collisions at the LHC provide direct sensitivity to the physics of jet quenching. In a sample of lead-lead collisions at sqrt(s) = 2.76 TeV corresponding to an integrated luminosity of approximately 7 inverse microbarns, ATLAS has measured jets with a calorimeter over the pseudorapidity interval |eta| < 2.1 and over the transverse momentum range 38 < pT < 210 GeV. Jets were reconstructed using the anti-kt algorithm with values for the distance parameter that determines the nominal jet radius of R = 0.2, 0.3, 0.4 and 0.5. The centrality dependence of the jet yield is characterized by the jet "central-to-peripheral ratio," Rcp. Jet production is found to be suppressed by approximately a factor of two in the 10% most central collisions relative to peripheral collisions. Rcp varies smoothly with centrality as characterized by the number of participating nucleons. The observed suppression is only weakly dependent on jet radius and transverse momentum. These results provide the first direct measurement of inclusive jet suppression in heavy ion collisions and complement previous measurements of dijet transverse energy imbalance at the LHC.

This paper presents the performance of the ATLAS muon reconstruction during the LHC run with $$pp$$ collisions at $$\sqrt{s}=7$$ –8 TeV in 2011–2012, focusing mainly on data collected in 2012. Measurements of the reconstruction efficiency and of the momentum scale and resolution, based on large reference samples of $${J/\psi \rightarrow \mu \mu }$$ , $$Z \rightarrow \mu \mu $$ and $${\Upsilon \rightarrow \mu \mu }$$ decays, are presented and compared to Monte Carlo simulations. Corrections to the simulation, to be used in physics analysis, are provided. Over most of the covered phase space (muon $$|\eta |<2.7$$ and $$5 \lesssim p_{\mathrm{T}}\lesssim 100$$ GeV) the efficiency is above $$99\,\%$$ and is measured with per-mille precision. The momentum resolution ranges from $$1.7\,\%$$ at central rapidity and for transverse momentum $$p_{\mathrm{T}}\simeq 10$$ GeV, to $$4\,\%$$ at large rapidity and $$p_{\mathrm{T}}\simeq 100$$ GeV. The momentum scale is known with an uncertainty of $$0.05\,\%$$ to $$0.2\,\%$$ depending on rapidity. A method for the recovery of final state radiation from the muons is also presented.

A search for the standard model Higgs boson is performed in the diphoton decay channel. The data used correspond to an integrated luminosity of 4.9 fb(-1) collected with the ATLAS detector at the Large Hadron Collider in proton-proton collisions at a center-of-mass energy of √s=7 TeV. In the diphoton mass range 110-150 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 110-150 GeV, this significance becomes 1.5 standard deviations. The standard model Higgs boson is excluded at 95% confidence level in the mass ranges of 113-115 GeV and 134.5-136 GeV.

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 %.

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 proton-proton collision data collected in 2011 at sqrt(s) = 7 TeV and corresponding to an integrated luminosity of 4.7/fb. Tag-and-probe methods using events with leptonic decays of W and Z bosons and J/psi 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.

In order to study further the long-range correlations ("ridge") observed recently in p+Pb collisions at sqrt(s_NN) =5.02 TeV, the second-order azimuthal anisotropy parameter of charged particles, v_2, has been measured with the cumulant method using the ATLAS detector at the LHC. In a data sample corresponding to an integrated luminosity of approximately 1 microb^(-1), the parameter v_2 has been obtained using two- and four-particle cumulants over the pseudorapidity range |eta|<2.5. The results are presented as a function of transverse momentum and the event activity, defined in terms of the transverse energy summed over 3.1<eta<4.9 in the direction of the Pb beam. They show features characteristic of collective anisotropic flow, similar to that observed in Pb+Pb collisions. A comparison is made to results obtained using two-particle correlation methods, and to predictions from hydrodynamic models of p+Pb collisions. Despite the small transverse spatial extent of the p+Pb collision system, the large magnitude of v_2 and its similarity to hydrodynamic predictions provide additional evidence for the importance of final-state effects in p+Pb reactions.

A bstract The distributions of event-by-event harmonic flow coefficients v n for n = 2- 4 are measured in $ \sqrt{{{s_{NN }}}} $ = 2 . 76 TeV Pb + Pb collisions using the ATLAS detector at the LHC. The measurements are performed using charged particles with transverse momentum p T &gt; 0 . 5 GeV and in the pseudorapidity range | η | &lt; 2 . 5 in a dataset of approximately 7 μ b −1 recorded in 2010. The shapes of the v n distributions suggest that the associated flow vectors are described by a two-dimensional Gaussian function in central collisions for v 2 and over most of the measured centrality range for v 3 and v 4 . Significant deviations from this function are observed for v 2 in mid-central and peripheral collisions, and a small deviation is observed for v 3 in mid-central collisions. In order to be sensitive to these deviations, it is shown that the commonly used multi-particle cumulants, involving four particles or more, need to be measured with a precision better than a few percent. The v n distributions are also measured independently for charged particles with 0 . 5 &lt; p T &lt; 1 GeV and p T &gt; 1 GeV. When these distributions are rescaled to the same mean values, the adjusted shapes are found to be nearly the same for these two p T ranges. The v n distributions are compared with the eccentricity distributions from two models for the initial collision geometry: a Glauber model and a model that includes corrections to the initial geometry due to gluon saturation effects. Both models fail to describe the experimental data consistently over most of the measured centrality range.

This paper describes a measurement of the $Z/\gamma^*$ boson transverse momentum spectrum using ATLAS proton-proton collision data at a centre-of-mass energy of $\sqrt{s}$ = 7 TeV at the LHC. The measurement is performed in the $Z/\gamma^* \rightarrow e^+e^-$ and $Z/\gamma^* \rightarrow \mu^+\mu^-$ channels, using data corresponding to an integrated luminosity of 4.7 fb$^{-1}$. Normalized differential cross sections as a function of the $Z/\gamma^*$ boson transverse momentum are measured for transverse momenta up to 800 GeV. The measurement is performed inclusively for $Z/\gamma^*$ 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.

Measurements of luminosity obtained using the ATLAS detector during early running of the Large Hadron Collider (LHC) at $\sqrt{s} = 7$ TeV are presented. The luminosity is independently determined using several detectors and multiple algorithms, each having different acceptances, systematic uncertainties and sensitivity to background. The ratios of the luminosities obtained from these methods are monitored as a function of time and of μ, the average number of inelastic interactions per bunch crossing. Residual time- and μ-dependence between the methods is less than 2% for 0<μ<2.5. Absolute luminosity calibrations, performed using beam separation scans, have a common systematic uncertainty of ±11%, dominated by the measurement of the LHC beam currents. After calibration, the luminosities obtained from the different methods differ by at most ±2%. The visible cross sections measured using the beam scans are compared to predictions obtained with the PYTHIA and PHOJET event generators and the ATLAS detector simulation.

A search for squarks and gluinos in final states containing jets, missing transverse momentum and no electrons or muons is presented. The data were recorded by the ATLAS experiment in sqrt(s) = 7 TeV proton-proton collisions at the Large Hadron Collider. No excess above the Standard Model background expectation was observed in 35 inverse picobarns of analysed data. Gluino masses below 500 GeV are excluded at the 95% confidence level in simplified models containing only squarks of the first two generations, a gluino octet and a massless neutralino. The exclusion increases to 870 GeV for equal mass squarks and gluinos. In MSUGRA/CMSSM models with tan(beta)= 3, A_0=0 and mu>0, squarks and gluinos of equal mass are excluded below 775 GeV. These are the most stringent limits to date.

The production cross sections of the inclusive Drell-Yan processes W±→ℓν and Z/γ∗→ℓℓ (ℓ=e, μ) are measured in proton-proton collisions at √s=7 TeV with the ATLAS detector. The cross sections are reported integrated over a fiducial kinematic range, extrapolated to the full range, and also evaluated differentially as a function of the W decay lepton pseudorapidity and the Z boson rapidity, respectively. Based on an integrated luminosity of about 35 pb−1 collected in 2010, the precision of these measurements reaches a few percent. The integrated and the differential W± and Z/γ∗ cross sections in the e and μ channels are combined, and compared with perturbative QCD calculations, based on a number of different parton distribution sets available at next-to-next-to-leading order.12 MoreReceived 26 September 2011DOI:https://doi.org/10.1103/PhysRevD.85.072004This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.© 2012 CERN, for the ATLAS Collaboration

A measurement of the production cross-section for top quark pairs ( $t\bar{t}$ ) in pp collisions at $\sqrt{s}=7\ \ \mbox{TeV}$ is presented using data recorded with the ATLAS detector at the Large Hadron Collider. Events are selected in two different topologies: single lepton (electron e or muon μ) with large missing transverse energy and at least four jets, and dilepton (ee, μμ or eμ) with large missing transverse energy and at least two jets. In a data sample of 2.9 pb−1, 37 candidate events are observed in the single-lepton topology and 9 events in the dilepton topology. The corresponding expected backgrounds from non- $t\bar{t}$ Standard Model processes are estimated using data-driven methods and determined to be 12.2±3.9 events and 2.5±0.6 events, respectively. The kinematic properties of the selected events are consistent with SM $t\bar{t}$ production. The inclusive top quark pair production cross-section is measured to be $$\sigma_{t\bar{t}}=145\pm31(\mathrm{stat.}) {}^{+42}_{-27}(\mathrm {syst.})\ \mbox{pb}.$$ The measurement agrees with perturbative QCD calculations.

The inclusive J/ψ production cross-section and fraction of J/ψ mesons produced in B-hadron decays are measured in proton–proton collisions at s=7 TeV with the ATLAS detector at the LHC, as a function of the transverse momentum and rapidity of the J/ψ, using 2.3 pb−1 of integrated luminosity. The cross-section is measured from a minimum pT of 1 GeV to a maximum of 70 GeV and for rapidities within |y|<2.4 giving the widest reach of any measurement of J/ψ production to date. The differential production cross-sections of prompt and non-prompt J/ψ are separately determined and are compared to Colour Singlet NNLO⋆, Colour Evaporation Model, and FONLL predictions.

The production of W bosons in association with two jets in proton–proton collisions at a centre-of-mass energy of has been analysed for the presence of double-parton interactions using data corresponding to an integrated luminosity of 36 pb−1, collected with the ATLAS detector at the Large Hadron Collider. The fraction of events arising from double-parton interactions, f(D)DP, has been measured through the pT balance between the two jets and amounts to f(D)DP = 0.08 ± 0.01 (stat.) ± 0.02 (sys.) for jets with transverse momentum pT > 20 GeV and rapidity |y| < 2.8. This corresponds to a measurement of the effective area parameter for hard double-parton interactions of σeff = 15 ± 3 (stat.) +5−3 (sys.) mb.

Inclusive jet and dijet cross sections have been measured in proton-proton collisions at a centre-of-mass energy of 7 TeV using the ATLAS detector at the Large Hadron Collider. The cross sections were measured using jets clustered with the anti-kT algorithm with parameters R=0.4 and R=0.6. These measurements are based on the 2010 data sample, consisting of a total integrated luminosity of 37 inverse picobarns. Inclusive jet double-differential cross sections are presented as a function of jet transverse momentum, in bins of jet rapidity. Dijet double-differential cross sections are studied as a function of the dijet invariant mass, in bins of half the rapidity separation of the two leading jets. The measurements are performed in the jet rapidity range |y|<4.4, covering jet transverse momenta from 20 GeV to 1.5 TeV and dijet invariant masses from 70 GeV to 5 TeV. The data are compared to expectations based on next-to-leading order QCD calculations corrected for non-perturbative effects, as well as to next-to-leading order Monte Carlo predictions. In addition to a test of the theory in a new kinematic regime, the data also provide sensitivity to parton distribution functions in a region where they are currently not well-constrained.

A search for squarks and gluinos in final states containing high-p T jets, missing transverse momentum and no electrons or muons is presented. The data were recorded in 2012 by the ATLAS experiment in $$ \sqrt{s}=8 $$ TeV proton-proton collisions at the Large Hadron Collider, with a total integrated luminosity of 20.3 fb−1. Results are interpreted in a variety of simplified and specific supersymmetry-breaking models assuming that R-parity is conserved and that the lightest neutralino is the lightest supersymmetric particle. An exclusion limit at the 95% confidence level on the mass of the gluino is set at 1330 GeV for a simplified model incorporating only a gluino and the lightest neutralino. For a simplified model involving the strong production of first- and second-generation squarks, squark masses below 850 GeV (440 GeV) are excluded for a massless lightest neutralino, assuming mass degenerate (single light-flavour) squarks. In mSUGRA/CMSSM models with tan β = 30, A 0 = −2m 0 and μ > 0, squarks and gluinos of equal mass are excluded for masses below 1700 GeV. Additional limits are set for non-universal Higgs mass models with gaugino mediation and for simplified models involving the pair production of gluinos, each decaying to a top squark and a top quark, with the top squark decaying to a charm quark and a neutralino. These limits extend the region of supersymmetric parameter space excluded by previous searches with the ATLAS detector.

A bstract A search for new phenomena in events with a high-energy jet and large missing transverse momentum is performed using data from proton-proton collisions at $ \sqrt{s}=7 $ TeV with the ATLAS experiment at the Large Hadron Collider. Four kinematic regions are explored using a dataset corresponding to an integrated luminosity of 4.7 fb −1 . No excess of events beyond expectations from Standard Model processes is observed, and limits are set on large extra dimensions and the pair production of dark matter particles.

A measurement of the total $pp$ cross section at the LHC at $\sqrt{s}=7$ TeV is presented. In a special run with high-$\beta^{\star}$ beam optics, an integrated luminosity of 80 $\mu$b$^{-1}$ was accumulated in order to measure the differential elastic cross section as a function of the Mandelstam momentum transfer variable $t$. The measurement is performed with the ALFA sub-detector of ATLAS. Using a fit to the differential elastic cross section in the $|t|$ range from 0.01 GeV$^2$ to 0.1 GeV$^2$ to extrapolate to $|t|\rightarrow 0$, the total cross section, $\sigma_{\mathrm{tot}}(pp\rightarrow X)$, is measured via the optical theorem to be: $$\sigma_{\mathrm{tot}}(pp\rightarrow X) = 95.35 \; \pm 0.38 \; ({\mbox{stat.}}) \pm 1.25 \; ({\mbox{exp.}}) \pm 0.37 \; (\mbox{extr.}) \; \mbox{mb},$$ where the first error is statistical, the second accounts for all experimental systematic uncertainties and the last is related to uncertainties in the extrapolation to $|t|\rightarrow 0$. In addition, the slope of the elastic cross section at small $|t|$ is determined to be $B = 19.73 \pm 0.14 \; ({\mbox{stat.}}) \pm 0.26 \; ({\mbox{syst.}}) \; \mbox{GeV}^{-2}$.

A search for scalar particles decaying via narrow resonances into two photons in the mass range 65-600 GeV is performed using 20.3 fb(-1) of √s 8 TeV pp collision data collected with the ATLAS detector at the Large Hadron Collider. The recently discovered Higgs boson is treated as a background. No significant evidence for an additional signal is observed. The results are presented as limits at the 95% confidence level on the production cross section of a scalar boson times branching ratio into two photons, in a fiducial volume where the reconstruction efficiency is approximately independent of the event topology. The upper limits set extend over a considerably wider mass range than previous searches.

A search for squarks and gluinos in events containing jets, missing transverse momentum and no electrons or muons is presented. The data were recorded in 2011 by the ATLAS experiment in s=7 TeV proton–proton collisions at the Large Hadron Collider. No excess above the Standard Model background expectation is observed in 1.04 fb−1 of data. Gluino and squark masses below 700 GeV and 875 GeV respectively are excluded at the 95% confidence level in simplified models containing only squarks of the first two generations, a gluino octet and a massless neutralino. The exclusion limit increases to 1075 GeV for squarks and gluinos of equal mass. In MSUGRA/CMSSM models with tanβ=10, A0=0 and μ>0, squarks and gluinos of equal mass are excluded for masses below 950 GeV. These limits extend the region of supersymmetric parameter space excluded by previous measurements.

The first measurements from proton–proton collisions recorded with the ATLAS detector at the LHC are presented. Data were collected in December 2009 using a minimum-bias trigger during collisions at a centre-of-mass energy of 900 GeV. The charged-particle multiplicity, its dependence on transverse momentum and pseudorapidity, and the relationship between mean transverse momentum and charged-particle multiplicity are measured for events with at least one charged particle in the kinematic range |η|<2.5 and pT>500 MeV. The measurements are compared to Monte Carlo models of proton–proton collisions and to results from other experiments at the same centre-of-mass energy. The charged-particle multiplicity per event and unit of pseudorapidity at η=0 is measured to be 1.333±0.003(stat.)±0.040(syst.), which is 5–15% higher than the Monte Carlo models predict.

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 (˜t1→c+˜χ01) 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 (m˜t1, m˜χ01) 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 ˜t1→b+ff′+˜χ01 and sbottom pair production with ˜b1→b+˜χ01, 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.5 MoreReceived 3 July 2014DOI:https://doi.org/10.1103/PhysRevD.90.052008This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.© 2014 CERN, for the ATLAS Collaboration

This Letter presents the first search for supersymmetry in final states containing one isolated electron or muon, jets, and missing transverse momentum from √s=7 TeV proton-proton collisions at the LHC. The data were recorded by the ATLAS experiment during 2010 and correspond to a total integrated luminosity of 35 pb(-1). No excess above the standard model background expectation is observed. Limits are set on the parameters of the minimal supergravity framework, extending previous limits. Within this framework, for A(0)=0 GeV, tanβ=3, and μ>0 and for equal squark and gluino masses, gluino masses below 700 GeV are excluded at 95% confidence level.

Using the ATLAS detector, a centrality-dependent suppression has been observed in the yield of J/ψ mesons produced in the collisions of lead ions at the Large Hadron Collider. In a sample of minimum-bias lead–lead collisions at a nucleon–nucleon centre of mass energy sNN=2.76TeV, corresponding to an integrated luminosity of about 6.7μb−1, J/ψ mesons are reconstructed via their decays to μ+μ− pairs. The measured J/ψ yield, normalized to the number of binary nucleon–nucleon collisions, is found to significantly decrease from peripheral to central collisions. The centrality dependence is found to be qualitatively similar to the trends observed at previous, lower energy experiments. The same sample is used to reconstruct Z bosons in the μ+μ− final state, and a total of 38 candidates are selected in the mass window of 66 to 116 GeV. The relative Z yields as a function of centrality are also presented, although no conclusion can be inferred about their scaling with the number of binary collisions, because of limited statistics. This analysis provides the first results on J/ψ and Z production in lead–lead collisions at the LHC.

Jet cross sections have been measured for the first time in proton-proton collisions at a centre-of-mass energy of 7 TeV using the ATLAS detector. The measurement uses an integrated luminosity of 17 nb-1 recorded at the Large Hadron Collider. The anti-kt algorithm is used to identify jets, with two jet resolution parameters, R = 0.4 and 0.6. The dominant uncertainty comes from the jet energy scale, which is determined to within 7% for central jets above 60 GeV transverse momentum. Inclusive single-jet differential cross sections are presented as functions of jet transverse momentum and rapidity. Dijet cross sections are presented as functions of dijet mass and the angular variable $\chi$. The results are compared to expectations based on next-to-leading-order QCD, which agree with the data, providing a validation of the theory in a new kinematic regime.

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.Received 16 September 2013DOI:https://doi.org/10.1103/PhysRevLett.112.041802This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.© 2014 CERN, for the ATLAS Collaboration

The results of a search for charged Higgs bosons are presented. The analysis is based on 4.6/fb of proton-proton collision data at sqrt(s) = 7 TeV collected by the ATLAS experiment at the Large Hadron Collider, using top quark pair events with a tau lepton in the final state. The data are consistent with the expected background from Standard Model processes. Assuming that the branching ratio of the charged Higgs boson to a tau lepton and a neutrino is 100%, this leads to upper limits on the branching ratio of top quark decays to a b quark and a charged Higgs boson between 5% and 1% for charged Higgs boson masses ranging from 90 GeV to 160 GeV, respectively. In the context of the mh-max scenario of the MSSM, tan(beta) above 12-26, as well as between 1 and 2-6, can be excluded for charged Higgs boson masses between 90 GeV and 150 GeV.

A measurement of the cross section for the inclusive production of isolated prompt photons in pp collisions at a centre-of-mass energy sqrt(s) = 7TeV is presented. The measurement covers the pseudorapidity ranges |eta|<1.37 and 1.52<|eta|<1.81 in the transverse energy range 15 < E_T <100 GeV. The results are based on an integrated luminosity of 880 nb-1, collected with the ATLAS detector at the Large Hadron Collider. Photon candidates are identified by combining information from the calorimeters and from the inner tracker. Residual background in the selected sample is estimated from data based on the observed distribution of the transverse isolation energy in a narrow cone around the photon candidate. The results are compared to predictions from next-to-leading order perturbative QCD calculations.

Measurements of inclusive jet production are performed in pp and Pb+Pb collisions at √(s)NN=2.76 TeV with the ATLAS detector at the LHC, corresponding to integrated luminosities of 4.0 and 0.14 nb(-1), respectively. The jets are identified with the anti-k(t) algorithm with R=0.4, and the spectra are measured over the kinematic range of jet transverse momentum 32<p(T)<500 GeV and absolute rapidity |y|<2.1 and as a function of collision centrality. The nuclear modification factor R(AA) is evaluated, and jets are found to be suppressed by approximately a factor of 2 in central collisions compared to pp collisions. The R(AA) shows a slight increase with p(T) and no significant variation with rapidity.

The Tile hadronic calorimeter of the ATLAS detector has undergone extensive testing in the experimental hall since its installation in late 2005. The readout, control and calibration systems have been fully operational since 2007 and the detector successfully collected data from the LHC single beams in 2008 and first collisions in 2009. This paper gives an overview of the Tile Calorimeter performance as measured using random triggers, calibration data, data from cosmic ray muons and single beam data. The detector operation status, noise characteristics and performance of the calibration systems are presented, as well as the validation of the timing and energy calibration carried out with minimum ionising cosmic ray muons data. The calibration systems' precision is well below the design of 1%. The determination of the global energy scale was performed with an uncertainty of 4%.

A QCD analysis is reported of ATLAS data on inclusive W(±) and Z boson production in pp collisions at the LHC, jointly with ep deep-inelastic scattering data from HERA. The ATLAS data exhibit sensitivity to the light quark sea composition and magnitude at Bjorken x∼0.01. Specifically, the data support the hypothesis of a symmetric composition of the light quark sea at low x. The ratio of the strange-to-down sea quark distributions is determined to be 1.00(-0.28)(+0.25) at absolute four-momentum transfer squared Q(2)=1.9 GeV(2) and x=0.023.

We report a measurement of the cross section of single top-quark production in the t-channel using 1.04 fb^-1 of pp collision data at sqrt(s) = 7 TeV recorded with the ATLAS detector at the LHC. Selected events feature one electron or muon, missing transverse momentum, and two or three jets, exactly one of them identified as originating from a b quark. The cross section is measured by fitting the distribution of a multivariate discriminant constructed with a neural network, yielding sigma(t)= 83 +/- 4 (stat.) +20 -19 (syst) pb which is in good agreement with the prediction of the Standard Model. Using the ratio of the measured to the theoretically predicted cross section and assuming that the top-quark-related CKM matrix elements obey the relation |V(tb)| &gt;&gt; |V(ts)|, |V(td)|, the coupling strength at the W-t-b vertex is determined to be |V(tb)| = 1.13 +0.14 -0.13. If it is assumed that |V(tb)| &lt;= 1 a lower limit of |V(tb)|&gt;0.75 is obtained at the 95% confidence level.

A search for new interactions and resonances produced in LHC proton–proton (pp) collisions at a centre-of-mass energy was performed with the ATLAS detector. Using a dataset with an integrated luminosity of 36 pb− 1, dijet mass and angular distributions were measured up to dijet masses of ∼3.5 TeV and were found to be in good agreement with Standard Model predictions. This analysis sets limits at 95% CL on various models for new physics: an excited quark is excluded for mass between 0.60 and 2.64 TeV, an axigluon hypothesis is excluded for axigluon masses between 0.60 and 2.10 TeV and quantum black holes are excluded in models with six extra space–time dimensions for quantum gravity scales between 0.75 and 3.67 TeV. Production cross section limits as a function of dijet mass are set using a simplified Gaussian signal model to facilitate comparisons with other hypotheses. Analysis of the dijet angular distribution using a novel technique simultaneously employing the dijet mass excludes quark contact interactions with a compositeness scale Λ below 9.5 TeV.

This Letter presents a search for new resonances decaying to final states with a vector boson produced in association with a high transverse momentum photon, Vγ, with V=W(→ℓν) or Z(→ℓ+ℓ−), where ℓ=e or μ. The measurements use 20.3 fb−1 of proton–proton collision data at a center-of-mass energy of s=8TeV recorded with the ATLAS detector. No deviations from the Standard Model expectations are found, and production cross section limits are set at 95% confidence level. Masses of the hypothetical aT and ωT states of a benchmark Low Scale Technicolor model are excluded in the ranges [275,960]GeV and [200,700]∪[750,890]GeV, respectively. Limits at 95% confidence level on the production cross section of a singlet scalar resonance decaying to Zγ final states have also been obtained for masses below 1180 GeV.

Measurements of two-particle correlation functions and the first five azimuthal harmonics, ${v}_{1}$ to ${v}_{5}$, are presented, using 28 ${\mathrm{nb}}^{\ensuremath{-}1}$ of $p+\text{Pb}$ collisions at a nucleon-nucleon center-of-mass energy of $\sqrt{{s}_{NN}}=5.02$ TeV measured with the ATLAS detector at the LHC. Significant long-range ``ridgelike'' correlations are observed for pairs with small relative azimuthal angle ($|\ensuremath{\Delta}\ensuremath{\phi}|&lt;\ensuremath{\pi}/3$) and back-to-back pairs ($|\ensuremath{\Delta}\ensuremath{\phi}|&gt;2\ensuremath{\pi}/3$) over the transverse momentum range $0.4&lt;{p}_{\mathrm{T}}&lt;12$ GeV and in different intervals of event activity. The event activity is defined by either the number of reconstructed tracks or the total transverse energy on the Pb-fragmentation side. The azimuthal structure of such long-range correlations is Fourier decomposed to obtain the harmonics ${v}_{n}$ as a function of ${p}_{\mathrm{T}}$ and event activity. The extracted ${v}_{n}$ values for $n=2$ to 5 decrease with $n$. The ${v}_{2}$ and ${v}_{3}$ values are found to be positive in the measured ${p}_{\mathrm{T}}$ range. The ${v}_{1}$ is also measured as a function of ${p}_{\mathrm{T}}$ and is observed to change sign around ${p}_{\mathrm{T}}\ensuremath{\approx}1.5$--2.0 GeV and then increase to about 0.1 for ${p}_{\mathrm{T}}&gt;4$ GeV. The ${v}_{2}({p}_{\mathrm{T}})$, ${v}_{3}({p}_{\mathrm{T}})$, and ${v}_{4}({p}_{\mathrm{T}})$ are compared to the ${v}_{n}$ coefficients in $\mathrm{Pb}+\mathrm{Pb}$ collisions at $\sqrt{{s}_{NN}}=2.76$ TeV with similar event multiplicities. Reasonable agreement is observed after accounting for the difference in the average ${p}_{\mathrm{T}}$ of particles produced in the two collision systems.

The inclusive top quark pair ([Formula: see text]) production cross-section [Formula: see text] has been measured in proton-proton collisions at [Formula: see text] and [Formula: see text] with the ATLAS experiment at the LHC, using [Formula: see text] events with an opposite-charge [Formula: see text] pair in the final state. The measurement was performed with the 2011 7 TeV dataset corresponding to an integrated luminosity of 4.6 [Formula: see text] and the 2012 8 TeV dataset of 20.3 [Formula: see text]. The numbers of events with exactly one and exactly two [Formula: see text]-tagged jets were counted and used to simultaneously determine [Formula: see text] and the efficiency to reconstruct and [Formula: see text]-tag a jet from a top quark decay, thereby minimising the associated systematic uncertainties. The cross-section was measured to be: [Formula: see text]where the four uncertainties arise from data statistics, experimental and theoretical systematic effects, knowledge of the integrated luminosity and of the LHC beam energy. The results are consistent with recent theoretical QCD calculations at next-to-next-to-leading order. Fiducial measurements corresponding to the experimental acceptance of the leptons are also reported, together with the ratio of cross-sections measured at the two centre-of-mass energies. The inclusive cross-section results were used to determine the top quark pole mass via the dependence of the theoretically predicted cross-section on [Formula: see text] giving a result of [Formula: see text][Formula: see text] GeV. By looking for an excess of [Formula: see text] production with respect to the QCD prediction, the results were also used to place limits on the pair-production of supersymmetric top squarks [Formula: see text] with masses close to the top quark mass, decaying via [Formula: see text] to predominantly right-handed top quarks and a light neutralino [Formula: see text], the lightest supersymmetric particle. Top squarks with masses between the top quark mass and 177 GeV are excluded at the 95 % confidence level.

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.

The ATLAS experiment at the LHC has measured the centrality dependence of charged particle pseudorapidity distributions over |eta| < 2 in lead-lead collisions at a nucleon-nucleon centre-of-mass energy of sqrt(s_NN) = 2.76 TeV. In order to include particles with transverse momentum as low as 30 MeV, the data were recorded with the central solenoid magnet off. Charged particles were reconstructed with two algorithms (2-point "tracklets" and full tracks) using information from the pixel detector only. The lead-lead collision centrality was characterized by the total transverse energy in the forward calorimeter in the range 3.2 < |eta| < 4.9. Measurements are presented of the per-event charged particle density distribution, dN_ch/deta, and the average charged particle multiplicity in the pseudorapidity interval |eta|<0.5 in several intervals of collision centrality. The results are compared to previous mid-rapidity measurements at the LHC and RHIC. The variation of the mid-rapidity charged particle yield per colliding nucleon pair with the number of participants is consistent with the lower sqrt(s_NN) results. The shape of the dN_ch/deta distribution is found to be independent of centrality within the systematic uncertainties of the measurement.

A combined search for the Standard Model Higgs boson with the ATLAS detector at the LHC is presented. The datasets used correspond to integrated luminosities from 4.6 fb^-1 to 4.9 fb^-1 of proton-proton collisions collected at sqrt(s) = 7 TeV in 2011. The Higgs boson mass ranges of 111.4 GeV to 116.6 GeV, 119.4 GeV to 122.1 GeV, and 129.2 GeV to 541 GeV are excluded at the 95% confidence level, while the range 120 GeV to 560 GeV is expected to be excluded in the absence of a signal. An excess of events is observed at Higgs boson mass hypotheses around 126 GeV with a local significance of 2.9 standard deviations (sigma). The global probability for the background to produce an excess at least as significant anywhere in the entire explored Higgs boson mass range of 110-600 GeV is estimated to be ~15%, corresponding to a significance of approximately one sigma.

A search for squarks and gluinos in final states containing jets, missing transverse momentum and no high-pT electrons or muons is presented. The data represent the complete sample recorded in 2011 by the ATLAS experiment in 7 TeV proton-proton collisions at the Large Hadron Collider, with a total integrated luminosity of 4.7 fb−1. No excess above the Standard Model background expectation is observed. Gluino masses below 860 GeV and squark masses below 1320 GeV are excluded at the 95% confidence level in simplified models containing only squarks of the first two generations, a gluino octet and a massless neutralino, for squark or gluino masses below 2 TeV, respectively. Squarks and gluinos with equal masses below 1410 GeV are excluded. In minimal supergravity/constrained minimal supersymmetric Standard Model models with tanβ=10, A0=0 and μ>0, squarks and gluinos of equal mass are excluded for masses below 1360 GeV. Constraints are also placed on the parameter space of supersymmetric models with compressed spectra. These limits considerably extend the region of supersymmetric parameter space excluded by previous measurements with the ATLAS detector.8 MoreReceived 4 August 2012DOI:https://doi.org/10.1103/PhysRevD.87.012008This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.© 2013 CERN, for the ATLAS Collaboration

A bstract Recent studies have highlighted the potential of jet substructure techniques to identify the hadronic decays of boosted heavy particles. These studies all rely upon the assumption that the internal substructure of jets generated by QCD radiation is well understood. In this article, this assumption is tested on an inclusive sample of jets recorded with the ATLAS detector in 2010, which corresponds to 35 pb −1 of pp collisions delivered by the LHC at $ \sqrt {s} = 7\;{\text{TeV}} $ . In a subsample of events with single pp collisions, measurements corrected for detector efficiency and resolution are presented with full systematic uncertainties. Jet invariant mass, k t splitting scales and N -subjettiness variables are presented for anti- k t R = 1.0 jets and Cambridge-Aachen R = 1.2 jets. Jet invariant-mass spectra for Cambridge-Aachen R = 1.2 jets after a splitting and filtering procedure are also presented. Leading-order parton-shower Monte Carlo predictions for these variables are found to be broadly in agreement with data. The dependence of mean jet mass on additional pp interactions is also explored.

A bstract The results of a search for pair production of supersymmetric partners of the Standard Model third-generation quarks are reported. This search uses 20.1 fb −1 of pp collisions at $ \sqrt{s}=8 $ TeV collected by the ATLAS experiment at the Large Hadron Collider. The lightest bottom and top squarks ( $ {{\widetilde{b}}_1} $ and $ {{\widetilde{t}}_1} $ respectively) are searched for in a final state with large missing transverse momentum and two jets identified as originating from b -quarks. No excess of events above the expected level of Standard Model background is found. The results are used to set upper limits on the visible cross section for processes beyond the Standard Model. Exclusion limits at the 95 % confidence level on the masses of the third-generation squarks are derived in phenomenological supersymmetric R -parity-conserving models in which either the bottom or the top squark is the lightest squark. The $ {{\widetilde{b}}_1} $ is assumed to decay via $ {{\widetilde{b}}_1}\to b_{{\chi 1}}^{{\sim 0}} $ and the $ {{\widetilde{t}}_1} $ via $ {{\widetilde{t}}_1}\to b_{{\chi 1}}^{{\sim \pm }} $ , with undetectable products of the subsequent decay of the $ _{{\chi 1}}^{{\sim \pm }} $ due to the small mass splitting between the $ _{{\chi 1}}^{{\sim \pm }} $ and the $ _{{\chi 1}}^{{\sim 0}} $ .

A search for new heavy particles manifested as resonances in two-jet final states is presented. The data were produced in 7 TeV proton-proton collisions by the LHC and correspond to an integrated luminosity of 315 nb⁻¹ collected by the ATLAS detector. No resonances were observed. Upper limits were set on the product of cross section and signal acceptance for excited-quark (q*) production as a function of q* mass. These exclude at the 95% C.L. the q* mass interval 0.30<m(q*)<1.26 TeV, extending the reach of previous experiments.

A search for the direct production of charginos and neutralinos in final states with three leptons and missing transverse momentum is presented. The analysis is based on 20.3 fb-1 of sqrt(s) = 8 TeV proton--proton collision data delivered by the Large Hadron Collider and recorded with the ATLAS detector. Observations are consistent with the Standard Model expectations and limits are set in R-parity-conserving phenomenological Minimal Supersymmetric Standard Models and in simplified supersymmetric models, significantly extending previous results. For simplified supersymmetric models of direct chargino ($\tilde\chi^\pm_1$) and next-to-lightest neutralino ($\tilde{\chi}_2^0$) production with decays to lightest neutralino ($\tilde{\chi}_1^0$) via either all three generations of sleptons, staus only, gauge bosons, or Higgs bosons, $\tilde\chi^\pm_1$ and $\tilde{\chi}_2^0$ masses are excluded up to 700 GeV, 380 GeV, 345 GeV, or 148 GeV respectively, for a massless $\tilde{\chi}_1^0$.

This Letter presents a search for the Standard Model Higgs boson in the decay channel H->ZZ(*)->l+l-l'+l'-, where l,l'=e or mu, using proton-proton collisions at sqrt(s) = 7 TeV recorded with the ATLAS detector and corresponding to an integrated luminosity of 4.8 fb^-1. The four-lepton mass distribution is compared with Standard Model background expectations to derive upper limits on the cross section of a Standard Model Higgs boson with a mass between 110 GeV and 600 GeV. The mass ranges 134-156 GeV, 182-233 GeV, 256-265 GeV and 268-415 GeV are excluded at the 95% confidence level. The largest upward deviations from the background-only hypothesis are observed for Higgs boson masses of 125, 244 and 500 GeV with local significances of 2.1, 2.2 and 2.1 standard deviations, respectively. Once the look-elsewhere effect is considered, none of these excesses are significant.

Measurements of the centrality and rapidity dependence of inclusive jet production in sNN=5.02 TeV proton–lead (p+Pb) collisions and the jet cross-section in s=2.76 TeV proton–proton collisions are presented. These quantities are measured in datasets corresponding to an integrated luminosity of 27.8 nb−1 and 4.0 pb−1, respectively, recorded with the ATLAS detector at the Large Hadron Collider in 2013. The p+Pb collision centrality was characterised using the total transverse energy measured in the pseudorapidity interval −4.9<η<−3.2 in the direction of the lead beam. Results are presented for the double-differential per-collision yields as a function of jet rapidity and transverse momentum (pT) for minimum-bias and centrality-selected p+Pb collisions, and are compared to the jet rate from the geometric expectation. The total jet yield in minimum-bias events is slightly enhanced above the expectation in a pT-dependent manner but is consistent with the expectation within uncertainties. The ratios of jet spectra from different centrality selections show a strong modification of jet production at all pT at forward rapidities and for large pT at mid-rapidity, which manifests as a suppression of the jet yield in central events and an enhancement in peripheral events. These effects imply that the factorisation between hard and soft processes is violated at an unexpected level in proton–nucleus collisions. Furthermore, the modifications at forward rapidities are found to be a function of the total jet energy only, implying that the violations may have a simple dependence on the hard parton–parton kinematics.

Pseudorapidity gap distributions in proton-proton collisions at $\sqrt {s} =7~\mbox{~TeV}$ are studied using a minimum bias data sample with an integrated luminosity of 7.1 μb−1. Cross sections are measured differentially in terms of Δη F , the larger of the pseudorapidity regions extending to the limits of the ATLAS sensitivity, at η=±4.9, in which no final state particles are produced above a transverse momentum threshold $p_{\mathrm{T}}^{\mathrm{cut}}$ . The measurements span the region 0<Δη F <8 for $200 \mbox{~MeV}< p_{\mathrm{T}}^{\mathrm {cut}} < 800 \mbox{~MeV}$ . At small Δη F , the data test the reliability of hadronisation models in describing rapidity and transverse momentum fluctuations in final state particle production. The measurements at larger gap sizes are dominated by contributions from the single diffractive dissociation process (pp→Xp), enhanced by double dissociation (pp→XY) where the invariant mass of the lighter of the two dissociation systems satisfies M Y ≲7 GeV. The resulting cross section is dσ/dΔη F ≈1 mb for Δη F ≳3. The large rapidity gap data are used to constrain the value of the Pomeron intercept appropriate to triple Regge models of soft diffraction. The cross section integrated over all gap sizes is compared with other LHC inelastic cross section measurements.

This Letter reports on a search for narrow high-mass resonances decaying into dilepton final states. The data were recorded by the ATLAS experiment in $pp$ collisions at $\sqrt{s}=7\text{ }\text{ }\mathrm{TeV}$ at the Large Hadron Collider and correspond to a total integrated luminosity of 1.08 $(1.21)\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ in the ${e}^{+}{e}^{\ensuremath{-}}$ (${\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$) channel. No statistically significant excess above the standard model expectation is observed and upper limits are set at the 95% C.L. on the cross section times branching fraction of ${Z}^{\ensuremath{'}}$ resonances and Randall-Sundrum gravitons decaying into dileptons as a function of the resonance mass. A lower mass limit of 1.83 TeV on the sequential standard model ${Z}^{\ensuremath{'}}$ boson is set. A Randall-Sundrum graviton with coupling $k/{\overline{M}}_{\mathrm{Pl}}=0.1$ is excluded at 95% C.L. for masses below 1.63 TeV.

This Letter presents evidence for the associated production of a W boson and a top quark using 2.05 fb−1 of pp collision data at s=7TeV accumulated with the ATLAS detector at the LHC. The analysis is based on the selection of the dileptonic final states with events featuring two isolated leptons, electron or muon, with significant transverse missing momentum and at least one jet. An approach based on boosted decision trees has been developed to improve the discrimination of single top-quark Wt events from background. A template fit to the final classifier distributions is performed to determine the cross-section. The result is incompatible with the background-only hypothesis at the 3.3σ level, the expected sensitivity assuming the Standard Model production rate being 3.4σ. The corresponding cross-section is determined and found to be σWt=16.8±2.9(stat)±4.9(syst)pb, in good agreement with the Standard Model expectation. From this result the CKM matrix element |Vtb|=1.03−0.19+0.16 is derived assuming that the Wt production through |Vts| and |Vtd| is small.

A bstract A search for heavy Majorana neutrinos in events containing a pair of high- p T leptons of the same charge and high- p T jets is presented. The search uses 20.3 fb −1 of pp collision data collected with the ATLAS detector at the CERN Large Hadron Collider with a centre-of-mass energy of $$ \sqrt{s}=8 $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msqrt> <mml:mi>s</mml:mi> </mml:msqrt> <mml:mo>=</mml:mo> <mml:mn>8</mml:mn> </mml:math> TeV. The data are found to be consistent with the background-only hypothesis based on the Standard Model expectation. In the context of a Type-I seesaw mechanism, limits are set on the production cross-section times branching ratio for production of heavy Majorana neutrinos in the mass range between 100 and 500 GeV. The limits are subsequently interpreted as limits on the mixing between the heavy Majorana neutrinos and the Standard Model neutrinos. In the context of a left-right symmetric model, limits on the production cross-section times branching ratio are set with respect to the masses of heavy Majorana neutrinos and heavy gauge bosons W R and Z ′.

A bstract Measurements of the production of jets of particles in association with a Z boson in pp collisions at $ \sqrt{s}=7 $ TeV are presented, using data corresponding to an integrated luminosity of 4.6 fb −1 collected by the ATLAS experiment at the Large Hadron Collider. Inclusive and differential jet cross sections in Z events, with Z decaying into electron or muon pairs, are measured for jets with transverse momentum p T &gt; 30 GeV and rapidity | y | &lt; 4 . 4. The results are compared to next-to-leading-order perturbative QCD calculations, and to predictions from different Monte Carlo generators based on leading-order and next-to-leading-order matrix elements supplemented by parton showers.

A search for a charged Higgs boson (H +) in $t\bar{t}$ decays is presented, where one of the top quarks decays via t→H + b, followed by H +→ two jets ( $c\bar{s}$ ). 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 $\sqrt{s} = 7~\mathrm {TeV}$ 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 $\mathcal{B}(H^{+} \rightarrow c\bar{s})=100~\%$ .

This letter reports on a search for hypothetical heavy neutrinos, N, and right-handed gauge bosons, WR, in events with two high transverse momentum leptons and at least one high transverse momentum hadronic jet. The results were obtained from data corresponding to an integrated luminosity of 2.1 fb^-1 collected in proton-proton collisions at sqrt(s) = 7 TeV with the ATLAS detector at the CERN Large Hadron Collider. No excess above the Standard Model background expectation is observed. Excluded mass regions for Majorana and Dirac neutrinos are presented using two approaches for interactions that violate lepton and lepton-flavour numbers. One approach uses an effective operator framework, the other approach is guided by the Left-Right Symmetric Model. The results described in this letter represent the most stringent limits to date on the masses of heavy neutrinos and WR bosons obtained in direct searches.

This paper presents a measurement of the W+W− production cross section in pp collisions at √s=7 TeV. The leptonic decay channels are analyzed using data corresponding to an integrated luminosity of 4.6 fb−1 collected with the ATLAS detector at the Large Hadron Collider. The W+W− production cross section σ(pp→W+W−+X) is measured to be 51.9±2.0(stat)±3.9(syst)±2.0(lumi) pb, compatible with the Standard Model prediction of 44.7+2.1−1.9 pb. A measurement of the normalized fiducial cross section as a function of the leading lepton transverse momentum is also presented. The reconstructed transverse momentum distribution of the leading lepton is used to extract limits on anomalous WWZ and WWγ couplings.3 MoreReceived 10 October 2012DOI:https://doi.org/10.1103/PhysRevD.87.112001This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.© 2013 CERN, for the ATLAS Collaboration

This paper presents cross sections for the production of a [Formula: see text] boson in association with jets, measured in proton-proton collisions at [Formula: see text] with the ATLAS experiment at the large hadron collider. With an integrated luminosity of [Formula: see text], this data set allows for an exploration of a large kinematic range, including jet production up to a transverse momentum of [Formula: see text] and multiplicities up to seven associated jets. The production cross sections for [Formula: see text] bosons are measured in both the electron and muon decay channels. Differential cross sections for many observables are also presented including measurements of the jet observables such as the rapidities and the transverse momenta as well as measurements of event observables such as the scalar sums of the transverse momenta of the jets. The measurements are compared to numerous QCD predictions including next-to-leading-order perturbative calculations, resummation calculations and Monte Carlo generators.

A bstract A search is presented for new particles decaying to large numbers (7 or more) of jets, with missing transverse momentum and no isolated electrons or muons. This analysis uses 20.3 fb −1 of pp collision data at $ \sqrt{s}=8 $ TeV collected by the ATLAS experiment at the Large Hadron Collider. The sensitivity of the search is enhanced by considering the number of b -tagged jets and the scalar sum of masses of large-radius jets in an event. No evidence is found for physics beyond the Standard Model. The results are interpreted in the context of various simplified supersymmetry-inspired models where gluinos are pair produced, as well as an mSUGRA/CMSSM model.

The results of a search for top squark (stop) pair production in final states with one isolated lepton, jets, and missing transverse momentum are reported. The analysis is performed with proton--proton collision data at $\sqrt{s} = 8$ TeV collected with the ATLAS detector at the LHC in 2012 corresponding to an integrated luminosity of $20$ fb$^{-1}$. The lightest supersymmetric particle (LSP) is taken to be the lightest neutralino which only interacts weakly and is assumed to be stable. The stop decay modes considered are those to a top quark and the LSP as well as to a bottom quark and the lightest chargino, where the chargino decays to the LSP by emitting a $W$ boson. A wide range of scenarios with different mass splittings between the stop, the lightest neutralino and the lightest chargino are considered, including cases where the $W$ bosons or the top quarks are off-shell. Decay modes involving the heavier charginos and neutralinos are addressed using a set of phenomenological models of supersymmetry. No significant excess over the Standard Model prediction is observed. A stop with a mass between $210$ and $640$ GeV decaying directly to a top quark and a massless LSP is excluded at $95$ % confidence level, and in models where the mass of the lightest chargino is twice that of the LSP, stops are excluded at $95$ % confidence level up to a mass of $500$ GeV for an LSP mass in the range of $100$ to $150$ GeV. Stringent exclusion limits are also derived for all other stop decay modes considered, and model-independent upper limits are set on the visible cross-section for processes beyond the Standard Model.

A search is reported for a neutral Higgs boson in the decay channel H→Zγ, Z→ℓ+ℓ− (ℓ=e,μ), using 4.5 fb−1 of pp collisions at s=7TeV and 20.3 fb−1 of pp collisions at s=8TeV, recorded by the ATLAS detector at the CERN Large Hadron Collider. The observed distribution of the invariant mass of the three final-state particles, mℓℓγ, is consistent with the Standard Model hypothesis in the investigated mass range of 120–150 GeV. For a Higgs boson with a mass of 125.5 GeV, the observed upper limit at the 95% confidence level is 11 times the Standard Model expectation. Upper limits are set on the cross section times branching ratio of a neutral Higgs boson with mass in the range 120–150 GeV between 0.13 and 0.5 pb for s=8TeV at 95% confidence level.

This article presents measurements of the t-channel single top-quark (t) and top-antiquark (¯t) total production cross sections σ(tq) and σ(¯tq), their ratio Rt=σ(tq)/σ(¯tq), and a measurement of the inclusive production cross section σ(tq+¯tq) in proton-proton collisions at √s=7 TeV at the LHC. Differential cross sections for the tq and ¯tq 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(¯tq)=23±1(stat)±3(syst) pb, Rt=2.04±0.13(stat)±0.12(syst), and σ(tq+¯tq)=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+¯tq) 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±0.07.11 MoreReceived 30 June 2014DOI:https://doi.org/10.1103/PhysRevD.90.112006This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.© 2014 CERN, for the ATLAS Collaboration

A search for excited states of the Bc(±) meson is performed using 4.9 fb(-1) of 7 TeV and 19.2 fb(-1) of 8 TeV pp collision data collected by the ATLAS experiment at the LHC. A new state is observed through its hadronic transition to the ground state, with the latter detected in the decay Bc(±)→J/ψπ(±). The state appears in the m(Bc(±)π(+)π(-))-m(Bc(±))-2m(π(±)) mass difference distribution with a significance of 5.2 standard deviations. The mass of the observed state is 6842±4±5 MeV, where the first error is statistical and the second is systematic. The mass and decay of this state are consistent with expectations for the second S-wave state of the Bc(±) meson, Bc(±)(2S).

Invariant mass distributions of jet pairs (dijets) produced in LHC proton–proton collisions at a centre-of-mass energy s=7 TeV have been studied using a data set corresponding to an integrated luminosity of 1.0 fb-1 recorded in 2011 by ATLAS. Dijet masses up to ∼4 TeV are observed in the data, and no evidence of resonance production over background is found. Limits are set at 95% C.L. for several new physics hypotheses: excited quarks are excluded for masses below 299 TeV, axigluons are excluded for masses below 3.32 TeV, and colour octet scalar resonances are excluded for masses below 1.92 TeV.

Measurements of charged particle distributions, sensitive to the underlying event, have been performed with the ATLAS detector at the LHC. The measurements are based on data collected using a minimum-bias trigger to select proton-proton collisions at center-of-mass energies of 900 GeV and 7 TeV. The "underlying event" is defined as those aspects of a hadronic interaction attributed not to the hard scattering process, but rather to the accompanying interactions of the rest of the proton. Three regions are defined in azimuthal angle with respect to the highest-pt charged particle in the event, such that the region transverse to the dominant momentum-flow is most sensitive to the underlying event. In each of these regions, distributions of the charged particle multiplicity, pt density, and average pt are measured. The data show a higher underlying event activity than that predicted by Monte Carlo models tuned to pre-LHC data.

The χ(b)(nP) quarkonium states are produced in proton-proton collisions at the Large Hadron Collider at sqrt[s] = 7 TeV and recorded by the ATLAS detector. Using a data sample corresponding to an integrated luminosity of 4.4 fb(-1), these states are reconstructed through their radiative decays to Υ(1S,2S) with Υ → μ+ μ-. In addition to the mass peaks corresponding to the decay modes χ(b)(1P,2P) → Υ(1S)γ, a new structure centered at a mass of 10.530 ± 0.005(stat) ± 0.009(syst) GeV is also observed, in both the Υ(1S)γ and Υ(2S)γ decay modes. This structure is interpreted as the χ(b)(3P) system.

Nearly 50 years ago, theoretical physicists proposed that a field permeates the universe and gives energy to the vacuum. This field was required to explain why some, but not all, fundamental particles have mass. Numerous precision measurements during recent decades have provided indirect support for the existence of this field, but one crucial prediction of this theory has remained unconfirmed despite 30 years of experimental searches: the existence of a massive particle, the standard model Higgs boson. The ATLAS experiment at the Large Hadron Collider at CERN has now observed the production of a new particle with a mass of 126 giga-electron volts and decay signatures consistent with those expected for the Higgs particle. This result is strong support for the standard model of particle physics, including the presence of this vacuum field. The existence and properties of the newly discovered particle may also have consequences beyond the standard model itself.

Azimuthal decorrelations between the two central jets with the largest transverse momenta are sensitive to the dynamics of events with multiple jets. We present a measurement of the normalized differential cross section based on the full dataset (L=36/pb) acquired by the ATLAS detector during the 2010 sqrt(s)=7 TeV proton-proton run of the LHC. The measured distributions include jets with transverse momenta up to 1.3 TeV, probing perturbative QCD in a high energy regime.

This Letter reports a measurement of the high-mass Drell-Yan differential cross-section in proton-proton collisions at a centre-of-mass energy of 7 TeV at the LHC. Based on an integrated luminosity of 4.9 /fb, the differential cross-section in the Z/gamma* to e+e- channel is measured with the ATLAS detector as a function of the invariant mass, Mee, in the range 116 < Mee < 1500 GeV, for a fiducial region in which both the electron and the positron have transverse momentum pT > 25 GeV and pseudorapidity eta < 2.5. A comparison is made to various event generators and to the predictions of perturbative QCD calculations at next-to-next-to-leading order.

Using 1.8 fb−1 of pp collisions at a center-of-mass energy of 7 TeV recorded by the ATLAS detector at the Large Hadron Collider, we present measurements of the production cross sections of Υ(1S,2S,3S) mesons. Υ mesons are reconstructed using the dimuon decay mode. Total production cross sections for pT<70 GeV and in the rapidity interval |yΥ|<2.25 are measured to be, 8.01±0.02±0.36±0.31 nb, 2.05±0.01±0.12±0.08 nb, and 0.92±0.01±0.07±0.04 nb, respectively, with uncertainties separated into statistical, systematic, and luminosity measurement effects. In addition, differential cross section times dimuon branching fractions for Υ(1S), Υ(2S), and Υ(3S) as a function of Υ transverse momentum pT and rapidity are presented. These cross sections are obtained assuming unpolarized production. If the production polarization is fully transverse or longitudinal with no azimuthal dependence in the helicity frame, the cross section may vary by approximately ±20%. If a nontrivial azimuthal dependence is considered, integrated cross sections may be significantly enhanced by a factor of 2 or more. We compare our results to several theoretical models of Υ meson production, finding that none provide an accurate description of our data over the full range of Υ transverse momenta accessible with this data set.12 MoreReceived 30 November 2012DOI:https://doi.org/10.1103/PhysRevD.87.052004This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.© 2013 CERN, for the ATLAS Collaboration

A measurement of spin correlation in tt[over ¯] production is reported using data collected with the ATLAS detector at the LHC, corresponding to an integrated luminosity of 2.1 fb(-1). Candidate events are selected in the dilepton topology with large missing transverse energy and at least two jets. The difference in azimuthal angle between the two charged leptons in the laboratory frame is used to extract the correlation between the top and antitop quark spins. In the helicity basis the measured degree of correlation corresponds to A(helicity)=0.40(-0.08)(+0.09), in agreement with the next-to-leading-order standard model prediction. The hypothesis of zero spin correlation is excluded at 5.1 standard deviations.

A bstract The ATLAS experiment at the LHC has measured the production cross section of events with two isolated photons in the final state, in proton-proton collisions at $ \sqrt{s}=7\;\mathrm{TeV} $ . The full data set collected in 2011, corresponding to an integrated luminosity of 4 . 9 fb −1 , is used. The amount of background, from hadronic jets and isolated electrons, is estimated with data-driven techniques and subtracted. The total cross section, for two isolated photons with transverse energies above 25 GeV and 22 GeV respectively, in the acceptance of the electromagnetic calorimeter (| η | &lt; 1 . 37 and 1 . 52 &lt; | η | &lt; 2 . 37) and with an angular separation Δ R &gt; 0 . 4, is $ 44.0_{-4.2}^{+3.2 } $ pb. The differential cross sections as a function of the di-photon invariant mass, transverse momentum, azimuthal separation, and cosine of the polar angle of the largest transverse energy photon in the Collins-Soper di-photon rest frame are also measured. The results are compared to the prediction of leading-order parton-shower and next-to-leading-order and next-to-next-to-leading-order parton-level generators.

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 fb−1 of pp collision data at $ \sqrt{s} $ = 8 TeV, 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.

The integrated and differential fiducial cross sections for the production of a W or Z boson in association with a high-energy photon are measured using pp collisions at sqrt{s} = 7 TeV. The analyses use a data sample with an integrated luminosity of 4.6 fb^{-1} collected by the ATLAS detector during the 2011 LHC data-taking period. Events are selected using leptonic decays of the W and Z bosons (W(e nu,mu nu) and Z(e+ e-, mu+ mu-, nu nubar)) with the requirement of an associated isolated photon. The data are used to test the electroweak sector of the Standard Model and search for evidence for new phenomena. The measurements are used to probe the anomalous WWgamma, ZZgamma and Zgammagamma triple-gauge-boson couplings and to search for the production of vector resonances decaying to Zgamma and Wgamma. No deviations from Standard Model predictions are observed and limits are placed on anomalous triple-gauge-boson couplings and on the production of new vector meson resonances.

A search for long-lived particles is performed using a data sample of 4.7 fb−1 from proton–proton collisions at a centre-of-mass energy s=7TeV collected by the ATLAS detector at the LHC. No excess is observed above the estimated background and lower limits, at 95% confidence level, are set on the mass of the long-lived particles in different scenarios, based on their possible interactions in the inner detector, the calorimeters and the muon spectrometer. Long-lived staus in gauge-mediated SUSY-breaking models are excluded up to a mass of 300 GeV for tanβ=5–20. Directly produced long-lived sleptons are excluded up to a mass of 278 GeV. R-hadrons, composites of gluino (stop, sbottom) and light quarks, are excluded up to a mass of 985 GeV (683 GeV, 612 GeV) when using a generic interaction model. Additionally two sets of limits on R-hadrons are obtained that are less sensitive to the interaction model for R-hadrons. One set of limits is obtained using only the inner detector and calorimeter observables, and a second set of limits is obtained based on the inner detector alone.

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 t¯t 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 t¯t system and of the top-quark transverse momentum. The measurements can also discriminate among different sets of parton distribution functions.5 MoreReceived 1 July 2014DOI:https://doi.org/10.1103/PhysRevD.90.072004This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical Society

A low-background inclusive search for new physics in events with same-sign dileptons is presented. The search uses proton-proton collisions corresponding to 20.3 fb$^{-1}$ of integrated luminosity taken in 2012 at a centre-of-mass energy of 8 TeV with the ATLAS detector at the LHC. Pairs of isolated leptons with the same electric charge and large transverse momenta of the type $e^{\pm}e^{\pm}, e^{\pm}μ^{\pm}$, and $μ^{\pm}μ^{\pm}$ are selected and their invariant mass distribution is examined. No excess of events above the expected level of Standard Model background is found. The results are used to set upper limits on the cross-sections for processes beyond the Standard Model. Limits are placed as a function of the dilepton invariant mass within a fiducial region corresponding to the signal event selection criteria. Exclusion limits are also derived for a specific model of doubly charged Higgs boson production.

Results are presented of a search for any particle(s) decaying to six or more jets in association with missing transverse momentum. The search is performed using 1.34fb−1 of $\sqrt {s}$ = 7 TeV proton-proton collisions recorded by the ATLAS detector during 2011. Data-driven techniques are used to determine the backgrounds in kinematic regions that require at least six, seven or eight jets, well beyond the multiplicities required in previous analyses. No evidence is found for physics beyond the Standard Model. The results are interpreted in the context of a supersymmetry model (MSUGRA/CMSSM) where they extend previous constraints.

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 $\sqrt{s}=7\text{ }\text{ }\mathrm{TeV}$. The measurement is based on $4.7\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ 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.

A bstract A measurement of the ZZ production cross section in proton-proton collisions at $ \sqrt{s}=7 $ TeV using data recorded by the ATLAS experiment at the Large Hadron Collider is presented. In a data sample corresponding to an integrated luminosity of 4.6 fb −1 collected in 2011, events are selected that are consistent either with two Z bosons decaying to electrons or muons or with one Z boson decaying to electrons or muons and a second Z boson decaying to neutrinos. The ZZ (*) → ℓ + ℓ − ℓ ′+ ℓ ′− and $ ZZ\to {\ell^{+}}{\ell^{-}}\nu \overline{\nu} $ cross sections are measured in restricted phase-space regions. These results are then used to derive the total cross section for ZZ events produced with both Z bosons in the mass range 66 to 116 GeV, $ \sigma_{ZZ}^{\mathrm{tot}} = 6.7 \pm 0.7\;\,\left( {\mathrm{stat}.} \right)\;\;_{-0.3}^{+0.4}\;\,\left( {\mathrm{syst}.} \right) \pm 0.3\;\,\left( {\mathrm{lumi}.} \right) $ pb, which is consistent with the Standard Model prediction of $ 5.89_{-0.18}^{+0.22 } $ pb calculated at next-to-leading order in QCD. The normalized differential cross sections in bins of various kinematic variables are presented. Finally, the differential event yield as a function of the transverse momentum of the leading Z boson is used to set limits on anomalous neutral triple gauge boson couplings in ZZ production.

A bstract This paper presents measurements of the polarization of W bosons in top quark decays, derived from $ t\overline t $ events with missing transverse momentum, one charged lepton and at least four jets, or two charged leptons and at least two jets. Data from pp collisions at a centre-of-mass energy of 7 TeV were collected with the ATLAS experiment at the LHC and correspond to an integrated luminosity of 1.04 fb −1 . The measured fractions of longitudinally, left- and right-handed polarization are F 0 = 0 . 67 ± 0 . 07, F L = 0 . 32 ± 0 . 04 and F R = 0 . 01 ± 0 . 05, in agreement with the Standard Model predictions. As the polarization of the W bosons in top quark decays is sensitive to the Wtb vertex Lorentz structure and couplings, the measurements were used to set limits on anomalous contributions to the Wtb couplings.

A study of W ± Z production in proton-proton collisions at $\sqrt{s} = 7~\mbox{TeV}$ is presented using data corresponding to an integrated luminosity of 4.6 fb−1 collected with the ATLAS detector at the Large Hadron Collider in 2011. In total, 317 candidates, with a background expectation of 68±10 events, are observed in double-leptonic decay final states with electrons, muons and missing transverse momentum. The total cross-section is determined to be $\sigma _{WZ}^{\mathrm {tot}}= 19.0^{+1.4}_{-1.3}\text{(stat.)}\pm0.9\text{(syst.)}\pm 0.4\text{(lumi.)}~\mbox{pb}$ , consistent with the Standard Model expectation of $17.6^{+1.1}_{-1.0}~\mbox{pb}$ . Limits on anomalous triple gauge boson couplings are derived using the transverse momentum spectrum of Z bosons in the selected events. The cross-section is also presented as a function of Z boson transverse momentum and diboson invariant mass.

A blind analysis searching for the decay Bs0→μ+μ− has been performed using proton–proton collisions at a centre-of-mass energy of 7 TeV recorded with the ATLAS detector at the LHC. With an integrated luminosity of 2.4 fb−1 no excess of events over the background expectation is found and an upper limit is set on the branching fraction BR(Bs0→μ+μ−)<2.2(1.9)×10−8 at 95% (90%) confidence level.

Inclusive multi-jet production is studied in proton–proton collisions at a center-of-mass energy of 7 TeV, using the ATLAS detector. The data sample corresponds to an integrated luminosity of 2.4 pb−1. Results on multi-jet cross sections are presented and compared to both leading-order plus parton-shower Monte Carlo predictions and to next-to-leading-order QCD calculations.

The results of a search for pair production of the scalar partners of bottom quarks in 2.05 fb(-1) of pp collisions at sqrt[s]=7 TeV using the ATLAS experiment are reported. Scalar bottom quarks are searched for in events with large missing transverse momentum and two jets in the final state, where both jets are identified as originating from a bottom quark. In an R-parity conserving minimal supersymmetric scenario, assuming that the scalar bottom quark decays exclusively into a bottom quark and a neutralino, 95% confidence-level upper limits are obtained in the b(1) - χ(1)(0) mass plane such that for neutralino masses below 60 GeV scalar bottom masses up to 390 GeV are excluded.

We present an update of a search for supersymmetry in final states containing jets, missing transverse momentum, and one isolated electron or muon, using $1.04\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ of proton-proton collision data at $\sqrt{s}=7\text{ }\text{ }\mathrm{TeV}$ recorded by the ATLAS experiment at the LHC in the first half of 2011. The analysis is carried out in four distinct signal regions with either three or four jets and variations on the (missing) transverse momentum cuts, resulting in optimized limits for various supersymmetry models. No excess above the standard model background expectation is observed. Limits are set on the visible cross section of new physics within the kinematic requirements of the search. The results are interpreted as limits on the parameters of the minimal supergravity framework, limits on cross sections of simplified models with specific squark and gluino decay modes, and limits on parameters of a model with bilinear $R$-parity violation.

A bstract This paper presents the application of a variety of techniques to study jet substructure. The performance of various modified jet algorithms, or jet grooming techniques, for several jet types and event topologies is investigated for jets with transverse momentum larger than 300 GeV. Properties of jets subjected to the mass-drop filtering, trimming, and pruning algorithms are found to have a reduced sensitivity to multiple proton-proton interactions, are more stable at high luminosity and improve the physics potential of searches for heavy boosted objects. Studies of the expected discrimination power of jet mass and jet substructure observables in searches for new physics are also presented. Event samples enriched in boosted W and Z bosons and top-quark pairs are used to study both the individual jet invariant mass scales and the efficacy of algorithms to tag boosted hadronic objects. The analyses presented use the full 2011 ATLAS dataset, corresponding to an integrated luminosity of 4.7 ± 0.1 fb −1 from proton-proton collisions produced by the Large Hadron Collider at a centre-of-mass energy of $ \sqrt{s}=7 $ TeV.