J. Alison

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.

A measurement of the Higgs boson mass is presented based on the combined data samples of the ATLAS and CMS experiments at the CERN LHC in the H→γγ and H→ZZ→4ℓ decay channels. The results are obtained from a simultaneous fit to the reconstructed invariant mass peaks in the two channels and for the two experiments. The measured masses from the individual channels and the two experiments are found to be consistent among themselves. The combined measured mass of the Higgs boson is m_{H}=125.09±0.21 (stat)±0.11 (syst) GeV.

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.

Combined ATLAS and CMS measurements of the Higgs boson production and decay rates, as well as constraints on its couplings to vector bosons and fermions, are presented. The combination is based on the analysis of five production processes, namely gluon fusion, vector boson fusion, and associated production with a W or a Z boson or a pair of top quarks, and of the six decay modes H → ZZ, W W , γγ, ττ, bb, and μμ. All results are reported assuming a value of 125.09 GeV for the Higgs boson mass, the result of the combined measurement by the ATLAS and CMS experiments. The analysis uses the CERN LHC proton-proton collision data recorded by the ATLAS and CMS experiments in 2011 and 2012, corresponding to integrated luminosities per experiment of approximately 5 fb−1 at $$ \sqrt{s}=7 $$ TeV and 20 fb−1 at $$ \sqrt{s}=8 $$ TeV. The Higgs boson production and decay rates measured by the two experiments are combined within the context of three generic parameterisations: two based on cross sections and branching fractions, and one on ratios of coupling modifiers. Several interpretations of the measurements with more model-dependent parameterisations are also given. The combined signal yield relative to the Standard Model prediction is measured to be 1.09 ± 0.11. The combined measurements lead to observed significances for the vector boson fusion production process and for the H → ττ decay of 5.4 and 5.5 standard deviations, respectively. The data are consistent with the Standard Model predictions for all parameterisations considered.

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.

This article documents the performance of the ATLAS muon identification and reconstruction using the LHC dataset recorded at $$\sqrt{s} = 13$$ TeV in 2015. Using a large sample of $$J/\psi \rightarrow \mu \mu $$ and $$Z\rightarrow \mu \mu $$ decays from 3.2 fb $$^{-1}$$ of pp collision data, measurements of the reconstruction efficiency, as well as of the momentum scale and resolution, are presented and compared to Monte Carlo simulations. The reconstruction efficiency is measured to be close to $$99~\%$$ over most of the covered phase space ( $$|\eta |<2.5$$ and $$5 < p_{\mathrm {T}} < 100$$ GeV). The isolation efficiency varies between 93 and $$100~\%$$ depending on the selection applied and on the momentum of the muon. Both efficiencies are well reproduced in simulation. In the central region of the detector, the momentum resolution is measured to be $$1.7~\%$$ ( $$2.3~\%$$ ) for muons from $$J/\psi \rightarrow \mu \mu $$ ( $$Z\rightarrow \mu \mu $$ ) decays, and the momentum scale is known with an uncertainty of $$0.05~\%$$ . In the region $$|\eta |>2.2$$ , the $$p_{\mathrm {T}} $$ resolution for muons from $$Z\rightarrow \mu \mu $$ decays is $$2.9~\%$$ while the precision of the momentum scale for low- $$p_{\mathrm {T}} $$ muons from $$J/\psi \rightarrow \mu \mu $$ decays is about $$0.2~\%$$ .

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.

Combined measurements of Higgs boson production cross sections and branching fractions are presented. The combination is based on the analyses of the Higgs boson decay modes $H \to \gamma\gamma$, $ZZ^\ast$, $WW^\ast$, $\tau\tau$, $b\bar{b}$, $\mu\mu$, searches for decays into invisible final states, and on measurements of off-shell Higgs boson production. Up to $79.8$ fb$^{-1}$ of proton-proton collision data collected at $\sqrt{s}=$ 13 TeV with the ATLAS detector are used. Results are presented for the gluon-gluon fusion and vector-boson fusion processes, and for associated production with vector bosons or top-quarks. The global signal strength is determined to be $\mu = 1.11^{+0.09}_{-0.08}$. The combined measurement yields an observed (expected) significance for the vector-boson fusion production process of $6.5\sigma$ ($5.3\sigma$). Measurements in kinematic regions defined within the simplified template cross section framework are also shown. The results are interpreted in terms of modifiers applied to the Standard Model couplings of the Higgs boson to other particles, and are used to set exclusion limits on parameters in two-Higgs-doublet models and in the simplified Minimal Supersymmetric Standard Model. No significant deviations from Standard Model predictions are observed.

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.

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.

The reconstruction of the signal from hadrons and jets emerging from the proton-proton collisions at the Large Hadron Collider (LHC) and entering the ATLAS calorimeters is based on a three-dimensional topological clustering of individual calorimeter cell signals. The cluster formation follows cell signal-significance patterns generated by electromagnetic and hadronic showers. In this, the clustering algorithm implicitly performs a topological noise suppression by removing cells with insignificant signals which are not in close proximity to cells with significant signals. The resulting topological cell clusters have shape and location information, which is exploited to apply a local energy calibration and corrections depending on the nature of the cluster. Topological cell clustering is established as a well-performing calorimeter signal definition for jet and missing transverse momentum reconstruction in ATLAS.

A detailed study is presented of the expected performance of the ATLAS detector. The reconstruction of tracks, leptons, photons, missing energy and jets is investigated, together with the performance of b-tagging and the trigger. The physics potential for a variety of interesting physics processes, within the Standard Model and beyond, is examined. The study comprises a series of notes based on simulations of the detector and physics processes, with particular emphasis given to the data expected from the first years of operation of the LHC at CERN.

Results of a search for new phenomena in final states with an energetic jet and large missing transverse momentum are reported. The search uses 20.3 fb $${}^{-1}$$ of $$\sqrt{s}=8$$ TeV data collected in 2012 with the ATLAS detector at the LHC. Events are required to have at least one jet with $$p_{\mathrm {T}}> 120$$ GeV and no leptons. Nine signal regions are considered with increasing missing transverse momentum requirements between $$E_{\mathrm {T}}^{\mathrm {miss}}>150$$ GeV and $$E_{\mathrm {T}}^{\mathrm {miss}}> 700$$ GeV. Good agreement is observed between the number of events in data and Standard Model expectations. The results are translated into exclusion limits on models with either large extra spatial dimensions, pair production of weakly interacting dark matter candidates, or production of very light gravitinos in a gauge-mediated supersymmetric model. In addition, limits on the production of an invisibly decaying Higgs-like boson leading to similar topologies in the final state are presented.

The jet energy scale (JES) and its systematic uncertainty are determined for jets measured with the ATLAS detector using proton-proton collision data with a centre-of-mass energy of [Formula: see text] TeV corresponding to an integrated luminosity of [Formula: see text][Formula: see text]. Jets are reconstructed from energy deposits forming topological clusters of calorimeter cells using the anti-[Formula: see text] algorithm with distance parameters [Formula: see text] or [Formula: see text], and are calibrated using MC simulations. A residual JES correction is applied to account for differences between data and MC simulations. This correction and its systematic uncertainty are estimated using a combination of in situ techniques exploiting the transverse momentum balance between a jet and a reference object such as a photon or a [Formula: see text] boson, for [Formula: see text] and pseudorapidities [Formula: see text]. The effect of multiple proton-proton interactions is corrected for, and an uncertainty is evaluated using in situ techniques. The smallest JES uncertainty of less than 1 % is found in the central calorimeter region ([Formula: see text]) for jets with [Formula: see text]. For central jets at lower [Formula: see text], the uncertainty is about 3 %. A consistent JES estimate is found using measurements of the calorimeter response of single hadrons in proton-proton collisions and test-beam data, which also provide the estimate for [Formula: see text] TeV. The calibration of forward jets is derived from dijet [Formula: see text] balance measurements. The resulting uncertainty reaches its largest value of 6 % for low-[Formula: see text] jets at [Formula: see text]. Additional JES uncertainties due to specific event topologies, such as close-by jets or selections of event samples with an enhanced content of jets originating from light quarks or gluons, are also discussed. The magnitude of these uncertainties depends on the event sample used in a given physics analysis, but typically amounts to 0.5-3 %.

A measurement of the production processes of the recently discovered Higgs boson is performed in the two-photon final state using 4.5 fb−1 of proton-proton collisions data at √s=7 TeV and 20.3 fb−1 at √s=8 TeV collected by the ATLAS detector at the Large Hadron Collider. The number of observed Higgs boson decays to diphotons divided by the corresponding Standard Model prediction, called the signal strength, is found to be μ=1.17±0.27 at the value of the Higgs boson mass measured by ATLAS, mH=125.4 GeV. The analysis is optimized to measure the signal strengths for individual Higgs boson production processes at this value of mH. They are found to be μggF=1.32±0.38, μVBF=0.8±0.7, μWH=1.0±1.6, μZH=0.1+3.7−0.1, and μt¯tH=1.6+2.7−1.8, for Higgs boson production through gluon fusion, vector-boson fusion, and in association with a W or Z boson or a top-quark pair, respectively. Compared with the previously published ATLAS analysis, the results reported here also benefit from a new energy calibration procedure for photons and the subsequent reduction of the systematic uncertainty on the diphoton mass resolution. No significant deviations from the predictions of the Standard Model are found.14 MoreReceived 1 September 2014DOI:https://doi.org/10.1103/PhysRevD.90.112015This 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 high-mass dielectron and dimuon resonances in the mass range of 250 GeV to 6 TeV is presented. The data were recorded by the ATLAS experiment in proton–proton collisions at a centre-of-mass energy of s=13 TeV during Run 2 of the Large Hadron Collider and correspond to an integrated luminosity of 139 fb−1. A functional form is fitted to the dilepton invariant-mass distribution to model the contribution from background processes, and a generic signal shape is used to determine the significance of observed deviations from this background estimate. No significant deviation is observed and upper limits are placed at the 95% confidence level on the fiducial cross-section times branching ratio for various resonance width hypotheses. The derived limits are shown to be applicable to spin-0, spin-1 and spin-2 signal hypotheses. For a set of benchmark models, the limits are converted into lower limits on the resonance mass and reach 4.5 TeV for the E6-motivated Zψ′ boson. Also presented are limits on Heavy Vector Triplet model couplings.

The large rate of multiple simultaneous proton-proton interactions, or pile-up, generated by the Large Hadron Collider in Run 1 required the development of many new techniques to mitigate the adverse effects of these conditions. This paper describes the methods employed in the ATLAS experiment to correct for the impact of pile-up on jet energy and jet shapes, and for the presence of spurious additional jets, with a primary focus on the large 20.3 [Formula: see text] data sample collected at a centre-of-mass energy of [Formula: see text]. The energy correction techniques that incorporate sophisticated estimates of the average pile-up energy density and tracking information are presented. Jet-to-vertex association techniques are discussed and projections of performance for the future are considered. Lastly, the extension of these techniques to mitigate the effect of pile-up on jet shapes using subtraction and grooming procedures is presented.

Studies of the spin, parity and tensor couplings of the Higgs boson in the [Formula: see text], [Formula: see text] and [Formula: see text] decay processes at the LHC are presented. The investigations are based on [Formula: see text] of pp collision data collected by the ATLAS experiment at [Formula: see text] TeV and [Formula: see text] TeV. The Standard Model (SM) Higgs boson hypothesis, corresponding to the quantum numbers [Formula: see text], is tested against several alternative spin scenarios, including non-SM spin-0 and spin-2 models with universal and non-universal couplings to fermions and vector bosons. All tested alternative models are excluded in favour of the SM Higgs boson hypothesis at more than 99.9 % confidence level. Using the [Formula: see text] and [Formula: see text] decays, the tensor structure of the interaction between the spin-0 boson and the SM vector bosons is also investigated. The observed distributions of variables sensitive to the non-SM tensor couplings are compatible with the SM predictions and constraints on the non-SM couplings are derived.

The observation of Higgs boson production in association with a top quark pair ($t\bar{t}H$), based on the analysis of proton-proton collision data at a centre-of-mass energy of 13 TeV recorded with the ATLAS detector at the Large Hadron Collider, is presented. Using data corresponding to integrated luminosities of up to 79.8 fb$^{-1}$, and considering Higgs boson decays into $b\bar{b}$, $WW^*$, $\tau\tau$, $\gamma\gamma$, and $ZZ^*$, the observed significance is 5.8 standard deviations, compared to an expectation of 4.9 standard deviations. Combined with the $t\bar{t}H$ searches using a dataset corresponding to integrated luminosities of 4.5 fb$^{-1}$ at 7 TeV and 20.3 fb$^{-1}$ at 8 TeV, the observed (expected) significance is 6.3 (5.1) standard deviations. Assuming Standard Model branching fractions, the total $t\bar{t}H$ production cross section at 13 TeV is measured to be 670 $\pm$ 90 (stat.) $^{+110}_{-100}$ (syst.) fb, in agreement with the Standard Model prediction.

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

A search is conducted for new resonant and non-resonant high-mass phenomena in dielectron and dimuon final states. The search uses 36.1 fb−1 of proton-proton collision data, collected at $$ \sqrt{s}=13 $$ TeV by the ATLAS experiment at the LHC in 2015 and 2016. No significant deviation from the Standard Model prediction is observed. Upper limits at 95% credibility level are set on the cross-section times branching ratio for resonances decaying into dileptons, which are converted to lower limits on the resonance mass, up to 4.1 TeV for the E6-motivated Z ′ . Lower limits on the qqℓℓ contact interaction scale are set between 2.4 TeV and 40 TeV, depending on the model.

ATLAS has measured two-particle correlations as a function of the relative azimuthal angle, Δϕ, and pseudorapidity, Δη, in sqrt[s]=13 and 2.76 TeV pp collisions at the LHC using charged particles measured in the pseudorapidity interval |η|<2.5. The correlation functions evaluated in different intervals of measured charged-particle multiplicity show a multiplicity-dependent enhancement at Δϕ∼0 that extends over a wide range of Δη, which has been referred to as the "ridge." Per-trigger-particle yields, Y(Δϕ), are measured over 2<|Δη|<5. For both collision energies, the Y(Δϕ) distribution in all multiplicity intervals is found to be consistent with a linear combination of the per-trigger-particle yields measured in collisions with less than 20 reconstructed tracks, and a constant combinatoric contribution modulated by cos(2Δϕ). The fitted Fourier coefficient, v_{2,2}, exhibits factorization, suggesting that the ridge results from per-event cos(2ϕ) modulation of the single-particle distribution with Fourier coefficients v_{2}. The v_{2} values are presented as a function of multiplicity and transverse momentum. They are found to be approximately constant as a function of multiplicity and to have a p_{T} dependence similar to that measured in p+Pb and Pb+Pb collisions. The v_{2} values in the 13 and 2.76 TeV data are consistent within uncertainties. These results suggest that the ridge in pp collisions arises from the same or similar underlying physics as observed in p+Pb collisions, and that the dynamics responsible for the ridge has no strong sqrt[s] dependence.

A measurement of the mass of the W boson is presented based on proton-proton collision data recorded in 2011 at a centre-of-mass energy of 7 TeV with the ATLAS detector at the LHC, and corresponding to 4.6fb-1 of integrated luminosity. The selected data sample consists of 7.8×106 candidates in the W→μν channel and 5.9×106 candidates in the W→eν channel. The W-boson mass is obtained from template fits to the reconstructed distributions of the charged lepton transverse momentum and of the W boson transverse mass in the electron and muon decay channels, yielding mW=80370±7(stat.)±11(exp. syst.)±14(mod. syst.)MeV=80370±19MeV, where the first uncertainty is statistical, the second corresponds to the experimental systematic uncertainty, and the third to the physics-modelling systematic uncertainty. A measurement of the mass difference between the W+ and W- bosons yields mW+-mW-=-29±28 MeV.

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 is presented for a high-mass Higgs boson in the [Formula: see text], [Formula: see text], [Formula: see text], and [Formula: see text] decay modes using the ATLAS detector at the CERN Large Hadron Collider. The search uses proton-proton collision data at a centre-of-mass energy of 8 TeV corresponding to an integrated luminosity of 20.3 fb[Formula: see text]. The results of the search are interpreted in the scenario of a heavy Higgs boson with a width that is small compared with the experimental mass resolution. The Higgs boson mass range considered extends up to [Formula: see text] for all four decay modes and down to as low as 140 [Formula: see text], depending on the decay mode. No significant excess of events over the Standard Model prediction is found. A simultaneous fit to the four decay modes yields upper limits on the production cross-section of a heavy Higgs boson times the branching ratio to [Formula: see text] boson pairs. 95 % confidence level upper limits range from 0.53 pb at [Formula: see text] GeV to 0.008 pb at [Formula: see text] GeV for the gluon-fusion production mode and from 0.31 pb at [Formula: see text] GeV to 0.009 pb at [Formula: see text] GeV for the vector-boson-fusion production mode. The results are also interpreted in the context of Type-I and Type-II two-Higgs-doublet models.

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 %.

Searches for the electroweak production of charginos, neutralinos and sleptons in final states characterized by the presence of two leptons (electrons and muons) and missing transverse momentum are performed using 20.3 fb−1 of proton-proton collision data at $ \sqrt{s} $ = 8 TeV recorded with the ATLAS experiment at the Large Hadron Collider. No significant excess beyond Standard Model expectations is observed. Limits are set on the masses of the lightest chargino, next-to-lightest neutralino and sleptons for different lightest-neutralino mass hypotheses in simplified models. Results are also interpreted in various scenarios of the phenomenological Minimal Supersymmetric Standard Model.

Results of a search for new phenomena in final states with an energetic jet and large missing transverse momentum are reported. The search uses proton--proton collision data corresponding to an integrated luminosity of 36.1 fb${}^{-1}$ at a centre-of-mass energy of 13 TeV collected in 2015 and 2016 with the ATLAS detector at the Large Hadron Collider. Events are required to have at least one jet with a transverse momentum above 250 GeV and no leptons ($e$ or $\mu$). Several signal regions are considered with increasing requirements on the missing transverse momentum above 250 GeV. Good agreement is observed between the number of events in data and Standard Model predictions. The results are translated into exclusion limits in models with pair-produced weakly interacting dark-matter candidates, large extra spatial dimensions, and supersymmetric particles in several compressed scenarios.

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.

The final ATLAS Run 1 measurements of Higgs boson production and couplings in the decay channel H→ZZ∗→ℓ+ℓ−ℓ'+ℓ'−, where ℓ, ℓ′=e or μ, are presented. These measurements were performed using pp collision data corresponding to integrated luminosities of 4.5 and 20.3 fb−1 at center-of-mass energies of 7 and 8 TeV, respectively, recorded with the ATLAS detector at the LHC. The H→ZZ∗→4ℓ signal is observed with a significance of 8.1 standard deviations, with an expectation of 6.2 standard deviations, at mH=125.36 GeV, the combined ATLAS measurement of the Higgs boson mass from the H→γγ and H→ZZ∗→4ℓ channels. The production rate relative to the Standard Model expectation, the signal strength, is measured in four different production categories in the H→ZZ∗→4ℓ channel. The measured signal strength, at this mass, and with all categories combined, is 1.44+0.40−0.33. The signal strength for Higgs boson production in gluon fusion or in association with t¯t or b¯b pairs is found to be 1.7+0.5−0.4, while the signal strength for vector-boson fusion combined with WH/ZH associated production is found to be 0.3+1.6−0.9.14 MoreReceived 22 August 2014DOI:https://doi.org/10.1103/PhysRevD.91.012006This 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.© 2015 CERN, for the ATLAS Collaboration

A search for the decay of the Standard Model Higgs boson into a $b\bar{b}$ pair when produced in association with a $W$ or $Z$ boson is performed with the ATLAS detector. The data, corresponding to an integrated luminosity of 79.8 $\mathrm{fb}^{-1}$ were collected in proton-proton collisions during Run 2 of the Large Hadron Collider at a centre-of-mass energy of 13 TeV. For a Higgs boson mass of 125 GeV, an excess of events over the expected background from other Standard Model processes is found with an observed (expected) significance of 4.9 (4.3) standard deviations. A combination with the results from other searches in Run 1 and in Run 2 for the Higgs boson in the $b\bar{b}$ decay mode is performed, which yields an observed (expected) significance of 5.4 (5.5) standard deviations, thus providing direct observation of the Higgs boson decay into $b$-quarks. The ratio of the measured event yield for a Higgs boson decaying into $b\bar{b}$ to the Standard Model expectation is $1.01 \pm 0.12 \mathrm{(stat.)} ^{+0.16}_{-0.15} \mathrm{(syst.)}$. Additionally, a combination of Run 2 results searching for the Higgs boson produced in association with a vector boson yields an observed (expected) significance of 5.3 (4.8) standard deviations.

A search for the electroweak production of charginos and sleptons decaying into final states with two electrons or muons is presented. The analysis is based on 139 fb$^{-1}$ of proton-proton collisions recorded by the ATLAS detector at the Large Hadron Collider at $\sqrt{s}=13$ TeV. Three $R$-parity-conserving scenarios where the lightest neutralino is the lightest supersymmetric particle are considered: the production of chargino pairs with decays via either $W$ bosons or sleptons, and the direct production of slepton pairs. The analysis is optimised for the first of these scenarios, but the results are also interpreted in the others. No significant deviations from the Standard Model expectations are observed and limits at 95 % confidence level are set on the masses of relevant supersymmetric particles in each of the scenarios. For a massless lightest neutralino, masses up to 420 GeV are excluded for the production of the lightest-chargino pairs assuming $W$-boson-mediated decays and up to 1 TeV for slepton-mediated decays, whereas for slepton-pair production masses up to 700 GeV are excluded assuming three generations of mass-degenerate sleptons.

Abstract The algorithms used by the ATLAS Collaboration during Run 2 of the Large Hadron Collider to identify jets containing b -hadrons are presented. The performance of the algorithms is evaluated in the simulation and the efficiency with which these algorithms identify jets containing b -hadrons is measured in collision data. The measurement uses a likelihood-based method in a sample highly enriched in $$t{\bar{t}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>t</mml:mi><mml:mover><mml:mrow><mml:mi>t</mml:mi></mml:mrow><mml:mrow><mml:mo>¯</mml:mo></mml:mrow></mml:mover></mml:mrow></mml:math> events. The topology of the $$t \rightarrow W b$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>t</mml:mi><mml:mo>→</mml:mo><mml:mi>W</mml:mi><mml:mi>b</mml:mi></mml:mrow></mml:math> decays is exploited to simultaneously measure both the jet flavour composition of the sample and the efficiency in a transverse momentum range from 20 to 600 GeV. The efficiency measurement is subsequently compared with that predicted by the simulation. The data used in this measurement, corresponding to a total integrated luminosity of 80.5 $$\hbox {fb}^{-1}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mtext>fb</mml:mtext><mml:mrow><mml:mo>-</mml:mo><mml:mn>1</mml:mn></mml:mrow></mml:msup></mml:math> , were collected in proton–proton collisions during the years 2015–2017 at a centre-of-mass energy $$\sqrt{s}=$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msqrt><mml:mi>s</mml:mi></mml:msqrt><mml:mo>=</mml:mo></mml:mrow></mml:math> 13 TeV. By simultaneously extracting both the efficiency and jet flavour composition, this measurement significantly improves the precision compared to previous results, with uncertainties ranging from 1 to 8% depending on the jet transverse momentum.

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.

The performance of the missing transverse momentum ( $$E_{\mathrm{T}}^{\mathrm{miss}}$$ ) reconstruction with the ATLAS detector is evaluated using data collected in proton–proton collisions at the LHC at a centre-of-mass energy of 13 TeV in 2015. To reconstruct $$E_{\mathrm{T}}^{\mathrm{miss}}$$ , fully calibrated electrons, muons, photons, hadronically decaying $$\tau \text {-leptons}$$ , and jets reconstructed from calorimeter energy deposits and charged-particle tracks are used. These are combined with the soft hadronic activity measured by reconstructed charged-particle tracks not associated with the hard objects. Possible double counting of contributions from reconstructed charged-particle tracks from the inner detector, energy deposits in the calorimeter, and reconstructed muons from the muon spectrometer is avoided by applying a signal ambiguity resolution procedure which rejects already used signals when combining the various $$E_{\mathrm{T}}^{\mathrm{miss}}$$ contributions. The individual terms as well as the overall reconstructed $$E_{\mathrm{T}}^{\mathrm{miss}}$$ are evaluated with various performance metrics for scale (linearity), resolution, and sensitivity to the data-taking conditions. The method developed to determine the systematic uncertainties of the $$E_{\mathrm{T}}^{\mathrm{miss}}$$ scale and resolution is discussed. Results are shown based on the full 2015 data sample corresponding to an integrated luminosity of $$3.2~\hbox {fb}^{-1}$$ .

This paper describes the reconstruction of electrons and photons with the ATLAS detector, as employed for measurements and searches exploiting the complete LHC Run 2 dataset. An improved energy clustering algorithm is introduced, and its implications for the measurement and identification of electrons and photons are discussed in detail. Corrections and calibrations that affect performance, including energy calibration, identification and isolation efficiencies, and the measurement of the charge of reconstructed electron candidates are determined using up to 81 fb$^{-1}$ of proton-proton collision data collected at $\sqrt{s}=$ 13 TeV between 2015 and 2017.

This Letter presents a measurement of the inelastic proton-proton cross section using 60 μb−1 of pp collisions at a center-of-mass energy √s of 13 TeV with the ATLAS detector at the LHC. Inelastic interactions are selected using rings of plastic scintillators in the forward region (2.07<|η|<3.86) of the detector. A cross section of 68.1±1.4 mb is measured in the fiducial region ξ=M2X/s>10−6, where MX is the larger invariant mass of the two hadronic systems separated by the largest rapidity gap in the event. In this ξ range the scintillators are highly efficient. For diffractive events this corresponds to cases where at least one proton dissociates to a system with MX>13 GeV. The measured cross section is compared with a range of theoretical predictions. When extrapolated to the full phase space, a cross section of 78.1±2.9 mb is measured, consistent with the inelastic cross section increasing with center-of-mass energy.Received 9 June 2016DOI:https://doi.org/10.1103/PhysRevLett.117.182002Published by the American Physical Society 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.© 2016 CERN, for the ATLAS CollaborationPhysics Subject Headings (PhySH)Research AreasParticle interactionsParticle productionQuark & gluon jetsTechniquesHadron collidersParticles & Fields

Dijet events are studied in the proton--proton collision dataset recorded at $\sqrt{s}=$13 TeV with the ATLAS detector at the Large Hadron Collider in 2015 and 2016, corresponding to integrated luminosities of 3.5 fb$^{-1}$ and 33.5 fb$^{-1}$ respectively. Invariant mass and angular distributions are compared to background predictions and no significant deviation is observed. For resonance searches, a new method for fitting the background component of the invariant mass distribution is employed. The dataset is then used to set upper limits at a 95% confidence level on a range of new physics scenarios. Excited quarks with masses below 6.0 TeV are excluded, and limits are set on quantum black holes, heavy W' bosons, W* bosons, and a range of masses and couplings in a Z' dark matter mediator model. Model-independent limits on signals with a Gaussian shape are also set, using a new approach allowing factorization of physics and detector effects. From the angular distributions, a scale of new physics in contact interaction models is excluded for scenarios with either constructive or destructive interference. These results represent a substantial improvement over those obtained previously with lower integrated luminosity.

Properties of the Higgs boson are measured in the two-photon final state using 36.1 fb−1 of proton–proton collision data recorded at √s=13 TeV by the ATLAS experiment at the Large Hadron Collider. Cross-section measurements for the production of a Higgs boson through gluon–gluon fusion, vector-boson fusion, and in association with a vector boson or a top-quark pair are reported. The signal strength, defined as the ratio of the observed to the expected signal yield, is measured for each of these production processes as well as inclusively. The global signal strength measurement of 0.99±0.14 improves on the precision of the ATLAS measurement at √s=7 and 8 TeV by a factor of two. Measurements of gluon–gluon fusion and vector-boson fusion productions yield signal strengths compatible with the Standard Model prediction. Measurements of simplified template cross sections, designed to quantify the different Higgs boson production processes in specific regions of phase space, are reported. The cross section for the production of the Higgs boson decaying to two isolated photons in a fiducial region closely matching the experimental selection of the photons is measured to be 55±10 fb, which is in good agreement with the Standard Model prediction of 64±2 fb. Furthermore, cross sections in fiducial regions enriched in Higgs boson production in vector-boson fusion or in association with large missing transverse momentum, leptons or top-quark pairs are reported. Differential and double-differential measurements are performed for several variables related to the diphoton kinematics as well as the kinematics and multiplicity of the jets produced in association with a Higgs boson. These differential cross sections are sensitive to higher order QCD corrections and properties of the Higgs boson, such as its spin and CP quantum numbers. No significant deviations from a wide array of Standard Model predictions are observed. Finally, the strength and tensor structure of the Higgs boson interactions are investigated using an effective Lagrangian, which introduces additional CP-even and CP-odd interactions. No significant new physics contributions are observed.36 MoreReceived 14 February 2018DOI:https://doi.org/10.1103/PhysRevD.98.052005Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.© 2018 CERN, for the ATLAS CollaborationPhysics Subject Headings (PhySH)Physical SystemsHiggs bosonsPhotonsPropertiesParticle propertiesTechniquesParticle data analysisParticles & Fields

Results of a search for new phenomena in final states with an energetic jet and large missing transverse momentum are reported. The search uses proton-proton collision data corresponding to an integrated luminosity of $3.2\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ at $\sqrt{s}=13\text{ }\text{ }\mathrm{TeV}$ collected in 2015 with the ATLAS detector at the Large Hadron Collider. Events are required to have at least one jet with a transverse momentum above 250 GeV and no leptons. Several signal regions are considered with increasing missing-transverse-momentum requirements between ${E}_{\mathrm{T}}^{\mathrm{miss}}&gt;250\text{ }\text{ }\mathrm{GeV}$ and ${E}_{\mathrm{T}}^{\mathrm{miss}}&gt;700\text{ }\text{ }\mathrm{GeV}$. Good agreement is observed between the number of events in data and Standard Model predictions. The results are translated into exclusion limits in models with large extra spatial dimensions, pair production of weakly interacting dark-matter candidates, and the production of supersymmetric particles in several compressed scenarios.

A search for heavy neutral Higgs bosons and Z′ bosons is performed using a data sample corresponding to an integrated luminosity of 36.1 fb−1 from proton-proton collisions at $$ \sqrt{s}=13 $$ TeV recorded by the ATLAS detector at the LHC during 2015 and 2016. The heavy resonance is assumed to decay to τ +τ − with at least one tau lepton decaying to final states with hadrons and a neutrino. The search is performed in the mass range of 0.2-2.25 TeV for Higgs bosons and 0.2-4.0 TeV for Z′ bosons. The data are in good agreement with the background predicted by the Standard Model. The results are interpreted in benchmark scenarios. In the context of the hMSSM scenario, the data exclude tan β > 1.0 for m A = 0.25 TeV and tan β > 42 for m A = 1.5 TeV at the 95% confidence level. For the Sequential Standard Model, Z SSM ′ with mZ′ < 2.42 TeV is excluded at 95% confidence level, while Z NU ′ with mZ ′ < 2.25 TeV is excluded for the non-universal G(221) model that exhibits enhanced couplings to third-generation fermions.

This paper describes the implementation and performance of a particle flow algorithm applied to 20.2 fb$^{-1}$ of ATLAS data from 8 TeV proton-proton collisions in Run 1 of the LHC. The algorithm removes calorimeter energy deposits due to charged hadrons from consideration during jet reconstruction, instead using measurements of their momenta from the inner tracker. This improves the accuracy of the charged-hadron measurement, while retaining the calorimeter measurements of neutral-particle energies. The paper places emphasis on how this is achieved, while minimising double-counting of charged-hadron signals between the inner tracker and calorimeter. The performance of particle flow jets, formed from the ensemble of signals from the calorimeter and the inner tracker, is compared to that of jets reconstructed from calorimeter energy deposits alone, demonstrating improvements in resolution and pile-up stability.

High-precision measurements by the ATLAS Collaboration are presented of inclusive W+→ℓ+ν , W-→ℓ-ν¯ and Z/γ∗→ℓℓ ( ℓ=e,μ ) Drell-Yan production cross sections at the LHC. The data were collected in proton-proton collisions at s=7TeV with an integrated luminosity of 4.6fb-1 . Differential W+ and W- cross sections are measured in a lepton pseudorapidity range |ηℓ|<2.5 . Differential Z/γ∗ cross sections are measured as a function of the absolute dilepton rapidity, for |yℓℓ|<3.6 , for three intervals of dilepton mass, mℓℓ , extending from 46 to 150GeV . The integrated and differential electron- and muon-channel cross sections are combined and compared to theoretical predictions using recent sets of parton distribution functions. The data, together with the final inclusive e±p scattering cross-section data from H1 and ZEUS, are interpreted in a next-to-next-to-leading-order QCD analysis, and a new set of parton distribution functions, ATLAS-epWZ16, is obtained. The ratio of strange-to-light sea-quark densities in the proton is determined more accurately than in previous determinations based on collider data only, and is established to be close to unity in the sensitivity range of the data. A new measurement of the CKM matrix element |Vcs| is also provided.

Measurements of the space charge density across the width of a plane parallel sample of degassed crosslinked polyethene (XLPE) have been made recently. Repeated at intervals throughout a period of applied high voltage, these measurements provide data on the dynamics of charge injection and space charge development in the sample under the influence of a stepped high DC voltage. In order to analyse these data, a simple model for bipolar charge injection, subsequent transport and trapping has been developed. This model is reported in this paper, and numerical simulations for charge density generated, as a function of position, time and field, are compared to the experimentally obtained profiles.

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 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

Results of a search for $H \to \tau \tau$ decays are presented, based on the full set of proton-proton collision data recorded by the ATLAS experiment at the LHC during 2011 and 2012. The data correspond to integrated luminosities of 4.5 $\rm{fb}^{-1}$ and 20.3 $\rm{fb}^{-1}$ at centre-of-mass energies of $\sqrt{s}$ = 7 TeV and $\sqrt{s}$ = 8 TeV respectively. All combinations of leptonic ($\tau \to \ell \nu \bar \nu$ with $\ell = e, \mu$) and hadronic ($\tau \to \rm{hadrons} \nu$) tau decays are considered. An excess of events over the expected background from other Standard Model processes is found with an observed (expected) significance of 4.5 (3.4) standard deviations. This excess provides evidence for the direct coupling of the recently discovered Higgs boson to fermions. The measured signal strength, normalised to the Standard Model expectation, of $\mu = 1.43 ^{+0.43}_{-0.37}$ is consistent with the predicted Yukawa coupling strength in the Standard Model.

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.

This Letter describes a model-agnostic search for pairs of jets (dijets) produced by resonant and non-resonant phenomena beyond the Standard Model in 3.6 fb$^{-1}$ of proton-proton collisions with a centre-of-mass energy of $\sqrt{s}=13$ TeV recorded by the ATLAS detector at the Large Hadron Collider. The distribution of the invariant mass of the two leading jets is examined for local excesses above a data-derived estimate of the smoothly falling prediction of the Standard Model. The data are also compared to a Monte Carlo simulation of Standard Model angular distributions derived from the rapidity of the two jets. No evidence of anomalous phenomena is observed in the data, which are used to exclude, at 95% CL, quantum black holes with threshold masses below 8.3 TeV, 8.1 TeV, or 5.3 TeV in three different benchmark scenarios; resonance masses below 5.2 TeV for excited quarks, 2.6 TeV in a $W^\prime$ model, a range of masses starting from $m_{Z^\prime}=1.5$ TeV and couplings from $g_q = 0.2$ in a $Z^\prime$ model; and contact interactions with a compositeness scale below 12.0 TeV and 17.5 TeV respectively for destructive and constructive interference between the new interaction and QCD processes. These results significantly extend the ATLAS limits obtained from 8 TeV data. Gaussian-shaped contributions to the mass distribution are also excluded if the effective cross-section exceeds values ranging from approximately 50-300 fb for masses below 2 TeV to 2-20 fb for masses above 4 TeV.

A search for the Standard Model Higgs boson produced in association with a top-quark pair, [Formula: see text], is presented. The analysis uses 20.3 fb-1 of pp collision data at [Formula: see text], collected with the ATLAS detector at the Large Hadron Collider during 2012. The search is designed for the [Formula: see text] decay mode and uses events containing one or two electrons or muons. In order to improve the sensitivity of the search, events are categorised according to their jet and b-tagged jet multiplicities. A neural network is used to discriminate between signal and background events, the latter being dominated by [Formula: see text]+jets production. In the single-lepton channel, variables calculated using a matrix element method are included as inputs to the neural network to improve discrimination of the irreducible [Formula: see text] background. No significant excess of events above the background expectation is found and an observed (expected) limit of 3.4 (2.2) times the Standard Model cross section is obtained at 95 % confidence level. The ratio of the measured [Formula: see text] signal cross section to the Standard Model expectation is found to be [Formula: see text] assuming a Higgs boson mass of 125[Formula: see text].

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.

Distributions of transverse momentum $$p_T^{\ell \ell }$$ and the related angular variable $$\phi ^*_\eta $$ of Drell–Yan lepton pairs are measured in 20.3 fb $$^{-1}$$ of proton–proton collisions at $$\sqrt{s}=8$$  TeV with the ATLAS detector at the LHC. Measurements in electron-pair and muon-pair final states are corrected for detector effects and combined. Compared to previous measurements in proton–proton collisions at $$\sqrt{s}=7$$  TeV, these new measurements benefit from a larger data sample and improved control of systematic uncertainties. Measurements are performed in bins of lepton-pair mass above, around and below the Z-boson mass peak. The data are compared to predictions from perturbative and resummed QCD calculations. For values of $$\phi ^*_\eta < 1$$ the predictions from the Monte Carlo generator ResBos are generally consistent with the data within the theoretical uncertainties. However, at larger values of $$\phi ^*_\eta $$ this is not the case. Monte Carlo generators based on the parton-shower approach are unable to describe the data over the full range of $$p_T^{\ell \ell }$$ while the fixed-order prediction of Dynnlo falls below the data at high values of $$p_T^{\ell \ell }$$ . ResBos and the parton-shower Monte Carlo generators provide a much better description of the evolution of the $$\phi ^*_\eta $$ and $$p_T^{\ell \ell }$$ distributions as a function of lepton-pair mass and rapidity than the basic shape of the data.

A search for the supersymmetric partners of quarks and gluons (squarks and gluinos) in final states containing hadronic jets and missing transverse momentum, but no electrons or muons, is presented. The data used in this search were recorded in 2015 and 2016 by the ATLAS experiment in $\sqrt{s}$=13 TeV proton--proton collisions at the Large Hadron Collider, corresponding to an integrated luminosity of 36.1 fb$^{-1}$. The results are interpreted in the context of various models where squarks and gluinos are pair-produced and the neutralino is the lightest supersymmetric particle. An exclusion limit at the 95\% confidence level on the mass of the gluino is set at 2.03 TeV for a simplified model incorporating only a gluino and the lightest neutralino, assuming the lightest neutralino is massless. For a simplified model involving the strong production of mass-degenerate first- and second-generation squarks, squark masses below 1.55 TeV are excluded if the lightest neutralino is massless. These limits substantially extend the region of supersymmetric parameter space previously excluded by searches with the ATLAS detector.

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.

Searches for both resonant and non-resonant Higgs boson pair production are performed in the $hh\to bb\tau\tau, \gamma\gamma WW^*$ final states using 20.3 fb$^{-1}$ of $pp$ collision data at a center-of-mass energy of 8 TeV recorded with the ATLAS detector at the Large Hadron Collider. No evidence of their production is observed and 95\% confidence level upper limits on the production cross sections are set. These results are then combined with the published results of the $hh\to \gamma\gamma bb, bbbb$ analyses. An upper limit of 0.69 (0.47) pb on the non-resonant Standard Model like $hh$ production is observed (expected), corresponding to 70 (48) times of the SM $gg\to hh$ cross section. For production via narrow resonances, cross section limits of $hh$ production from a heavy Higgs boson decay are set as a function of the heavy Higgs boson mass. The observed (expected) limits range from 2.1 (1.1) pb at 260 GeV to 0.011 (0.018) pb at 1000 GeV. These results are interpreted in the context of two simplified scenarios of the Minimal Supersymmetric Standard Model.

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 is performed for narrow resonances decaying into WW, WZ, or ZZ boson pairs using 20.3 fb−1 of proton-proton collision data at a centre-of-mass energy of $$ \sqrt{s}=8 $$ TeV recorded with the ATLAS detector at the Large Hadron Collider. Diboson resonances with masses in the range from 1.3 to 3.0 TeV are sought after using the invariant mass distribution of dijets where both jets are tagged as a boson jet, compatible with a highly boosted W or Z boson decaying to quarks, using jet mass and substructure properties. The largest deviation from a smoothly falling background in the observed dijet invariant mass distribution occurs around 2 TeV in the WZ channel, with a global significance of 2.5 standard deviations. Exclusion limits at the 95% confidence level are set on the production cross section times branching ratio for the WZ final state of a new heavy gauge boson, W′, and for the WW and ZZ final states of Kaluza-Klein excitations of the graviton in a bulk Randall-Sundrum model, as a function of the resonance mass. W′ bosons with couplings predicted by the extended gauge model in the mass range from 1.3 to 1.5 TeV are excluded at 95% confidence level.

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 search for doubly charged Higgs bosons with pairs of prompt, isolated, highly energetic leptons with the same electric charge is presented. The search uses a proton-proton collision data sample at a centre-of-mass energy of 13 TeV corresponding to 36.1 $\mathrm{fb}^{-1}$ of integrated luminosity recorded in 2015 and 2016 by the ATLAS detector at the LHC. This analysis focuses on the decays $H^{\pm\pm}\rightarrow e^{\pm}e^{\pm}$, $H^{\pm\pm}\rightarrow e^{\pm}\mu^{\pm}$ and $H^{\pm\pm}\rightarrow \mu^{\pm}\mu^{\pm}$, fitting the dilepton mass spectra in several exclusive signal regions. No significant evidence of a signal is observed and corresponding limits on the production cross-section are derived at 95% confidence level. The observed lower limit on the mass of a doubly charged Higgs boson only coupling to left-handed leptons ($e$,$\mu$) varies from 770 GeV to 870 GeV (850 GeV expected) for $B(H^{\pm\pm}\rightarrow \ell^{\pm}\ell^{\pm})$ = 100% and both the expected and observed mass limits are above 450 GeV for $B(H^{\pm\pm}\rightarrow \ell^{\pm}\ell^{\pm})$ = 10% and any combination of partial branching ratios.

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.

Dark matter particles, if sufficiently light, may be produced in decays of the Higgs boson. This Letter presents a statistical combination of searches for $H\to\textrm{invisible}$ decays where $H$ is produced according to the Standard Model via vector boson fusion, $Z(\ell\ell)H$, and $W\!/\!Z(\textrm{had})H$, all performed with the ATLAS detector using 36.1 fb$^{-1}$ of $pp$ collisions at a center-of-mass energy of $\sqrt s = 13$ TeV at the LHC. In combination with the results at $\sqrt s = 7$ and 8 TeV, an exclusion limit on the $H\to\textrm{invisible}$ branching ratio of $0.26 (0.17^{+0.07}_{-0.05})$ at 95% confidence level is observed (expected).

A search for a heavy, CP-odd Higgs boson, $A$, decaying into a $Z$ boson and a 125 GeV Higgs boson, $h$, with the ATLAS detector at the LHC is presented. The search uses proton--proton collision data at a centre-of-mass energy of 8 TeV corresponding to an integrated luminosity of 20.3 fb$^{-1}$. Decays of CP-even $h$ bosons to $\tau\tau$ or $bb$ pairs with the $Z$ boson decaying to electron or muon pairs are considered, as well as $h \rightarrow bb$ decays with the $Z$ boson decaying to neutrinos. No evidence for the production of an $A$ boson in these channels is found and the 95% confidence level upper limits derived for $\sigma (gg\rightarrow A) \times \mbox{BR}(A \rightarrow Zh) \times \mbox{BR}(h \rightarrow f\bar{f})$ are 0.098--0.013 pb for $f=\tau$ and 0.57--0.014 pb for $f=b$ in a range of $m_A =$ 220--1000 GeV. The results are combined and interpreted in the context of two-Higgs doublet models.

The ATLAS Inner Detector is a composite tracking system consisting of silicon pixels, silicon strips and straw tubes in a 2 T magnetic field. Its installation was completed in August 2008 and the detector took part in data- taking with single LHC beams and cosmic rays. The initial detector operation, hardware commissioning and in-situ calibrations are described. Tracking performance has been measured with 7.6 million cosmic-ray events, collected using a tracking trigger and reconstructed with modular pattern-recognition and fitting software. The intrinsic hit efficiency and tracking trigger efficiencies are close to 100%. Lorentz angle measurements for both electrons and holes, specific energy-loss calibration and transition radiation turn-on measurements have been performed. Different alignment techniques have been used to reconstruct the detector geometry. After the initial alignment, a transverse impact parameter resolution of 22.1+/-0.9 {\mu}m and a relative momentum resolution {\sigma}p/p = (4.83+/-0.16) \times 10-4 GeV-1 \times pT have been measured for high momentum tracks.