Topological cell clustering in the ATLAS calorimeters and its performance in LHC Run 1.

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.

Identification of boosted, hadronically decaying W bosons and comparisons with ATLAS data taken at [Formula: see text] TeV.

This paper reports a detailed study of techniques for identifying boosted, hadronically decaying W bosons using 20.3 fb[Formula: see text] of proton-proton collision data collected by the ATLAS detector at the LHC at a centre-of-mass energy [Formula: see text]. A range of techniques for optimising the signal jet mass resolution are combined with various jet substructure variables. The results of these studies in Monte Carlo simulations show that a simple pairwise combination of groomed jet mass and one substructure variable can provide a 50 % efficiency for identifying W bosons with transverse momenta larger than 200 GeV while maintaining multijet background efficiencies of 2-4 % for jets with the same transverse momentum. These signal and background efficiencies are confirmed in data for a selection of tagging techniques.

Observation of Long-Range Elliptic Azimuthal Anisotropies in sqrt[s]=13 and 2.76 TeV pp Collisions with the ATLAS Detector.

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.

Search for direct top squark pair production in final states with two tau leptons in pp collisions at [Formula: see text] TeV with the ATLAS detector.

A search for direct pair production of the supersymmetric partner of the top quark, decaying via a scalar tau to a nearly massless gravitino, has been performed using 20 fb[Formula: see text] of proton-proton collision data at [Formula: see text]. The data were collected by the ATLAS experiment at the LHC in 2012. Top squark candidates are searched for in events with either two hadronically decaying tau leptons, one hadronically decaying tau and one light lepton, or two light leptons. No significant excess over the Standard Model expectation is found. Exclusion limits at [Formula: see text] confidence level are set as a function of the top squark and scalar tau masses. Depending on the scalar tau mass, ranging from the [Formula: see text] LEP limit to the top squark mass, lower limits between 490 and [Formula: see text] are placed on the top squark mass within the model considered.

Measurement of the charge asymmetry in top-quark pair production in the lepton-plus-jets final state in pp collision data at [Formula: see text] with the ATLAS detector.

This paper reports inclusive and differential measurements of the [Formula: see text] charge asymmetry [Formula: see text] in [Formula: see text] of [Formula: see text][Formula: see text] collisions recorded by the ATLAS experiment at the Large Hadron Collider at CERN. Three differential measurements are performed as a function of the invariant mass, transverse momentum and longitudinal boost of the [Formula: see text] system. The [Formula: see text] pairs are selected in the single-lepton channels (e or [Formula: see text]) with at least four jets, and a likelihood fit is used to reconstruct the [Formula: see text] event kinematics. A Bayesian unfolding procedure is performed to infer the asymmetry at parton level from the observed data distribution. The inclusive [Formula: see text] charge asymmetry is measured to be [Formula: see text] (stat. [Formula: see text] syst.). The inclusive and differential measurements are compatible with the values predicted by the Standard Model.

Search for an additional, heavy Higgs boson in the [Formula: see text] decay channel at [Formula: see text] in [Formula: see text] collision data with the ATLAS detector.

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.

Search for flavour-changing neutral current top-quark decays to [Formula: see text] in [Formula: see text] collision data collected with the ATLAS detector at [Formula: see text] TeV.

A search for the flavour-changing neutral-current decay [Formula: see text] is presented. Data collected by the ATLAS detector during 2012 from proton-proton collisions at the Large Hadron Collider at a centre-of-mass energy of [Formula: see text] TeV, corresponding to an integrated luminosity of 20.3 fb[Formula: see text], are analysed. Top-quark pair-production events with one top quark decaying through the [Formula: see text] ([Formula: see text]) channel and the other through the dominant Standard Model mode [Formula: see text] are considered as signal. Only the decays of the Z boson to charged leptons and leptonic W boson decays are used. No evidence for a signal is found and an observed (expected) upper limit on the [Formula: see text] branching ratio of [Formula: see text] ([Formula: see text]) is set at the 95 % confidence level.

Search for single top-quark production via flavour-changing neutral currents at 8 TeV with the ATLAS detector.

A search for single top-quark production via flavour-changing neutral current processes from gluon plus up- or charm-quark initial states in proton-proton collisions at the LHC is presented. Data collected with the ATLAS detector in 2012 at a centre-of-mass energy of 8 TeV and corresponding to an integrated luminosity of 20.3 fb[Formula: see text] are used. Candidate events for a top quark decaying into a lepton, a neutrino and a jet are selected and classified into signal- and background-like candidates using a neural network. No signal is observed and an upper limit on the production cross-section multiplied by the [Formula: see text] branching fraction is set. The observed 95 % CL limit is [Formula: see text] and the expected 95 % CL limit is [Formula: see text]. The observed limit can be interpreted as upper limits on the coupling constants of the flavour-changing neutral current interactions divided by the scale of new physics [Formula: see text] and [Formula: see text] and on the branching fractions [Formula: see text] and [Formula: see text].

Study of the [Formula: see text] and [Formula: see text] decays with the ATLAS detector.

The decays [Formula: see text] and [Formula: see text] are studied with the ATLAS detector at the LHC using a dataset corresponding to integrated luminosities of 4.9 and 20.6 fb[Formula: see text] of pp collisions collected at centre-of-mass energies [Formula: see text] TeV and 8 TeV, respectively. Signal candidates are identified through [Formula: see text] and [Formula: see text] decays. With a two-dimensional likelihood fit involving the [Formula: see text] reconstructed invariant mass and an angle between the [Formula: see text] and [Formula: see text] candidate momenta in the muon pair rest frame, the yields of [Formula: see text] and [Formula: see text], and the transverse polarisation fraction in [Formula: see text] decay are measured. The transverse polarisation fraction is determined to be [Formula: see text], and the derived ratio of the branching fractions of the two modes is [Formula: see text], where the first error is statistical and the second is systematic. Finally, a sample of [Formula: see text] decays is used to derive the ratios of branching fractions [Formula: see text] and [Formula: see text], where the third error corresponds to the uncertainty of the branching fraction of [Formula: see text] decay. The available theoretical predictions are generally consistent with the measurement.

Searches for scalar leptoquarks in pp collisions at [Formula: see text] = 8 TeV with the ATLAS detector.

Searches for pair-produced scalar leptoquarks are performed using 20 fb[Formula: see text] of proton-proton collision data provided by the LHC and recorded by the ATLAS detector at [Formula: see text] TeV. Events with two electrons (muons) and two or more jets in the final state are used to search for first (second)-generation leptoquarks. The results from two previously published ATLAS analyses are interpreted in terms of third-generation leptoquarks decaying to [Formula: see text] and [Formula: see text] final states. No statistically significant excess above the Standard Model expectation is observed in any channel and scalar leptoquarks are excluded at 95 % CL with masses up to [Formula: see text] 1050 GeV for first-generation leptoquarks, [Formula: see text] 1000 GeV for second-generation leptoquarks, [Formula: see text] 625 GeV for third-generation leptoquarks in the [Formula: see text] channel, and 200 [Formula: see text] 640 GeV in the [Formula: see text] channel.

Measurements of fiducial cross-sections for [Formula: see text] production with one or two additional b-jets in pp collisions at [Formula: see text]=8 TeV using the ATLAS detector.

Fiducial cross-sections for [Formula: see text] production with one or two additional b-jets are reported, using an integrated luminosity of 20.3 fb[Formula: see text] of proton-proton collisions at a centre-of-mass energy of 8 TeV at the Large Hadron Collider, collected with the ATLAS detector. The cross-section times branching ratio for [Formula: see text] events with at least one additional b-jet is measured to be 950 [Formula: see text] 70 (stat.) [Formula: see text] (syst.) fb in the lepton-plus-jets channel and 50 [Formula: see text] 10 (stat.) [Formula: see text] (syst.) fb in the [Formula: see text] channel. The cross-section times branching ratio for events with at least two additional b-jets is measured to be 19.3 [Formula: see text] 3.5 (stat.) [Formula: see text] 5.7 (syst.) fb in the dilepton channel ([Formula: see text], [Formula: see text], and  ee) using a method based on tight selection criteria, and 13.5 [Formula: see text] 3.3 (stat.) [Formula: see text] 3.6 (syst.) fb using a looser selection that allows the background normalisation to be extracted from data. The latter method also measures a value of 1.30 [Formula: see text] 0.33 (stat.) [Formula: see text] 0.28 (syst.)% for the ratio of [Formula: see text] production with two additional b-jets to [Formula: see text] production with any two additional jets. All measurements are in good agreement with recent theory predictions.

Measurements of the Higgs boson production and decay rates and coupling strengths using pp collision data at [Formula: see text] and 8 TeV in the ATLAS experiment.

Combined analyses of the Higgs boson production and decay rates as well as its coupling strengths to vector bosons and fermions are presented. The combinations include the results of the analyses of the [Formula: see text] and [Formula: see text] decay modes, and the constraints on the associated production with a pair of top quarks and on the off-shell coupling strengths of the Higgs boson. The results are based on the LHC proton-proton collision datasets, with integrated luminosities of up to 4.7 [Formula: see text] at [Formula: see text] TeV and 20.3 [Formula: see text] at [Formula: see text] TeV, recorded by the ATLAS detector in 2011 and 2012. Combining all production modes and decay channels, the measured signal yield, normalised to the Standard Model expectation, is [Formula: see text]. The observed Higgs boson production and decay rates are interpreted in a leading-order coupling framework, exploring a wide range of benchmark coupling models both with and without assumptions on the Higgs boson width and on the Standard Model particle content in loop processes. The data are found to be compatible with the Standard Model expectations for a Higgs boson at a mass of 125.36 GeV for all models considered.

Performance of pile-up mitigation techniques for jets in [Formula: see text] collisions at [Formula: see text] TeV using the ATLAS detector.

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.

Search for new phenomena in events with at least three photons collected in pp collisions at [Formula: see text] = 8 TeV with the ATLAS detector.

Results of a search for new phenomena in events with at least three photons are reported. Data from proton-proton collisions at a centre-of-mass energy of 8 TeV, corresponding to an integrated luminosity of 20.3 fb[Formula: see text], were collected with the ATLAS detector at the LHC. The observed data are well described by the Standard Model. Limits at the 95 % confidence level on new phenomena are presented based on the rate of events in an inclusive signal region and a restricted signal region targeting the rare decay [Formula: see text], as well as di-photon and tri-photon resonance searches. For a Standard Model Higgs boson decaying to four photons via a pair of intermediate pseudoscalar particles (a), limits are found to be [Formula: see text] for 10 GeV [Formula: see text] 62 GeV. Limits are also presented for Higgs boson-like scalars (H) for [Formula: see text] 125 GeV, and for a [Formula: see text] decaying to three photons via [Formula: see text]. Additionally, the observed limit on the branching ratio of the Z boson decay to three photons is found to be BR[Formula: see text], a result five times stronger than the previous result from LEP.

Probing lepton flavour violation via neutrinoless [Formula: see text] decays with the ATLAS detector.

This article presents the sensitivity of the ATLAS experiment to the lepton-flavour-violating decays of [Formula: see text]. A method utilising the production of [Formula: see text] leptons via [Formula: see text] decays is used. This method is applied to the sample of 20.3 fb[Formula: see text] of pp collision data at a centre-of-mass energy of 8 TeV collected by the ATLAS experiment at the LHC in 2012. No event is observed passing the selection criteria, and the observed (expected) upper limit on the [Formula: see text] lepton branching fraction into three muons, [Formula: see text], is [Formula: see text] ([Formula: see text]) at 90 % confidence level.

A new method to distinguish hadronically decaying boosted Z bosons from W bosons using the ATLAS detector.

The distribution of particles inside hadronic jets produced in the decay of boosted W and Z bosons can be used to discriminate such jets from the continuum background. Given that a jet has been identified as likely resulting from the hadronic decay of a boosted W or Z boson, this paper presents a technique for further differentiating Z bosons from W bosons. The variables used are jet mass, jet charge, and a b-tagging discriminant. A likelihood tagger is constructed from these variables and tested in the simulation of [Formula: see text] for bosons in the transverse momentum range 200 GeV [Formula: see text] 400 GeV in [Formula: see text] TeV pp collisions with the ATLAS detector at the LHC. For Z-boson tagging efficiencies of [Formula: see text], 50, and [Formula: see text], one can achieve [Formula: see text]-boson tagging rejection factors ([Formula: see text]) of 1.7, 8.3 and 1000, respectively. It is not possible to measure these efficiencies in the data due to the lack of a pure sample of high [Formula: see text], hadronically decaying Z bosons. However, the modelling of the tagger inputs for boosted W bosons is studied in data using a [Formula: see text]-enriched sample of events in 20.3 fb[Formula: see text] of data at [Formula: see text] TeV. The inputs are well modelled within uncertainties, which builds confidence in the expected tagger performance.

Measurement of the differential cross-sections of prompt and non-prompt production of [Formula: see text] and [Formula: see text] in pp collisions at [Formula: see text] and 8 TeV with the ATLAS detector.

The production rates of prompt and non-prompt [Formula: see text] and [Formula: see text] mesons in their dimuon decay modes are measured using 2.1 and 11.4 fb[Formula: see text] of data collected with the ATLAS experiment at the Large Hadron Collider, in proton-proton collisions at [Formula: see text] and 8 respectively. Production cross-sections for prompt as well as non-prompt sources, ratios of [Formula: see text] to [Formula: see text] production, and the fractions of non-prompt production for [Formula: see text] and [Formula: see text] are measured as a function of meson transverse momentum and rapidity. The measurements are compared to theoretical predictions.

Measurement of the transverse momentum and [Formula: see text] distributions of Drell-Yan lepton pairs in proton-proton collisions at [Formula: see text] TeV with the ATLAS detector.

Distributions of transverse momentum [Formula: see text] and the related angular variable [Formula: see text] of Drell–Yan lepton pairs are measured in 20.3 fb[Formula: see text] of proton–proton collisions at [Formula: see text] 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 [Formula: see text] 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 [Formula: see text] the predictions from the Monte Carlo generator ResBos are generally consistent with the data within the theoretical uncertainties. However, at larger values of [Formula: see text] 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 [Formula: see text] while the fixed-order prediction of Dynnlo falls below the data at high values of [Formula: see text]. ResBos and the parton-shower Monte Carlo generators provide a much better description of the evolution of the [Formula: see text] and [Formula: see text] distributions as a function of lepton-pair mass and rapidity than the basic shape of the data.

Reconstruction of hadronic decay products of tau leptons with the ATLAS experiment.

This paper presents a new method of reconstructing the individual charged and neutral hadrons in tau decays with the ATLAS detector. The reconstructed hadrons are used to classify the decay mode and to calculate the visible four-momentum of reconstructed tau candidates, significantly improving the resolution with respect to the calibration in the existing tau reconstruction. The performance of the reconstruction algorithm is optimised and evaluated using simulation and validated using samples of [Formula: see text] and [Formula: see text]+jets events selected from proton-proton collisions at a centre-of-mass energy [Formula: see text], corresponding to an integrated luminosity of 5 [Formula: see text].

Measurements of top-quark pair differential cross-sections in the lepton+jets channel in pp collisions at [Formula: see text] using the ATLAS detector.

Measurements of normalized differential cross-sections of top-quark pair production are presented as a function of the top-quark, [Formula: see text] system and event-level kinematic observables in proton-proton collisions at a centre-of-mass energy of [Formula: see text]. The observables have been chosen to emphasize the [Formula: see text] production process and to be sensitive to effects of initial- and final-state radiation, to the different parton distribution functions, and to non-resonant processes and higher-order corrections. The dataset corresponds to an integrated luminosity of 20.3 fb[Formula: see text], recorded in 2012 with the ATLAS detector at the CERN Large Hadron Collider. Events are selected in the lepton+jets channel, requiring exactly one charged lepton and at least four jets with at least two of the jets tagged as originating from a b-quark. The measured spectra are corrected for detector effects and are compared to several Monte Carlo simulations. The results are in fair agreement with the predictions over a wide kinematic range. Nevertheless, most generators predict a harder top-quark transverse momentum distribution at high values than what is observed in the data. Predictions beyond NLO accuracy improve the agreement with data at high top-quark transverse momenta. Using the current settings and parton distribution functions, the rapidity distributions are not well modelled by any generator under consideration. However, the level of agreement is improved when more recent sets of parton distribution functions are used.