Search

Correlations of two flow harmonics vn and vm via three- and four-particle cumulants are measured in 13 TeV pp, 5.02 TeV p+Pb, and 2.76 TeV peripheral Pb+Pb collisions with the ATLAS detector at the LHC. The goal is to understand the multi-particle nature of the long-range collective phenomenon in these collision systems. The large non-flow background from dijet production present in the standard cumulant method is suppressed using a method of subevent cumulants involving two, three and four subevents separated in pseudorapidity. The results show a negative correlation between v2and v3and a positive correlation between v2 and v4 for all collision systems and over the full multiplicity range. However, the magnitudes of the correlations are found to depend on the event multiplicity, the choice of transverse momentum range and collision system. The relative correlation strength, obtained by normalisation of the cumulants with the 〈v2n〉from a two-particle correlation analysis, is similar in the three collision systems and depends weakly on the event multiplicity and transverse momentum. These results based on the subevent methods provide strong evidence of a similar long-range multi-particle collectivity in pp, p+Pb and peripheral Pb+Pb collisions.

A search for doubly charged scalar bosons decaying into W boson pairs is presented. It uses a data sample from proton–proton collisions corresponding to an integrated luminosity of 36.1 fb−1 collected by the ATLAS detector at the LHC at a centre-of-mass energy of 13 TeV in 2015 and 2016. This search is guided by a model that includes an extension of the Higgs sector through a scalar triplet, leading to a rich phenomenology that includes doubly charged scalar bosons H±±. Those bosons are produced in pairs in proton–proton collisions and decay predominantly into electroweak gauge bosons H±±→W±W±. Experimental signatures with several leptons, missing transverse energy and jets are explored. No significant deviations from the Standard Model predictions are found. The parameter space of the benchmark model is excluded at 95% confidence level for H±± bosons with masses between 200 and 220 GeV.

A search for a pair of neutral, scalar bosons with each decaying into two W bosons is presented using 36.1 fb−1 of proton-proton collision data at a centre-of-mass energy of 13 TeV recorded with the ATLAS detector at the Large Hadron Collider. This search uses three production models: non-resonant and resonant Higgs boson pair production and resonant production of a pair of heavy scalar particles. Three final states, classified by the number of leptons, are analysed: two same-sign leptons, three leptons, and four leptons. No significant excess over the expected Standard Model backgrounds is observed. An observed (expected) 95% confidence-level upper limit of 160 (120) times the Standard Model prediction of non-resonant Higgs boson pair production cross-section is set from a combined analysis of the three final states. Upper limits are set on the production cross-section times branching ratio of a heavy scalar X decaying into a Higgs boson pair in the mass range of 260 GeV ≤ mX ≤ 500 GeV and the observed (expected) limits range from 9.3 (10) pb to 2.8 (2.6) pb. Upper limits are set on the production cross-section times branching ratio of a heavy scalar X decaying into a pair of heavy scalars S for mass ranges of 280 GeV ≤ mX ≤ 340 GeV and 135 GeV ≤ mS ≤ 165 GeV and the observed (expected) limits range from 2.5 (2.5) pb to 0.16 (0.17) pb.

This Letter presents a search for new light resonances decaying to pairs of quarks and produced in association with a high-pT photon or jet. The dataset consists of proton–proton collisions with an integrated luminosity of 36.1 fb−1 at a centre-of-mass energy of √s=13 TeV recorded by the ATLAS detector at the Large Hadron Collider. Resonance candidates are identified as massive large-radius jets with substructure consistent with a particle decaying into a quark pair. The mass spectrum of the candidates is examined for local excesses above background. No evidence of a new resonance is observed in the data, which are used to exclude the production of a lepto-phobic axial-vector Z′boson.

We report a search for Higgs bosons that are produced via vector boson fusion and subsequently decay into invisible particles. The experimental signature is an energetic jet pair with invariant mass of O(1) TeV and O(100) GeV missing transverse momentum. The analysis uses 36.1fb−1of ppcollision data at √s=13 TeV recorded by the ATLAS detector at the LHC. In the signal region the 2252 observed events are consistent with the background estimation. Assuming a 125 GeV scalar particle with Standard Model cross sections, the upper limit on the branching fraction of the Higgs boson decay into invisible particles is 0.37 at 95% confidence level where 0.28 was expected. This limit is interpreted in Higgs portal models to set bounds on thewimp–nucleon scattering cross section. We also consider invisible decays of additional scalar bosons with masses up to 3 TeV for which the upper limits on the cross section times branching fraction are in the range of 0.3–1.7 pb.

A search for new charged massive gauge bosons, W′, is performed with the ATLAS detector at the LHC. Data were collected in proton–proton collisions at a center-of-mass energy of √s=13 TeV and correspond to an integrated luminosity of 36.1fb−1. This analysis searches for W′ bosons in the W′ → tb ̄ decay channel in final states with an electron or muon plus jets. The search covers resonance masses between 0.5 and 5.0 TeV and considers right-handed W′ bosons. No significant deviation from the Standard Model (SM) expectation is observed and upper limits are set on the W′ → tb ̄ cross section times branching ratio and the W′ boson effective couplings as a function of the W′boson mass. For right-handed W′ bosons with coupling to the SM particles equal to the SM weak coupling constant, masses below 3.15 TeV are excluded at the 95% confidence level. This search is also combined with a previously published ATLAS result for W′ → tb ̄ in the fully hadronic final state. Using the combined searches, right-handed W′ bosons with masses below 3.25 TeV are excluded at the 95% confidence level.

This paper describes a strategy for a general search used by the ATLAS Collaboration to find potential indications of new physics. Events are classified according to their final state into many event classes. For each event class an automated search algorithm tests whether the data are compatible with the Monte Carlo simulated expectation in several distributions sensitive to the effects of new physics. The significance of a deviation is quantified using pseudo-experiments. A data selection with a significant deviation defines a signal region for a dedicated follow-up analysis with an improved background expectation. The analysis of the data-derived signal regions on a new dataset allows a statistical interpretation without the large look-elsewhere effect. The sensitivity of the approach is discussed using Standard Model processes and benchmark signals of new physics. As an example, results are shown for 3.2 fb−1 of proton–proton collision data at a centre-of-mass energy of 13 TeV collected with the ATLAS detector at the LHC in 2015, in which more than 700 event classes and more than 105 regions have been analysed. No significant deviations are found and consequently no data-derived signal regions for a follow-up analysis have been defined.

A measurement of production cross sections of the Higgs boson in proton-proton collisions is presented in the H→ττ decay channel. The analysis is performed using 36.1 fb−1 of data recorded by the ATLAS experiment at the Large Hadron Collider at a center-of-mass energy of √s=13 TeV. All combinations of leptonic (τ→ℓv¯v with ℓ=e,μ) and hadronic (τ→hadrons v) τ decays are considered. The H→ττ signal over the expected background from other Standard Model processes is established with an observed (expected) significance of 4.4 (4.1) standard deviations. Combined with results obtained using data taken at 7 and 8 TeV center-of-mass energies, the observed (expected) significance amounts to 6.4 (5.4) standard deviations and constitutes an observation of H→ττ decays. Using the data taken at √s=13 TeV, the total cross section in the H→ττ decay channel is measured to be 3.77+0.60−0.59 (stat) +0.87−0.74 (syst) pb, for a Higgs boson of mass 125 GeV assuming the relative contributions of its production modes as predicted by the Standard Model. Total cross sections in the H→ττ decay channel are determined separately for vector-boson-fusion production and gluon-gluon-fusion production to be σVBFH→ττ=0.28±0.09 (stat) +0.11−0.09 (syst) pb and σggFH→ττ=3.1±1.0 (stat) +1.6−1.3 (syst) pb, respectively. Similarly, results of a fit are reported in the framework of simplified template cross sections. All measurements are in agreement with Standard Model expectations.

Measurements of the yield and nuclear modification factor, RAA, for inclusive jet production are performed using 0.49 nb−1of Pb+Pb data at √sNN=5.02 TeV and 25 pb−1of ppdata at √s=5.02 TeV with the ATLAS detector at the LHC. Jets are reconstructed with the anti-kt algorithm with radius parameter R =0.4 and are measured over the transverse momentum range of 40–1000 GeV in six rapidity intervals covering |y| < 2.8. The magnitude of RAA increases with increasing jet transverse momentum, reaching a value of approximately 0.6 at 1 TeVin the most central collisions. The magnitude of RAAalso increases towards peripheral collisions. The value of RAA is independent of rapidity at low jet transverse momenta, but it is observed to decrease with increasing rapidity at high transverse momenta

A search for Higgs boson pair production in the bb¯¯bb¯¯ final state is carried out with up to 36.1 fb−1 of LHC proton-proton collision data collected at √s=13 TeV with the ATLAS detector in 2015 and 2016. Three benchmark signals are studied: a spin-2 graviton decaying into a Higgs boson pair, a scalar resonance decaying into a Higgs boson pair, and Standard Model non-resonant Higgs boson pair production. Two analyses are carried out, each implementing a particular technique for the event reconstruction that targets Higgs bosons reconstructed as pairs of jets or single boosted jets. The resonance mass range covered is 260–3000 GeV. The analyses are statistically combined and upper limits on the production cross section of Higgs boson pairs times branching ratio to bb¯¯bb¯¯ are set in each model. No significant excess is observed; the largest deviation of data over prediction is found at a mass of 280 GeV, corresponding to 2.3 standard deviations globally. The observed 95% confidence level upper limit on the non-resonant production is 13 times the Standard Model prediction.

Measurements of fiducial integrated and differential cross sections for inclusive W+ , W− and Z boson production are reported. They are based on 25.0 ± 0.5pb−1 of pp collision data at √s=5.02 TeV collected with the ATLAS detector at the CERN Large Hadron Collider. Electron and muon decay channels are analysed, and the combined W+ , W− and Z integrated cross sections are found to be σW+=2266±9 (stat)±29 (syst)±43 (lumi) pb, σW−=1401±7 (stat)±18 (syst)±27 (lumi) pb, and σZ=374.5±3.4 (stat)±3.6 (syst)±7.0 (lumi) pb, in good agreement with next-to-next-to-leading-order QCD cross-section calculations. These measurements serve as references for Pb+Pb interactions at the LHC at √sNN=5.02 TeV.

A search for decays of the Higgs boson into a pair of new spin-zero particles, H→aa, where the a-bosons decay into a b-quark pair and a muon pair, is presented. The search uses 36.1 fb−1 of proton–proton collision data at √s=13 TeV recorded 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% confidence level are placed on the branching ratio (σH/σSM) ×B(H→aa →bbμμ), ranging from 1.2×10−4 to 8.4 ×10−4 in the a-boson mass range of 20–60 GeV. Model-independent limits are set on the visible production cross-section times the branching ratio to the bbμμ final state for new physics, σvis(X)×B(X→bbμμ), ranging from 0.1 fb to 0.73 fb for mμμ between 18 and 62 GeV.

Higgs boson production cross-sections in proton–proton collisions are measured in the H→WW*→eνμν decay channel. The proton–proton collision data were produced at the Large Hadron Collider at a centre-of-mass energy of 13 TeV and recorded by the ATLAS detector in 2015 and 2016, corresponding to an integrated luminosity of 36.1fb−1. The product of the H→WW*branching fraction times the gluon–gluon fusion and vector-boson fusion cross-sections are measured to be 11.4+1.2−1.1(stat.)+1.8−1.7(syst.) pb and 0.50+0.24−0.22(stat.) ±0.17(syst.) pb, respectively, in agreement with Standard Model predictions

A search for four-top-quark production, tt¯tt¯, is presented. It is based on proton-proton collision data with a center-of-mass energy √s=13 TeV collected by the ATLAS detector at the Large Hadron Collider during the years 2015 and 2016, corresponding to an integrated luminosity of 36.1 fb−1. Data are analyzed in both the single-lepton and opposite-sign dilepton channels, characterized by the presence of one or two isolated electrons or muons with high-transverse momentum and multiple jets. A data-driven method is used to estimate the dominant background from top-quark pair production in association with jets. No significant excess above the Standard Model expectation is observed. The result is combined with the previous same-sign dilepton and multilepton searches carried out by the ATLAS Collaboration and an observed (expected) upper limit of 5.3 (2.1) times the four-top-quark Standard Model cross section is obtained at 95% confidence level. Additionally, an upper limit on the anomalous four-top-quark production cross section is set in the context of an effective field theory model.

Measurements of the W ± →ℓ ± ν and Z→ℓ + ℓ − production cross sections (where ℓ ± =e ± ,μ ± ) in proton-proton collisions at √s=13 TeV are presented using data recorded by the ATLAS experiment at the Large Hadron Collider, corresponding to a total integrated luminosity of 81 pb −1 The total inclusive W ± -boson production cross sections times the single-lepton-flavour branching ratios are σ tot W + =11.83±0.02(stat)±0.32(sys)±0.25(lumi) nb and σ tot W − =8.79±0.02(stat)±0.24(sys)±0.18(lumi) nb for W + and W − , respectively. The total inclusive Z -boson production cross section times leptonic branching ratio, within the invariant mass window 66

Results of a search for new phenomena in events with large missing transverse momentum and a Higgs boson decaying to two photons are reported. Data from proton-proton collisions at a center-of-mass energy of 8 TeV and corresponding to an integrated luminosity of 20.3 fb⁻¹ have been collected with the ATLAS detector at the LHC. The observed data are well described by the expected Standard Model backgrounds. Upper limits on the cross section of events with large missing transverse momentum and a Higgs boson candidate are also placed. Exclusion limits are presented for models of physics beyond the Standard Model featuring dark-matter candidates.

A search has been performed for pair production of heavy vector-like down-type (B ) quarks. The analysis explores the lepton-plus-jets final state, characterized by events with one isolated charged lepton (electron or muon), significant missing transverse momentum and multiple jets. One or more jets are required to be tagged as arising from b -quarks, and at least one pair of jets must be tagged as arising from the hadronic decay of an electroweak boson. The analysis uses the full data sample of pp collisions recorded in 2012 by the ATLAS detector at the LHC, operating at a center-of-mass energy of 8 TeV, corresponding to an integrated luminosity of 20.3 fb −1 . No significant excess of events is observed above the expected background. Limits are set on vector-like B production, as a function of the B branching ratios, assuming the allowable decay modes are B→Wt/Zb/Hb . In the chiral limit with a branching ratio of 100% for the decay B→Wt , the observed (expected) 95% CL lower limit on the vector-like B mass is 810 GeV (760 GeV). In the case where the vector-like B quark has branching ratio values corresponding to those of an SU(2) singlet state, the observed (expected) 95% CL lower limit on the vector-like B mass is 640 GeV (505 GeV). The same analysis, when used to investigate pair production of a colored, charge 5/3 exotic fermion T 5/3 , with subsequent decay T 5/3 →Wt , sets an observed (expected) 95% CL lower limit on the T 5/3 mass of 840 GeV (780 GeV).

A search for new particles that decay into top quark pairs is reported. The search is performed with the ATLAS experiment at the LHC using an integrated luminosity of 20.3 fb⁻¹ of proton-proton collision data collected at a centre-of-mass energy of √s=8 TeV. The lepton-plus-jets final state is used, where the top pair decays to W + bW − b ¯ , with one W boson decaying leptonically and the other hadronically. The invariant mass spectrum of top quark pairs is examined for local excesses or deficits that are inconsistent with the Standard Model predictions. No evidence for a top quark pair resonance is found, and 95% confidence-level limits on the production rate are determined for massive states in benchmark models. The upper limits on the cross-section times branching ratio of a narrow Z ′ boson decaying to top pairs range from 4.2 pb to 0.03 pb for resonance masses from 0.4 TeV to 3.0 TeV. A narrow leptophobic topcolour Z ′ boson with mass below 1.8 TeV is excluded. Upper limits are set on the cross-section times branching ratio for a broad colour-octet resonance with Γ/m= 15% decaying to tt ¯ . These range from 2.5 pb to 0.03 pb for masses from 0.4 TeV to 3.0 TeV. A Kaluza-Klein excitation of the gluon in a Randall-Sundrum model is excluded for masses below 2.2 TeV.

A search is presented for narrow diboson resonances decaying to WW or WZ in the final state where one W boson decays leptonically (to an electron or a muon plus a neutrino) and the other W/Z boson decays hadronically. The analysis is performed using an integrated luminosity of 20.3 fb −1 of pp collisions at √s = 8 TeV collected by the ATLAS detector at the LHC. No evidence for resonant diboson production is observed, and resonance masses below 700 GeV and 1490 GeV are excluded at 95% confidence level for the spin-2 Randall-Sundrum bulk graviton G ∗ with coupling constant of 1.0 and the extended gauge model W ′ boson respectively.

This paper presents measurements from the ATLAS experiment of the forward-backward asymmetry in the reaction pp→Z/γ ∗ →l + l − , with l being electrons or muons, and the extraction of the effective weak mixing angle. The results are based on the full set of data collected in 2011 in pp collisions at the LHC at √ s = 7 TeV, corresponding to an integrated luminosity of 4.8 fb¯¹. The measured asymmetry values are found to be in agreement with the corresponding Standard Model predictions. The combination of the muon and electron channels yields a value of the effective weak mixing angle of 0.2308±0.0005(stat.)±0.0006(syst.)±0.0009(PDF) , where the first uncertainty corresponds to data statistics,the second to systematic effects and the third to knowledge of the parton density functions. This result agrees with the current world average from the Particle Data Group fit.

This Letter presents measurements of correlated production of nearby jets in Pb+Pb collisions at √sNN =2.76 TeV using the ATLAS detector at the Large Hadron Collider. The measurement was performed using 0.14 nb⁻¹ of data recorded in 2011. The production of correlated jet pairs was quantified using the rate, R ΔR , of "neighbouring" jets that accompany "test" jets within a given range of angular distance, ΔR , in the pseudorapidity--azimuthal angle plane. The jets were measured in the ATLAS calorimeter and were reconstructed using the anti-k t algorithm with radius parameters d=0.2 , 0.3 , and 0.4 . R ΔR was measured in different Pb+Pb collision centrality bins, characterized by the total transverse energy measured in the forward calorimeters. A centrality dependence of R ΔR is observed for all three jet radii with R ΔR found to be lower in central collisions than in peripheral collisions. The ratios formed by the R ΔR values in different centrality bins and the values in the 40-80 % centrality bin are presented.

This paper presents a search for Higgs bosons decaying to four leptons, either electrons or muons, via one or two light exotic gauge bosons Z d , H→ZZ d →4ℓ or H→Z d Z d →4ℓ . The search was performed using pp collision data corresponding to an integrated luminosity of about 20 fb⁻¹ at the center-of-mass energy of √s=8 TeV recorded with the ATLAS detector at the Large Hadron Collider. The observed data are well described by the Standard Model prediction. Upper bounds on the branching ratio of H→ZZ d →4ℓ and on the kinetic mixing parameter between the Z d and the Standard Model hypercharge gauge boson are set in the range (1 --9)×10⁻⁵ and (4 --17)×10⁻² respectively, at 95% confidence level assuming the Standard Model branching ratio of H→ZZ ∗ →4ℓ , for Z d masses between 15 and 55 GeV. Upper bounds on the effective mass mixing parameter between the Z and the Z d are also set using the branching ratio limits in the H→ZZ d →4ℓ search, and are in the range (1.5 --8.7)×10⁻⁴ for 15

Many extensions of the Standard Model posit the existence of heavy particles with long lifetimes. This article presents the results of a search for events containing at least one long-lived particle that decays at a significant distance from its production point into two leptons or into five or more charged particles. This analysis uses a data sample of proton-proton collisions at √s = 8 TeV corresponding to an integrated luminosity of 20.3 fb⁻¹ collected in 2012 by the ATLAS detector operating at the Large Hadron Collider. No events are observed in any of the signal regions, and limits are set on model parameters within supersymmetric scenarios involving R-parity violation, split supersymmetry, and gauge mediation. In some of the search channels, the trigger and search strategy are based only on the decay products of individual long-lived particles, irrespective of the rest of the event. In these cases, the provided limits can easily be reinterpreted in different scenarios.

A search for new phenomena in LHC proton-proton collisions at a center-of-mass energy of √s=8 TeV was performed with the ATLAS detector using an integrated luminosity of 17.3 fb¯¹. The angular distributions are studied in events with at least two jets; the highest dijet mass observed is 5.5 TeV. All angular distributions are consistent with the predictions of the Standard Model. In a benchmark model of quark contact interactions, a compositeness scale below 8.1 TeV in a destructive interference scenario and 12.0 TeV in a constructive interference scenario is excluded at 95 % CL; median expected limits are 8.9 TeV for the destructive interference scenario and 14.1 TeV for the constructive interference scenario.

An observation of the Λ 0 b →ψ(2S)Λ 0 decay and a comparison of its branching fraction with that of the Λ 0 b →J/ψΛ 0 decay has been made with the ATLAS detector in proton--proton collisions at s √ =8 TeV at the LHC using an integrated luminosity of 20.6 fb −1 . The J/ψ and ψ(2S) mesons are reconstructed in their decays to a muon pair, while the Λ 0 →pπ − decay is exploited for the Λ 0 baryon reconstruction. The Λ 0 b baryons are reconstructed with transverse momentum p T >10 GeV and pseudorapidity |η

A search for pair production of vector-like quarks, both up-type (T ) and down-type (B ), as well as for four-top-quark production, is presented. The search is based on pp collisions at √s=8 TeV recorded in 2012 with the ATLAS detector at the CERN Large Hadron Collider and corresponding to an integrated luminosity of 20.3 fb⁻¹. Data are analysed in the lepton-plus-jets final state, characterised by an isolated electron or muon with high transverse momentum, large missing transverse momentum and multiple jets. Dedicated analyses are performed targeting three cases: a T quark with significant branching ratio to a W boson and a b -quark (TT ¯ →Wb +X), and both a T quark and a B quark with significant branching ratio to a Higgs boson and a third-generation quark (TT ¯ →Ht +X and BB ¯ →Hb +X respectively). No significant excess of events above the Standard Model expectation is observed, and 95% CL lower limits are derived on the masses of the vector-like T and B quarks under several branching ratio hypotheses assuming contributions from T→Wb , Zt , Ht and B→Wt , Zb , Hb decays. The 95% CL observed lower limits on the T quark mass range between 715 GeV and 950 GeV for all possible values of the branching ratios into the three decay modes, and are the most stringent constraints to date. Additionally, the most restrictive upper bounds on four-top-quark production are set in a number of new physics scenarios.

This Letter reports evidence of triple gauge boson production pp→W(ℓν)γγ+X , which is accessible for the first time with the 8 TeV LHC data set. The fiducial cross section for this process is measured in a data sample corresponding to an integrated luminosity of 20.3 fb¯¹, collected by the ATLAS detector in 2012. Events are selected using the W boson decay to eν or μν as well as requiring two isolated photons. The measured cross section is used to set limits on anomalous quartic gauge couplings in the high diphoton mass region.

Searches are performed for resonant and non-resonant Higgs boson pair production in the γγbb final state using 20 fb⁻¹ of proton-proton collisions at a center-of-mass energy of 8 TeV recorded with the ATLAS detector at the CERN Large Hadron Collider. A 95% confidence level upper limit on the cross section times branching ratio of non-resonant production is set at 2.2 pb, while the expected limit is 1.0 pb. The difference derives from a modest excess of events, corresponding to 2.4 standard deviations from the background-only hypothesis. The limit observed in the search for a narrow X→hh resonance ranges between 0.7 and 3.5 pb as a function of the resonance mass.

A search for heavy long-lived multi-charged particles is performed using the ATLAS detector at the LHC. Data collected in 2012 at √s=8 TeV from pp collisions corresponding to an integrated luminosity of 20.3 fb¯¹ are examined. Particles producing anomalously high ionisation, consistent with long-lived massive particles with electric charges from |q|=2e to |q|=6e are searched for. No signal candidate events are observed, and 95\% confidence level cross-section upper limits are interpreted as lower mass limits for a Drell--Yan production model. The mass limits range between 660 and 785 GeV.

The top quark mass was measured in the channels tt ¯ →lepton+jets and tt ¯ →dilepton (lepton=e,μ ) based on ATLAS data recorded in 2011. The data were taken at the LHC with a proton--proton centre-of-mass energy of √s=7 TeV and correspond to an integrated luminosity of 4.6fb −1 . The tt ¯ →lepton+jets analysis uses a three-dimensional template technique which determines the top quark mass together with a global jet energy scale factor (JSF), and a relative b -to-light-jet energy scale factor (bJSF), where the terms b -jets and light-jets refer to jets originating from b -quarks and u,d,c,s -quarks or gluons, respectively. The analysis of the tt ¯ →dilepton channel exploits a one-dimensional template method using the m ℓb observable, defined as the average invariant mass of the two lepton+b -jet pairs in each event. The top quark mass is measured to be 172.33±0.75(stat)±1.02(syst) GeV, and 173.79±0.54(stat)±1.30(syst) GeV in the tt ¯ →lepton+jets and tt ¯ →dilepton channels, respectively. The combination of the two results yields m top =172.99±0.48(stat)±0.78(syst) GeV, with a total uncertainty of 0.91 GeV. 1.02(syst) GeV, and 173.79 ± 0.54(stat) ± 1.30(syst) GeV in the t ¯t → lepton+jets and t ¯t → dilepton channels, respectively. The combination of the two results yields mtop = 172.99 ± 0.48(stat) ± 0.78(syst) GeV, with a total uncertainty of 0.91 GeV.

Measurements of the total and differential cross sections of Higgs boson production are performed using 20.3 fb⁻¹ of pp collisions produced by the Large Hadron Collider at a center-of-mass energy of √s=8 TeV and recorded by the ATLAS detector. Cross sections are obtained from measured H→γγ and H→ZZ ∗ →4ℓ event yields, which are combined accounting for detector efficiencies, fiducial acceptances and branching fractions. Differential cross sections are reported as a function of Higgs boson transverse momentum, Higgs boson rapidity, number of jets in the event, and transverse momentum of the leading jet. The total production cross section is determined to be σ pp→H =33.0±5.3(stat)±1.6(sys)pb . The measurements are compared to state-of-the-art predictions.

A search for the associated production of the Higgs boson with a top quark pair is performed in multilepton final states using 20.3 fb⁻¹ of proton-proton collision data recorded by the ATLAS experiment at √s=8 TeV at the Large Hadron Collider. Five final states, targeting the decays H→WW∗, ττ , and ZZ∗ , are examined for the presence of the Standard Model (SM) Higgs boson: two same-charge light leptons (e or μ ) without a hadronically decaying τ lepton; three light leptons; two same-charge light leptons with a hadronically decaying τ lepton; four light leptons; and one light lepton and two hadronically decaying τ leptons. No significant excess of events is observed above the background expectation. The best fit for the tt ¯ H production cross section, assuming a Higgs boson mass of 125 GeV, is 2.1 +1.4 −1.2 times the SM expectation, and the observed (expected) upper limit at the 95% confidence level is 4.7 (2.4) times the SM rate. The p -value for compatibility with the background-only hypothesis is 1.8σ ; the expectation in the presence of a Standard Model signal is 0.9σ .

The normalized differential cross section for top-quark pair production in association with at least one jet is studied as a function of the inverse of the invariant mass of the tt ¯ +1 -jet system. This distribution can be used for a precise determination of the top-quark mass since gluon radiation depends on the mass of the quarks. The experimental analysis is based on proton-proton collision data collected by the ATLAS detector at the LHC with a centre-of-mass energy of 7 TeV corresponding to an integrated luminosity of 4.6 fb⁻¹. The selected events were identified using the lepton+jets top-quark-pair decay channel, where lepton refers to either an electron or a muon. The observed distribution is compared to a theoretical prediction at next-to-leading-order accuracy in quantum chromodynamics using the pole-mass scheme. With this method, the measured value of the top-quark pole mass, m pole t , is: m pole t =173.7 ± 1.5 (stat.) ± 1.4 (syst.) +1.0 −0.5 (theory) GeV. This result represents the most precise measurement of the top-quark pole mass to date.

This Letter reports a measurement of the exclusive γγ→ℓ + ℓ − (ℓ=e,μ) cross-section in proton-proton collisions at a centre-of-mass energy of 7 TeV by the ATLAS experiment at the LHC, based on an integrated luminosity of 4.6 fb⁻¹ . For the electron or muon pairs satisfying exclusive selection criteria, a fit to the dilepton acoplanarity distribution is used to extract the fiducial cross-sections. The cross-section in the electron channel is determined to be σ excl. γγ→e + e − =0.428±0.035(stat.)±0.018(syst.) pb for a phase-space region with invariant mass of the electron pairs greater than 24 GeV, in which both electrons have transverse momentum p T >12 GeV and pseudorapidity |η|10 GeV and pseudorapidity |η

Charged-particle spectra obtained in 0.15 nb¯¹ of Pb+Pb interactions at √sNN =2.76 TeV and 4.2 pb¯¹ of pp interactions at √s =2.76 TeV with the ATLAS detector at the LHC are presented in a wide transverse momentum (0.5

Many extensions of the Standard Model predict the existence of charged heavy long-lived particles, such as R -hadrons or charginos. These particles, if produced at the Large Hadron Collider, should be moving non-relativistically and are therefore identifiable through the measurement of an anomalously large specific energy loss in the ATLAS pixel detector. Measuring heavy long-lived particles through their track parameters in the vicinity of the interaction vertex provides sensitivity to metastable particles with lifetimes from 0.6 ns to 30 ns. A search for such particles with the ATLAS detector at the Large Hadron Collider is presented, based on a data sample corresponding to an integrated luminosity of 18.4 fb¯¹ of pp collisions at √ s= 8 TeV. No significant deviation from the Standard Model background expectation is observed, and lifetime-dependent upper limits on R -hadrons and chargino production are set. Gluino R -hadrons with 10 ns lifetime and masses up to 1185 GeV are excluded at 95% confidence level, and so are charginos with 15 ns lifetime and masses up to 482 GeV.

The ATLAS Collaboration measures the inclusive production of Z bosons via their decays into electron and muon pairs in p+Pb collisions at s NN − − − √ =5.02TeV at the Large Hadron Collider. The measurements are made using data corresponding to integrated luminosities of 29.4 and 28.1 nb −1 for Z→ee and Z→μμ , respectively. The results from the two channels are consistent and combined to obtain a cross section times the Z→ℓℓ branching ratio, integrated over the rapidity region ∣ ∣ y ∗ Z

Correlations between the elliptic or triangular flow coefficients vm (m =2 or 3) and other flow harmonics vn (n =2 to 5) are measured using √sNN=2.76 TeV Pb+Pb collision data collected in 2010 by the ATLAS experiment at the LHC, corresponding to an integrated lumonisity of 7 μ b¯¹. The vm -vn correlations are measured in midrapidity as a function of centrality, and, for events within the same centrality interval, as a function of event ellipticity or triangularity defined in a forward rapidity region. For events within the same centrality interval, v₃ is found to be anticorrelated with v₂ and this anticorrelation is consistent with similar anticorrelations between the corresponding eccentricities ϵ₂ and ϵ₃. On the other hand, it is observed that v₄ increases strongly with v₂, and v₅ increases strongly with both v₂ and v₃. The trend and strength of the v m -v n correlations for n =4 and 5 are found to disagree with ϵm -ϵn correlations predicted by initial-geometry models. Instead, these correlations are found to be consistent with the combined effects of a linear contribution to vn and a nonlinear term that is a function of v 2 2 or of v₂v₃, as predicted by hydrodynamic models. A simple two-component fit is used to separate these two contributions. The extracted linear and nonlinear contributions to v₄ and v₅ are found to be consistent with previously measured event-plane correlations.

An analysis is presented of events containing jets including at least one b -tagged jet, sizeable missing transverse momentum, and at least two leptons including a pair of the same electric charge, with the scalar sum of the jet and lepton transverse momenta being large. A data sample with an integrated luminosity of 20.3 fb⁻¹ of pp collisions at √s=8 TeV recorded by the ATLAS detector at the Large Hadron Collider is used. Standard Model processes rarely produce these final states, but there are several models of physics beyond the Standard Model that predict an enhanced rate of production of such events; the ones considered here are production of vector-like quarks, enhanced four-top-quark production, pair production of chiral b ′ -quarks, and production of two positively charged top quarks. Eleven signal regions are defined; subsets of these regions are combined when searching for each class of models. In the three signal regions primarily sensitive to positively charged top quark pair production, the data yield is consistent with the background expectation. There are more data events than expected from background in the set of eight signal regions defined for searching for vector-like quarks and chiral b ′ -quarks, but the significance of the discrepancy is less than two standard deviations. The discrepancy reaches 2.5 standard deviations in the set of five signal regions defined for searching for four-top-quark production. The results are used to set 95% CL limits on various models.