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 ¼ 125.09 AE 0.21 ðstatÞ AE 0.11 ðsystÞ GeV. DOI: 10.1103/PhysRevLett.114.191803 PACS numbers: 14.80.Bn, 13.85.Qk The study of the mechanism of electroweak symmetry breaking is one of the principal goals of the CERN LHC program. In the standard model (SM), this symmetry breaking is achieved through the introduction of a complex doublet scalar field, leading to the prediction of the Higgs boson H [1-6], whose mass m H is, however, not predicted by the theory. In 2012, the ATLAS and CMS Collaborations at the LHC announced the discovery of a particle with Higgs-boson-like properties and a mass of about 125 GeV [7][8][9]. The discovery was based primarily on mass peaks observed in the γγ and ZZ → l þ l − l 0þ l 0−(denoted H → ZZ → 4l for simplicity) decay channels, where one or both of the Z bosons can be off shell and where l and l 0 denote an electron or muon. With m H known, all properties of the SM Higgs boson, such as its production cross section and partial decay widths, can be predicted. Increasingly precise measurements [10][11][12][13] have established that all observed properties of the new particle, including its spin, parity, and coupling strengths to SM particles are consistent within the uncertainties with those expected for the SM Higgs boson.The ATLAS and CMS Collaborations have independently measured m H using the samples of proton-proton collision data collected in 2011 and 2012, commonly referred to as LHC Run 1. The analyzed samples correspond to approximately 5 fb −1 of integrated luminosity at ffiffi ffi s p ¼ 7 TeV, and 20 fb −1 at ffiffi ffi s p ¼ 8 TeV, for each experiment. Combined results in the context of the separate experiments, as well as those in the individual channels, are presented in Refs. [12,[14][15][16].This Letter describes a combination of the Run 1 data from the two experiments, leading to improved precision for m H . Besides its intrinsic importance as a fundamental parameter, improved knowledge of m H yields more precise predictions for the other Higgs boson properties. Furthermore, the combined mass measurement provides a first step towards combinations of other quantities, such as the couplings. In the SM, m H is related to the values of the masses of the W boson and top quark through loopinduced effects. Taking into account other measured SM quantities, the comparison of the measurements of the Higgs boson, W boson, and top quark masses can be used to directly test the consistency of the SM [17] and thus to search for evidence of physics beyond the SM.The combination is performed usin...

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+JP=0+ hypothesis is compared with alternative hypotheses using the Higgs boson decays H→γγH→γγ, H→ZZ⁎→4ℓH→ZZ⁎→4ℓ and H→WW⁎→ℓνℓνH→WW⁎→ℓνℓν, 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 √s=8TeV. For the H→ZZ⁎→4ℓH→ZZ⁎→4ℓ decay mode the dataset corresponding to an integrated luminosity of 4.6 fb−1 collected at √s=7TeV is included. The data are compatible with the Standard Model JP=0+JP=0+ quantum numbers for the Higgs boson, whereas all alternative hypotheses studied in this Letter, namely some specific JP=0−,1+,1−,2+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+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 preferre

This paper presents the electron and photon energy calibration achieved with the ATLAS detector using about 25 fb −1 of LHC proton-proton collision data taken at centre-of-mass energies of √ s = 7 and 8 TeV. The reconstruction of electron and photon energies is optimised using multivariate algorithms. The response of the calorimeter layers is equalised in data and simulation, and the longitudinal profile of the electromagnetic showers is exploited to estimate the passive material in front of the calorimeter and reoptimise the detector simulation. After all corrections, the Z resonance is used to set the absolute energy scale. For electrons from Z decays, the achieved calibration is typically accurate to 0.05 % in most of the detector acceptance, rising to 0.2 % in regions with large amounts of passive material. The remaining inaccuracy is less than 0.2-1 % for electrons with a transverse energy of 10 GeV, and is on average 0.3 % for photons. The detector resolution is determined with a relative inaccuracy of less than 10 % for electrons and photons up to 60 GeV transverse energy, rising to 40 % for transverse energies above 500 GeV.

Measurement of the Z/γ * boson transverse momentum distribution in pp collisions at √ s = 7 TeV with the ATLAS detectorThe ATLAS collaboration E-mail: atlas.publications@cern.chAbstract: This paper describes a measurement of the Z/γ * boson transverse momentum spectrum using ATLAS proton-proton collision data at a centre-of-mass energy of √ s = 7 TeV at the LHC. The measurement is performed in the Z/γ * → e + e − and Z/γ * → µ + µ − channels, using data corresponding to an integrated luminosity of 4.7 fb −1 . Normalized differential cross sections as a function of the Z/γ * boson transverse momentum are measured for transverse momenta up to 800 GeV. The measurement is performed inclusively for Z/γ * 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 ATLAS collaboration 31 Keywords: Hadron-Hadron Scattering IntroductionThe transverse momentum distribution of W and Z bosons produced in hadronic collisions is a traditional probe of strong interaction dynamics. The low transverse momentum (p T ) range is governed by initial-state parton radiation (ISR) and the intrinsic transverse momentum of the initial-state partons inside the proton, and modeled using soft-gluon resummation [1] or parton shower models [2,3]. Quark-gluon scattering dominates at high p T and is described by perturbative QCD [4][5][6]. The correct modelling of the vector boson p T distribution is important in many physics analyses at the LHC for which the production of W or Z bosons constitutes a significant background. Moreover, it is crucial for a precise measurement of the W boson mass. The transverse momentum distribution also probes the gluon density of the proton [7]. Vector boson p T distribution measurements were published by ATLAS [8, 9] and CMS [10] based on 35-40 pb −1 of proton-proton collisions at a centre-of-mass energy of √ s = 7 TeV. The typical precision of these measurements is 4% to 10%.-1 - JHEP09(2014)145This paper presents a measurement of the normalized Z boson transverse momentum distribution (p Z T ) with the ATLAS detector, in the Z/γ * → e + e − and Z/γ * → µ + µ − channels, using LHC proton-proton collision data taken in 2011 at a centre-of-mass energy of √ s = 7 TeV and corresponding to an integrated luminosity of 4.7 fb −1 [11]. The large integrated luminosity allows the measurement to be performed in three different Z boson rapidity (y Z ) bins, probing the transverse momentum dynamics over a wide range of the initial-state parton momentum fraction. With respect to previous results, the present analysis aims at reduced uncertainties, finer binning and extended measurement range.Reconstructed from the final-state lepton kinematics, p Z T is affected by lepton energy and momentum measurement uncertainties. To minimize the impact of these uncertainties, the φ η observable 1 was introduced as an alternative probe of p Z T [12], pioneered at the Tev...

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 ffiffi ffi s p ¼ 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.