Measurements of Higgs boson production and couplings in the four-lepton channel in pp collisions at center-of-mass energies of 7 and 8 TeV with the ATLAS detector

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 fb−1 and 20.3 fb−1 at center-of-mass energies of 7 TeV 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 at 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 tt¯ or bb¯ 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.

Measurements of the nuclear modification factor for jets in Pb+Pb collisions at sNN =2.76TeV with the ATLAS detector

Measurements of inclusive jet production are performed in pp and Pb+Pb collisions at sNN−−−√=2.76 TeV with the ATLAS detector at the LHC, corresponding to integrated luminosities of 4.0 pb−1 and 0.14 nb−1 , respectively. The jets are identified with the anti-kt algorithm with R=0.4, and the spectra are measured over the kinematic range of jet transverse momentum 32collisions compared to pp collisions. The RAA shows a slight increase with pT and no significant variation with rapidity.

Observation of top-quark pair production in association with a photon and measurement of the t t ¯ γ production cross section in pp collisions at s =7 TeV using the ATLAS detector

A search is performed for top-quark pairs (tt¯) produced together with a photon (γ) with transverse momentum >20 GeV using a sample of tt¯ candidate events in final states with jets, missing transverse momentum, and one isolated electron or muon. The dataset used corresponds to an integrated luminosity of 4.59 fb−1 of proton--proton collisions at a center-of-mass energy of 7 TeV recorded by the ATLAS detector at the CERN Large Hadron Collider. In total 140 and 222 tt¯γ candidate events are observed in the electron and muon channels, to be compared to the expectation of 79±26 and 120±39 non-tt¯γ background events respectively. The production of tt¯γ events is observed with a significance of 5.3 standard deviations away from the null hypothesis. The tt¯γ production cross section times the branching ratio (BR) of the single-lepton decay channel is measured in a fiducial kinematic region within the ATLAS acceptance. The measured value is σfidtt¯γ=63±8(stat.)+17−13(syst.)±1(lumi.) fb per lepton flavor, in good agreement with the leading-order theoretical calculation normalized to the next-to-leading-order theoretical prediction of 48±10 fb.

Search for a CP-odd Higgs boson decaying to Zh in pp collisions at s=8TeV with the ATLAS detector

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 ττ or bb pairs with the Z boson decaying to electron or muon pairs are considered, as well as h→bbh→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 View the MathML sourceσ(gg→A)×BR(A→Zh)×BR(h→ff¯) are 0.098–0.013 pb for f=τf=τ and 0.57–0.014 pb for f=bf=b in a range of mA=220–1000 GeVmA=220–1000 GeV. The results are combined and interpreted in the context of two-Higgs-doublet models.

Search for direct pair production of a chargino and a neutralino decaying to the 125 GeV Higgs boson in root s=8 TeV pp collisions with the ATLAS detector

A search is presented for the direct pair production of a chargino and a neutralino pp→χ~±1χ~02, where the chargino decays to the lightest neutralino and the W boson, χ~±1→χ~01(W±→ℓ±ν), while the neutralino decays to the lightest neutralino and the 125 GeV Higgs boson, χ~02→χ~01(h→bb/γγ/ℓ±νqq). The final states considered for the search have large missing transverse momentum, an isolated electron or muon, and one of the following: either two jets identified as originating from bottom quarks, or two photons, or a second electron or muon with the same electric charge. The analysis is based on 20.3 fb−1 of s√=8 TeV proton-proton collision data delivered by the Large Hadron Collider and recorded with the ATLAS detector. Observations are consistent with the Standard Model expectations, and limits are set in the context of a simplified supersymmetric model.

Search for high-mass diphoton resonances in pp collisions at s =8TeV with the ATLAS detector

This article describes a search for high-mass resonances decaying to a pair of photons using a sample of 20.3  fb−1 of pp collisions at s√=8  TeV recorded with the ATLAS detector at the Large Hadron Collider. The data are found to be in agreement with the Standard Model prediction, and limits are reported in the framework of the Randall-Sundrum model. This theory leads to the prediction of graviton states, the lightest of which could be observed at the Large Hadron Collider. A lower limit of 2.66 (1.41) TeV at 95% confidence level is set on the mass of the lightest graviton for couplings of k/M¯¯¯¯Pl=0.1 (0.01).

Search for massive supersymmetric particles decaying to many jets using the ATLAS detector in pp collisions at s√=8TeV

Results of a search for decays of massive particles to fully hadronic final states are presented. This search uses 20.3 fb−1 of data collected by the ATLAS detector in s√=8TeV proton--proton collisions at the LHC. Signatures based on high jet multiplicities without requirements on the missing transverse momentum are used to search for R-parity-violating supersymmetric gluino pair production with subsequent decays to quarks. The analysis is performed using a requirement on the number of jets, in combination with separate requirements on the number of b-tagged jets, as well as a topological observable formed from the scalar sum of the mass values of large-radius jets in the event. Results are interpreted in the context of all possible branching ratios of direct gluino decays to various quark flavors. No significant deviation is observed from the expected Standard Model backgrounds estimated using jet-counting as well as data-driven templates of the total-jet-mass spectra. Gluino pair decays to ten or more quarks via intermediate neutralinos are excluded for a gluino with mass mg~<1TeV for a neutralino mass mχ~01=500GeV. Direct gluino decays to six quarks are excluded for mg~<917GeV for light-flavor final states, and results for various flavor hypotheses are presented.

Search for new phenomena in events with a photon and missing transverse momentum in pp collisions at s√=8 TeV with the ATLAS detector

Results of a search for new phenomena in events with an energetic photon and large missing transverse momentum with the ATLAS experiment at the LHC are reported. Data were collected in proton--proton collisions at a center-of-mass energy of 8 TeV and correspond to an integrated luminosity of 20.3 fb−1. The observed data are well described by the expected Standard Model backgrounds. The expected (observed) upper limit on the fiducial cross section for the production of such events is 6.1 (5.3) fb at 95% confidence level. Exclusion limits are presented on models of new phenomena with large extra spatial dimensions, supersymmetric quarks, and direct pair production of dark-matter candidates.

Search for Scalar-Charm Pair Production in pp Collisions at s√=8 TeV with the ATLAS Detector

The results of a dedicated search for pair production of scalar partners of charm quarks are reported. The search is based on an integrated luminosity of 20.3fb−1 of pp collisions at s√=8 TeV recorded with the ATLAS detector at the LHC. The search is performed using events with large missing transverse momentum and at least two jets, where the two leading jets are each tagged as originating from c-quarks. Events containing isolated electrons or muons are vetoed. In an R-parity-conserving minimal supersymmetric scenario in which a single scalar-charm state is kinematically accessible, and where it decays exclusively into a charm quark and a neutralino, 95% confidence-level upper limits are obtained in the scalar-charm—neutralino mass plane such that, for neutralino masses below 200 GeV, scalar-charm masses up to 490 GeV are excluded.

Search for s-channel single top-quark production in proton-proton collisions at s√=8 TeV with the ATLAS detector

This Letter presents a search at the LHC for s-channel single top-quark production in proton-proton collisions at a centre-of-mass energy of 8 TeV. The analyzed data set was recorded by the ATLAS detector and corresponds to an integrated luminosity of 20.3 fb−1. Selected events contain one charged lepton, large missing transverse momentum and exactly two b-tagged jets. A multivariate event classifier based on boosted decision trees is developed to discriminate s-channel single top-quark events from the main background contributions. The signal extraction is based on a binned maximum-likelihood fit of the output classifier distribution. The analysis leads to an upper limit on the s-channel single top-quark production cross-section of 14.6 pb at the 95% confidence level. The fit gives a cross-section of σs=5.0±4.3 pb, consistent with the Standard Model expectation.

Search for W'→tb- in the lepton plus jets final state in proton-proton collisions at a centre-of-mass energy of s=8 TeV with the ATLAS detector

A search for new charged massive gauge bosons, called W′, is performed with the ATLAS detector at the LHC, in proton--proton collisions at a centre-of-mass energy of s√ = 8 TeV, using a dataset corresponding to an integrated luminosity of 20.3 fb−1. This analysis searches for W′ bosons in the W′→tb¯ decay channel in final states with electrons or muons, using a multivariate method based on boosted decision trees. The search covers masses between 0.5 and 3.0 TeV, for right-handed or left-handed W′ bosons. No significant deviation from the Standard Model expectation is observed and limits are set on the W′→tb¯ cross-section times branching ratio and on the W′-boson effective couplings as a function of the W′-boson mass using the CLs procedure. For a left-handed (right-handed) W′ boson, masses below 1.70 (1.92) TeV are excluded at 95% confidence level.

Simultaneous measurements of the tt¯, W+W−, and Z/γ∗→ττ production cross-sections in pp collisions at s√=7 TeV with the ATLAS detector

Simultaneous measurements of the tt¯, W+W−, and Z/γ∗→ττ production cross-sections using an integrated luminosity of 4.6 fb−1 of pp collisions at s√=7 TeV collected by the ATLAS detector at the LHC are presented. Events are selected with two high transverse momentum leptons consisting of an oppositely charged electron and muon pair. The three processes are separated using the distributions of the missing transverse momentum of events with zero and greater than zero jet multiplicities. Measurements of the fiducial cross-section are presented along with results that quantify for the first time the underlying correlations in the predicted and measured cross-sections due to proton parton distribution functions. These results indicate that the correlated NLO predictions for tt¯ and Z/γ∗→ττ significantly underestimate the data, while those at NNLO generally describe the data well. The full cross-sections are measured to be σ(tt¯)=181.2±2.8+9.7−9.5±3.3±3.3 pb, σ(W+W−)=53.3±2.7+7.3−8.0±1.0±0.5 pb, and σ(Z/γ∗→ττ)=1174±24+72−87±21±9 pb, where the cited uncertainties are due to statistics, systematic effects, luminosity and the LHC beam energy measurement, respectively.

Measurement of charged-particle spectra in Pb+Pb collisions at √sNN = 2.76TeV with the ATLAS detector at the LHC

Charged-particle spectra obtained in 0.15nb−1 of Pb+Pb interactions at sNN−−−√=2.76TeV and 4.2pb−1 of pp interactions at s√=2.76TeV with the ATLAS detector at the LHC are presented in a wide transverse momentum (0.5collisions, the spectra are presented as a function of collision centrality, which is determined by the response of the forward calorimeter located on both sides of the interaction point. The nuclear modification factors RAA and RCP are presented in detail as function of centrality, pT and η. They show a distinct pT-dependence with a pronounced minimum at about 7GeV. Above 60GeV, RAA is consistent with a plateau at a centrality-dependent value, within the uncertainties. The value is 0.55±0.01(stat.)±0.04(syst.) in the most central collisions. The RAA distribution is consistent with flat |η| dependence over the whole transverse momentum range in all centrality classes.

Measurement of the forward-backward asymmetry of electron and muon pair-production in pp collisions at s√ = 7 TeV with the ATLAS detector

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.

Search for heavy lepton resonances decaying to a Z boson and a lepton in pp collisions at s√=8 TeV with the ATLAS detector

A search for heavy leptons decaying to a Z boson and an electron or a muon is presented. The search is based on pp collision data taken at s√=8 TeV by the ATLAS experiment at the CERN Large Hadron Collider, corresponding to an integrated luminosity of 20.3 fb−1. Three high-transverse-momentum electrons or muons are selected, with two of them required to be consistent with originating from a Z boson decay. No significant excess above Standard Model background predictions is observed, and 95% confidence level limits on the production cross section of high-mass trilepton resonances are derived. The results are interpreted in the context of vector-like lepton and type-III seesaw models. For the vector-like lepton model, most heavy lepton mass values in the range 114-176 GeV are excluded. For the type-III seesaw model, most mass values in the range 100-468 GeV are excluded.