Search for heavy Majorana neutrinos with the ATLAS detector in pp collisions at s√=8 TeV

A search for heavy Majorana neutrinos in events containing a pair of high-pT leptons of the same charge and high-pT jets is presented. The search uses 20.3fb−1 of pp collision data collected with the ATLAS detector at the CERN Large Hadron Collider with a centre-of-mass energy of s√=8 TeV. The data are found to be consistent with the background-only hypothesis based on the Standard Model expectation. In the context of a Type-I seesaw mechanism, limits are set on the production cross-section times branching ratio for production of heavy Majorana neutrinos in the mass range between 100 and 500 GeV. The limits are subsequently interpreted as limits on the mixing between the heavy Majorana neutrinos and the Standard Model neutrinos. In the context of a left-right symmetric model, limits on the production cross-section times branching ratio are set with respect to the masses of heavy Majorana neutrinos and heavy gauge bosons WR and Z′.

A search for high-mass resonances decaying to τ+τ− in pp collisions at s√=8 TeV with the ATLAS detector

A search for high-mass resonances decaying into τ+τ− final states using proton-proton collisions at s√=8 TeV produced by the Large Hadron Collider is presented. The data were recorded with the ATLAS detector and correspond to an integrated luminosity of 19.5-20.3 fb−1. No statistically significant excess above the Standard Model expectation is observed; 95% credibility upper limits are set on the cross section times branching fraction of Z′ resonances decaying into τ+τ− pairs as a function of the resonance mass. As a result, Z′ bosons of the Sequential Standard Model with masses less than 2.02 TeV are excluded at 95% credibility. The impact of the fermionic couplings on the Z′ acceptance is investigated and limits are also placed on a Z′ model that exhibits enhanced couplings to third-generation fermions.

Search for invisible particles produced in association with single-top-quarks in proton-proton collisions at s√ = 8 TeV with the ATLAS detector

A search for the production of single-top-quarks in association with missing energy is performed in proton--proton collisions at a centre-of-mass energy of s√ = 8 TeV with the ATLAS experiment at the Large Hadron Collider using data collected in 2012, corresponding to an integrated luminosity of 20.3 fb−1. In this search, the W boson from the top quark is required to decay into an electron or a muon and a neutrino. No deviation from the Standard Model prediction is observed, and upper limits are set on the production cross-section for resonant and non-resonant production of an invisible exotic state in association with a right-handed top quark. In the case of resonant production, for a spin-0 resonance with a mass of 500 GeV, an effective coupling strength above 0.15 is excluded at 95% confidence level for the top quark and an invisible spin-1/2 state with mass between 0 GeV and 100 GeV. In the case of non-resonant production, an effective coupling strength above 0.2 is excluded at 95% confidence level for the top quark and an invisible spin-1 state with mass between 0 GeV and 657 GeV.

Search for new phenomena in final states with an energetic jet and large missing transverse momentum in pp collisions at s√=8 TeV with the ATLAS detector

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 s√=8 TeV data collected in 2012 with the ATLAS detector at the LHC. Events are required to have at least one jet with pT>120 GeV and no leptons. Nine signal regions are considered with increasing missing transverse momentum requirements between EmissT>150 GeV and EmissT>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 large extra spatial dimensions, pair production of weakly interacting dark matter candidates, and 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.

Search for pair-produced long-lived neutral particles decaying in the ATLAS hadronic calorimeter in pp collisions at s√ = 8 TeV

The ATLAS detector at the Large Hadron Collider at CERN is used to search for the decay of a scalar boson to a pair of long-lived particles, neutral under the Standard Model gauge group, in 20.3 fb−1 of data collected in proton--proton collisions at s√ = 8 TeV. This search is sensitive to long-lived particles that decay to Standard Model particles producing jets at the outer edge of the ATLAS electromagnetic calorimeter or inside the hadronic calorimeter. No significant excess of events is observed. Limits are reported on the product of the scalar boson production cross section times branching ratio into long-lived neutral particles as a function of the proper lifetime of the particles. Limits are reported for boson masses from 100 GeV to 900 GeV, and a long-lived neutral particle mass from 10 GeV to 150 GeV.

Search for production of WW/WZ resonances decaying to a lepton, neutrino and jets in pp collisions at s√ = 8 TeV with the ATLAS detector

A search is presented for a 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.

Search for resonant diboson production in the ℓℓqq¯ final state in pp collisions at s√=8 TeV with the ATLAS detector

This paper reports on a search for narrow resonances in diboson production in the ℓℓqq¯ final state using pp collision data corresponding to an integrated luminosity of 20fb−1 collected at s√=8 TeV with the ATLAS detector at the Large Hadron Collider. No significant excess of data events over the Standard Model expectation is observed. Upper limits at the 95% confidence level are set on the production cross section times branching ratio for Kaluza--Klein gravitons predicted by the Randall--Sundrum model and for Extended Gauge Model W' bosons. These results lead to the exclusion of mass values below 740 GeV and 1590 GeV for the graviton and W' boson respectively.

Search for squarks and gluinos in events with isolated leptons, jets and missing transverse momentum at s√=8 TeV with the ATLAS detector

The results of a search for supersymmetry in final states containing at least one isolated lepton (electron or muon), jets and large missing transverse momentum with the ATLAS detector at the Large Hadron Collider (LHC) are reported. The search is based on proton-proton collision data at a centre-of-mass energy s√=8 TeV collected in 2012, corresponding to an integrated luminosity of 20 fb−1. No significant excess above the Standard Model expectation is observed. Limits are set on the parameters of a minimal universal extra dimensions model, excluding a compactification radius of 1/Rc=950 GeV for a cut-off scale times radius (ΛRc) of approximately 30, as well as on sparticle masses for various supersymmetric models. Depending on the model, the search excludes gluino masses up to 1.32 TeV and squark masses up to 840 GeV.

Search for the bb¯ decay of the Standard Model Higgs boson in associated (W/Z)H production with the ATLAS detector

A search for the bb¯ decay of the Standard Model Higgs boson is performed with the ATLAS experiment using the full dataset recorded at the LHC in Run 1. The integrated luminosities used from pp collisions at s√=7 and 8 TeV are 4.7 and 20.3 fb−1, respectively. The processes considered are associated (W/Z)H production, where W→eν/μν, Z→ee/μμ and Z→νν. The observed (expected) deviation from the background-only hypothesis corresponds to a significance of 1.4 (2.6) standard deviations and the ratio of the measured signal yield to the Standard Model expectation is found to be μ=0.52±0.32(stat.)±0.24(syst.) for a Higgs boson mass of 125.36 GeV. The analysis procedure is validated by a measurement of the yield of (W/Z)Z production with Z→bb¯ in the same final states as for the Higgs boson search, from which the ratio of the observed signal yield to the Standard Model expectation is found to be 0.74±0.09(stat.)±0.14(syst.).

Search for W′→tb→qqbb Decays in pp Collisions at s√ = 8 TeV with the ATLAS Detector

A search for a massive W′ gauge boson is performed with the ATLAS detector at the LHC in pp collisions at a centre-of-mass energy of s√ = 8 TeV, corresponding to 20.3 fb−1 of integrated luminosity. This analysis is done in the W′→tb→qqbb mode for W′ masses above 1.5 TeV, where the W′ decay products are highly boosted. Novel jet substructure techniques are used to identify jets from high-momentum top quarks to ensure high sensitivity, independent of W′ mass, up to 3 TeV; b-tagging is also used to identify jets originating from b-quarks. The data are consistent with Standard Model background-only expectations, and upper limits at 95% confidence level are set on the W′→tb cross section times branching ratio ranging from 0.16 pb to 0.33 pb for left-handed W′ bosons, and ranging from 0.10 pb to 0.21 pb for W′ bosons with purely right-handed couplings. Upper limits at 95% confidence level are set on the W′-boson coupling to tb as a function of the W′ mass using an effective field theory approach, which is independent of details of particular models predicting a W′ boson.

Searches for heavy long-lived charged particles with the ATLAS detector in proton--proton collisions at s√ = 8 TeV

Searches for heavy long-lived charged particles are performed using a data sample of 19.8 fb−1 from proton--proton collisions at a centre-of-mass energy of s√ = 8 TeV collected by the ATLAS detector at the Large Hadron Collider. No excess is observed above the estimated background and limits are placed on the mass of long-lived particles in various supersymmetric models. Long-lived tau sleptons in models with gauge-mediated symmetry breaking are excluded up to masses between 440 and 385 GeV for tan(beta) between 10 and 50, with a 290 GeV limit in the case where only direct tau slepton production is considered. In the context of simplified LeptoSUSY models, where sleptons are stable and have a mass of 300 GeV, squark and gluino masses are excluded up to a mass of 1500 and 1360 GeV, respectively. Directly produced charginos, in simplified models where they are nearly degenerate to the lightest neutralino, are excluded up to a mass of 620 GeV. R-hadrons, composites containing a gluino, bottom squark or top squark, are excluded up to a mass of 1270, 845 and 900 GeV, respectively, using the full detector; and up to a mass of 1260, 835 and 870 GeV using an approach disregarding information from the muon spectrometer.

Measurement of the top-quark mass in the fully hadronic decay channel from ATLAS data at s√=7 TeV

The mass of the top quark is measured in a data set corresponding to 4.6 fb−1 of proton--proton collisions with centre-of-mass energy s√=7 TeV collected by the ATLAS detector at the LHC. Events consistent with hadronic decays of top--antitop quark pairs with at least six jets in the final state are selected. The substantial background from multijet production is modelled with data-driven methods that utilise the number of identified b-quark jets and the transverse momentum of the sixth leading jet, which have minimal correlation. The top-quark mass is obtained from template fits to the ratio of three-jet to dijet mass. The three-jet mass is calculated from the three jets of a top-quark decay. Using these three jets the dijet mass is obtained from the two jets of the W boson decay. The top-quark mass obtained from this fit is thus less sensitive to the uncertainty in the energy measurement of the jets. A binned likelihood fit yields a top-quark mass of mt = 175.1 ± 1.4 (stat.) ± 1.2 (syst.) GeV.

Identification and energy calibration of hadronically decaying tau leptons with the ATLAS experiment in pp collisions at s√=8 TeV

This paper describes the trigger and offline reconstruction, identification and energy calibration algorithms for hadronic decays of tau leptons employed for the data collected from pp collisions in 2012 with the ATLAS detector at the LHC center-of-mass energy s√ = 8 TeV. The performance of these algorithms is measured in most cases with Z decays to tau leptons using the full 2012 dataset, corresponding to an integrated luminosity of 20.3 fb−1. An uncertainty on the offline reconstructed tau energy scale of 2% to 4%, depending on transverse energy and pseudorapidity, is achieved using two independent methods. The offline tau identification efficiency is measured with a precision of 2.5% for hadronically decaying tau leptons with one associated track, and of 4% for the case of three associated tracks, inclusive in pseudorapidity and for a visible transverse energy greater than 20 GeV. For hadronic tau lepton decays selected by offline algorithms, the tau trigger identification efficiency is measured with a precision of 2% to 8%, depending on the transverse energy. The performance of the tau algorithms, both offline and at the trigger level, is found to be stable with respect to the number of concurrent proton--proton interactions and has supported a variety of physics results using hadronically decaying tau leptons at ATLAS.

Determination of spin and parity of the Higgs boson in the WW∗→eνμν decay channel with the ATLAS detector

Studies of the spin and parity quantum numbers of the Higgs boson in the WW∗→eνμν final state are presented, based on proton--proton collision data collected by the ATLAS detector at the Large Hadron Collider, corresponding to an integrated luminosity of 20.3 fb−1 at a centre-of-mass energy of s√=8 TeV. The Standard Model spin-parity JCP=0++ hypothesis is compared with alternative hypotheses for both spin and CP. The case where the observed resonance is a mixture of the Standard-Model-like Higgs boson and CP-even (JCP=0++) or CP-odd (JCP=0+−) Higgs boson in scenarios beyond the Standard Model is also studied. The data are found to be consistent with the Standard Model prediction and limits are placed on alternative spin and CP hypotheses, including CP mixing in different scenarios.

Measurement of the production of neighbouring jets in lead-lead collisions at √sNN=2.76 TeV with the ATLAS detector

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, RΔR, 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-kt algorithm with radius parameters d=0.2, 0.3, and 0.4. RΔR was measured in different Pb+Pb collision centrality bins, characterized by the total transverse energy measured in the forward calorimeters. A centrality dependence of RΔR is observed for all three jet radii with RΔR found to be lower in central collisions than in peripheral collisions. The ratios formed by the RΔR values in different centrality bins and the values in the 40--80 % centrality bin are presented.