Evidence for single top-quark production in the s-channel in proton–proton collisions at √s = 8 TeV with the ATLAS detector using the Matrix Element Method

Wed 23 Aug 2017 09:40:04 KRAT

This Letter presents evidence for single top-quark production in the s -channel using proton–proton collisions at a centre-of-mass energy of 8 TeV8 TeV with the ATLAS detector at the CERN Large Hadron Collider. The analysis is performed on events containing one isolated electron or muon, large missing transverse momentum and exactly two b -tagged jets in the final state. The analysed data set corresponds to an integrated luminosity of 20.3 fb−120.3 fb−1. The signal is extracted using a maximum-likelihood fit of a discriminant which is based on the matrix element method and optimized in order to separate single-top-quark s-channel events from the main background contributions, which are top-quark pair production and W boson production in association with heavy-flavour jets. The measurement leads to an observed signal significance of 3.2 standard deviations and a measured cross-section of View the MathML sourceσs=4.8±0.8(stat.)−1.3+1.6(syst.) pb, which is consistent with the Standard Model expectation. The expected significance for the analysis is 3.9 standard deviations.

Muon reconstruction performance of the ATLAS detector in proton-proton collision data at √s = 13 TeV

Wed 23 Aug 2017 09:40:03 KRAT

This article documents the performance of the ATLAS muon identification and reconstruction using the LHC dataset recorded at √s=13 TeV in 2015. Using a large sample of J/ψ→μμ and Z→μμ decays from 3.2 fb−1 of pp collision data, measurements of the reconstruction efficiency, as well as of the momentum scale and resolution, are presented and compared to Monte Carlo simulations. The reconstruction efficiency is measured to be close to 99 % over most of the covered phase space (|η|<2.5 and 52.2, the pT resolution for muons from Z→μμ decays is 2.9 % while the precision of the momentum scale for low-pT muons from J/ψ→μμ decays is about 0.2 %.

Search for new phenomena in dijet mass and angular distributions from pp collisions at √s = 13 TeV with the ATLAS detector

Wed 23 Aug 2017 09:40:03 KRAT

This Letter describes a model-agnostic search for pairs of jets (dijets) produced by resonant and non-resonant phenomena beyond the Standard Model in 3.6 fb−13.6 fb−1 of proton–proton collisions with a centre-of-mass energy of View the MathML sources=13 TeV recorded by the ATLAS detector at the Large Hadron Collider. The distribution of the invariant mass of the two leading jets is examined for local excesses above a data-derived estimate of the smoothly falling prediction of the Standard Model. The data are also compared to a Monte Carlo simulation of Standard Model angular distributions derived from the rapidity of the two jets. No evidence of anomalous phenomena is observed in the data, which are used to exclude, at 95% CL, quantum black holes with threshold masses below 8.3 TeV, 8.1 TeV, or 5.1 TeV5.1 TeV in three different benchmark scenarios; resonance masses below 5.2 TeV for excited quarks, 2.6 TeV in a W′W′ model, a range of masses starting from mZ′=1.5 TeVmZ′=1.5 TeV and couplings from gq=0.2gq=0.2 in a Z′Z′ model; and contact interactions with a compositeness scale below 12.0 TeV and 17.5 TeV respectively for destructive and constructive interference between the new interaction and QCD processes. These results significantly extend the ATLAS limits obtained from 8 TeV data. Gaussian-shaped contributions to the mass distribution are also excluded if the effective cross-section exceeds values ranging from approximately 50–300 fb for masses below 2 TeV to 2–20 fb for masses above 4 TeV.

Measurement of the transverse momentum and φη* distributions of Drell-Yan lepton pairs in proton-proton collisions at √s = 8 TeV with the ATLAS detector

Wed 23 Aug 2017 09:40:01 KRAT

Distributions of transverse momentum pℓℓT and the related angular variable ϕ∗η of DrellΓÇôYan lepton pairs are measured in 20.3┬áfb−1 of protonΓÇôproton collisions at √s=8┬áTeV 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 protonΓÇôproton collisions at √s=7┬áTeV, 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 ϕ∗η<1 the predictions from the Monte Carlo generator ResBos are generally consistent with the data within the theoretical uncertainties. However, at larger values of ϕ∗η 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 pℓℓT while the fixed-order prediction of Dynnlo falls below the data at high values of pℓℓT. ResBos and the parton-shower Monte Carlo generators provide a much better description of the evolution of the ϕ∗η and pℓℓT distributions as a function of lepton-pair mass and rapidity than the basic shape of the data.

Measurement of the dependence of transverse energy production at large pseudorapidity on the hard-scattering kinematics of proton–proton collisions at √s = 2.76 TeV with ATLAS

Wed 23 Aug 2017 09:39:57 KRAT

The relationship between jet production in the central region and the underlying-event activity in a pseudorapidity-separated region is studied in 4.0 pb−1 of View the MathML sources=2.76 TeVpp collision data recorded with the ATLAS detector at the LHC. The underlying event is characterised through measurements of the average value of the sum of the transverse energy at large pseudorapidity downstream of one of the protons, which are reported here as a function of hard-scattering kinematic variables. The hard scattering is characterised by the average transverse momentum and pseudorapidity of the two highest transverse momentum jets in the event. The dijet kinematics are used to estimate, on an event-by-event basis, the scaled longitudinal momenta of the hard-scattered partons in the target and projectile beam-protons moving toward and away from the region measuring transverse energy, respectively. Transverse energy production at large pseudorapidity is observed to decrease with a linear dependence on the longitudinal momentum fraction in the target proton and to depend only weakly on that in the projectile proton. The results are compared to the predictions of various Monte Carlo event generators, which qualitatively reproduce the trends observed in data but generally underpredict the overall level of transverse energy at forward pseudorapidity.

Combination of searches for WW, WZ, and ZZ resonances in pp collisions at √s = 8 TeV with the ATLAS detector

Wed 23 Aug 2017 09:39:53 KRAT

The ATLAS experiment at the CERN Large Hadron Collider has performed searches for new, heavy bosons decaying to WW, WZ and ZZ final states in multiple decay channels using 20.3 fb−120.3 fb−1 of pp collision data at View the MathML sources=8 TeV. In the current study, the results of these searches are combined to provide a more stringent test of models predicting heavy resonances with couplings to vector bosons. Direct searches for a charged diboson resonance decaying to WZ in the ℓνℓ′ℓ′ℓνℓ′ℓ′ (ℓ=μ,eℓ=μ,e), View the MathML sourceℓℓqq¯, View the MathML sourceℓνqq¯ and fully hadronic final states are combined and upper limits on the rate of production times branching ratio to the WZ bosons are compared with predictions of an extended gauge model with a heavy W′W′ boson. In addition, direct searches for a neutral diboson resonance decaying to WW and ZZ in the View the MathML sourceℓℓqq¯, View the MathML sourceℓνqq¯, and fully hadronic final states are combined and upper limits on the rate of production times branching ratio to the WW and ZZ bosons are compared with predictions for a heavy, spin-2 graviton in an extended Randall–Sundrum model where the Standard Model fields are allowed to propagate in the bulk of the extra dimension.

Measurements of four-lepton production in pp collisions at √s = 8 TeV with the ATLAS detector

Wed 23 Aug 2017 09:39:51 KRAT

The four-lepton (4ℓ , ℓ=e,μℓ=e,μ) production cross section is measured in the mass range from 80 to 1000 GeV using 20.3 fb−1 of data in pp collisions at View the MathML sources=8 TeV collected with the ATLAS detector at the LHC. The 4ℓ events are produced in the decays of resonant Z and Higgs bosons and the non-resonant ZZ continuum originating from View the MathML sourceqq¯, gg, and qg initial states. A total of 476 signal candidate events are observed with a background expectation of 26.2±3.626.2±3.6 events, enabling the measurement of the integrated cross section and the differential cross section as a function of the invariant mass and transverse momentum of the four-lepton system.

Search for new phenomena in final states with large jet multiplicities and missing transverse momentum with ATLAS using √s = 13 TeV proton-proton collisions

Wed 23 Aug 2017 09:33:01 KRAT

Results are reported of a search for new phenomena, such as supersymmetric particle production, that could be observed in high-energy proton–proton collisions. Events with large numbers of jets, together with missing transverse momentum from unobserved particles, are selected. The data analysed were recorded by the ATLAS experiment during 2015 using the 13 TeV centre-of-mass proton–proton collisions at the Large Hadron Collider, and correspond to an integrated luminosity of 3.2 fb−1. The search selected events with various jet multiplicities from ≥7 to ≥10 jets, and with various b-jet multiplicity requirements to enhance sensitivity. No excess above Standard Model expectations is observed. The results are interpreted within two supersymmetry models, where gluino masses up to 1400 GeV are excluded at 95% confidence level, significantly extending previous limits.

Measurement of the charged-particle multiplicity inside jets from √s = 8 TeV pp collisions with the ATLAS detector

Tue 13 Mar 2018 11:23:55 KRAT

The number of charged particles inside jets is a widely used discriminant for identifying the quark or gluon nature of the initiating parton and is sensitive to both the perturbative and non-perturbative components of fragmentation. This paper presents a measurement of the average number of charged particles with pT>500 MeV inside high-momentum jets in dijet events using 20.3 fb−1 of data recorded with the ATLAS detector in pp collisions at s√=8 TeV collisions at the LHC. The jets considered have transverse momenta from 50 GeV up to and beyond 1.5 TeV. The reconstructed charged-particle track multiplicity distribution is unfolded to remove distortions from detector effects and the resulting charged-particle multiplicity is compared to several models. Furthermore, quark and gluon jet fractions are used to extract the average charged-particle multiplicity for quark and gluon jets separately.

Search for the standard model Higgs boson produced in association with a vector boson and decaying into a tau pair in pp collisions at √s=8 TeV with the ATLAS detector

Tue 04 Dec 2018 12:47:18 KRAT

A search for the standard model Higgs boson produced in association with a vector boson with the decay H→ττ is presented. The data correspond to 20.3 fb−1 of integrated luminosity from proton-proton collisions at √s=8 TeV recorded by the ATLAS experiment at the LHC during 2012. The data agree with the background expectation, and 95% confidence-level upper limits are placed on the cross section of this process. The observed (expected) limit, expressed in terms of the signal strength μ

Measurement of the correlation between the polar angles of leptons from top quark decays in the helicity basis at √s = 7 TeV using the ATLAS detector

Thu 22 Mar 2018 09:47:57 KRAT

A measurement of the correlations between the polar angles of leptons from the decay of pair-produced t and ¯t quarks in the helicity basis is reported, using proton-proton collision data collected by the ATLAS detector at the LHC. The dataset corresponds to an integrated luminosity of 4.6 fb−1 at a center-of-mass energy of √s=7 TeV collected during 2011. Candidate events are selected in the dilepton topology with large missing transverse momentum and at least two jets. The angles θ1 and θ2 between the charged leptons and the direction of motion of the parent quarks in the t¯t rest frame are sensitive to the spin information, and the distribution of cosθ1⋅cosθ2 is sensitive to the spin correlation between the t and ¯t quarks. The distribution is unfolded to parton level and compared to the next-to-leading order prediction. A good agreement is observed.

Beam-induced and cosmic-ray backgrounds observed in the ATLAS detector during the LHC 2012 proton-proton running period

Thu 15 Mar 2018 09:41:01 KRAT

This paper discusses various observations on beam-induced and cosmic-ray backgrounds in the ATLAS detector during the LHC 2012 proton-proton run. Building on published results based on 2011 data, the correlations between background and residual pressure of the beam vacuum are revisited. Ghost charge evolution over 2012 and its role for backgrounds are evaluated. New methods to monitor ghost charge with beam-gas rates are presented and observations of LHC abort gap population by ghost charge are discussed in detail. Fake jets from colliding bunches and from ghost charge are analysed with improved methods, showing that ghost charge in individual radio-frequency buckets of the LHC can be resolved. Some results of two short periods of dedicated cosmic-ray background data-taking are shown; in particular cosmic-ray muon induced fake jet rates are compared to Monte Carlo simulations and to the fake jet rates from beam background. A thorough analysis of a particular LHC fill, where abnormally high background was observed, is presented. Correlations between backgrounds and beam intensity losses in special fills with very high β* are studied.

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

Thu 15 Mar 2018 09:40:58 KRAT

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 (W'

Search for dark matter produced in association with a Higgs boson decaying to two bottom quarks in pp collisions at √s = 8 TeV with the ATLAS detector

Thu 15 Mar 2018 09:40:54 KRAT

This article reports on a search for dark matter pair production in association with a Higgs boson decaying to a pair of bottom quarks, using data from 20.3 fb −1 of p p collisions at a center-of-mass energy of 8 TeV collected by the ATLAS detector at the LHC. The decay of the Higgs boson is reconstructed as a high-momentum b¯b system with either a pair of small-radius jets, or a single large-radius jet with substructure. The observed data are found to be consistent with the expected Standard Model backgrounds. Model-independent upper limits are placed on the visible cross sections for events with a Higgs boson decaying into b¯b and large missing transverse momentum with thresholds ranging from 150 to 400 GeV. Results are interpreted using a simplified model with a Z′ gauge boson decaying into different Higgs bosons predicted in a two-Higgs-doublet model, of which the heavy pseudoscalar Higgs decays into a pair of dark matter particles. Exclusion limits are also presented for the mass scales of various effective field theory operators that describe the interaction between dark matter particles and the Higgs boson.

Search for metastable heavy charged particles with large ionization energy loss in pp collisions at √s = 13 TeV using the ATLAS experiment

Sat 10 Mar 2018 12:34:47 KRAT

This paper presents a search for massive charged long-lived particles produced in pp collisions at s√= 13 TeV at the LHC using the ATLAS experiment. The dataset used corresponds to an integrated luminosity of 3.2 fb−1. Many extensions of the Standard Model predict the existence of massive charged long-lived particles, such as R-hadrons. These massive particles are expected to be produced with a velocity significantly below the speed of light, and therefore to have a specific ionization higher than any Standard Model particle of unit charge at high momenta. The Pixel subsystem of the ATLAS detector is used to measure the ionization energy loss of reconstructed charged particles and to search for such highly ionizing particles. The search presented here has much greater sensitivity than a similar search performed using the ATLAS detector in the s√= 8 TeV dataset, thanks to the increase in expected signal cross-section due to the higher center-of-mass energy of collisions, to an upgraded detector with a new silicon layer close to the interaction point, and to analysis improvements. No significant deviation from Standard Model background expectations is observed, and lifetime-dependent upper limits on R-hadron production cross-sections and masses are set. Gluino R-hadrons with lifetimes above 0.4 ns and decaying to qq¯ plus a 100 GeV neutralino are excluded at the 95% confidence level, with lower mass limit ranging between 740 GeV and 1590 GeV. In the case of stable R-hadrons the lower mass limit at the 95% confidence level is 1570 GeV.

Search for the electroweak production of supersymmetric particles in √s = 8 TeV pp collisions with the ATLAS detector

Mon 02 Apr 2018 10:59:21 KRAT

The ATLAS experiment has performed extensive searches for the electroweak production of charginos, neutralinos, and staus. This article summarizes and extends the search for electroweak supersymmetry with new analyses targeting scenarios not covered by previously published searches. New searches use vector-boson fusion production, initial-state radiation jets, and low-momentum lepton final states, as well as multivariate analysis techniques to improve the sensitivity to scenarios with small mass splittings and low-production cross sections. Results are based on 20 fb−1 of proton-proton collision data at √s=8 TeV recorded with the ATLAS experiment at the Large Hadron Collider. No significant excess beyond Standard Model expectations is observed. The new and existing searches are combined and interpreted in terms of 95% confidence-level exclusion limits in simplified models, where a single production process and decay mode is assumed, as well as within phenomenological supersymmetric models.

Measurement of the differential cross-section of highly boosted top quarks as a function of their transverse momentum in √s = 8 TeV proton-proton collisions using the ATLAS detector

Mon 02 Apr 2018 10:59:08 KRAT

The differential cross-section for pair production of top quarks with high transverse momentum is measured in

Performance of b-jet identification in the ATLAS experiment

Mon 02 Apr 2018 10:24:11 KRAT

The identification of jets containing b hadrons is important for the physics programme of the ATLAS experiment at the Large Hadron Collider. Several algorithms to identify jets containing b hadrons are described, ranging from those based on the reconstruction of an inclusive secondary vertex or the presence of tracks with large impact parameters to combined tagging algorithms making use of multi-variate discriminants. An independent b-tagging algorithm based on the reconstruction of muons inside jets as well as the b-tagging algorithm used in the online trigger are also presented. The b-jet tagging efficiency, the c-jet tagging efficiency and the mistag rate for light flavour jets in data have been measured with a number of complementary methods. The calibration results are presented as scale factors defined as the ratio of the efficiency (or mistag rate) in data to that in simulation. In the case of b jets, where more than one calibration method exists, the results from the various analyses have been combined taking into account the statistical correlation as well as the correlation of the sources of systematic uncertainty.

Search for magnetic monopoles and stable particles with high electric charges in 8 TeV pp collisions with the ATLAS detector

Mon 02 Apr 2018 10:24:06 KRAT

A search for highly ionizing particles produced in proton-proton collisions at 8 TeV center-of-mass energy is performed by the ATLAS Collaboration at the CERN Large Hadron Collider. The data set used corresponds to an integrated luminosity of 7.0 fb−1. A customized trigger significantly increases the sensitivity, permitting a search for such particles with charges and energies beyond what was previously accessible. No events were found in the signal region, leading to production cross section upper limits in the mass range 200–2500 GeV for magnetic monopoles with magnetic charge in the range 0.5gD<|g|<2.0gD, where gD is the Dirac charge, and for stable particles with electric charge in the range 10<|z|<60. Model-dependent limits are presented in given pair-production scenarios, and model-independent limits are presented in fiducial regions of particle energy and pseudorapidity.

Centrality, rapidity, and transverse momentum dependence of isolated prompt photon production in lead-lead collisions at √sNN = 2.76 TeV measured with the ATLAS detector

Mon 02 Apr 2018 10:23:59 KRAT

Prompt photon production in √sNN=2.76-TeV Pb+Pb collisions has been measured by the ATLAS experiment at the Large Hadron Collider using data collected in 2011 with an integrated luminosity of 0.14 nb−1. Inclusive photon yields, scaled by the mean nuclear thickness function, are presented as a function of collision centrality and transverse momentum in two pseudorapidity intervals, |η|<1.37 and1.52≤|η|<2.37. The scaled yields in the two pseudorapidity intervals, as well as the ratios of the forward yields to those at midrapidity, are compared to the expectations from next-to-leading-order perturbative QCD (pQCD) calculations. The measured cross sections agree well with the predictions for proton-proton collisions within statistical and systematic uncertainties. Both the yields and the ratios are also compared to two other pQCD calculations, one which uses the isospin content appropriate to colliding lead nuclei and another which includes nuclear modifications to the nucleon parton distribution functions.

Measurement of the charge asymmetry in top-quark pair production in the lepton-plus-jets final state in pp collision data at √s = 8 TeV with the ATLAS detector

Fri 29 Sep 2017 16:42:41 KRAT

This paper reports inclusive and differential measurements of the tˉt charge asymmetry AC in 20.3 fb−1 of √s=8 TeVpp 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 tˉt system. The tˉt pairs are selected in the single-lepton channels (e or μ) with at least four jets, and a likelihood fit is used to reconstruct the tˉt event kinematics. A Bayesian unfolding procedure is performed to infer the asymmetry at parton level from the observed data distribution. The inclusive tˉt charge asymmetry is measured to be AC=0.009±0.005 (stat. + syst.). The inclusive and differential measurements are compatible with the values predicted by the Standard Model.

Search for heavy long-lived charged R-hadrons with the ATLAS detector in 3.2 fb−1 of proton-proton collision data at √s = 13 TeV

Fri 29 Sep 2017 16:42:23 KRAT

A search for heavy long-lived charged R-hadrons is reported using a data sample corresponding to 3.2 fb−1 of proton–proton collisions at View the MathML sources=13 TeV collected by the ATLAS experiment at the Large Hadron Collider at CERN. The search is based on observables related to large ionisation losses and slow propagation velocities, which are signatures of heavy charged particles travelling significantly slower than the speed of light. No significant deviations from the expected background are observed. Upper limits at 95% confidence level are provided on the production cross section of long-lived R-hadrons in the mass range from 600 GeV to 2000 GeV and gluino, bottom and top squark masses are excluded up to 1580 GeV, 805 GeV and 890 GeV, respectively.

Search for TeV-scale gravity signatures in high-mass final states with leptons and jets with the ATLAS detector at √s=13TeV

Fri 29 Sep 2017 16:42:22 KRAT

A search for physics beyond the Standard Model, in final states with at least one high transverse momentum charged lepton (electron or muon) and two additional high transverse momentum leptons or jets, is performed using 3.2 fb−1 of proton–proton collision data recorded by the ATLAS detector at the Large Hadron Collider in 2015 at View the MathML sources=13 TeV. The upper end of the distribution of the scalar sum of the transverse momenta of leptons and jets is sensitive to the production of high-mass objects. No excess of events beyond Standard Model predictions is observed. Exclusion limits are set for models of microscopic black holes with two to six extra dimensions.

Search for an additional, heavy Higgs boson in the H → ZZ decay channel at √ s = 8 TeV in pp collision data with the ATLAS detector

Fri 29 Sep 2017 16:42:04 KRAT

A search is presented for a high-mass Higgs boson in the H→ZZ→ℓ+ℓ−ℓ+ℓ−, H→ZZ→ℓ+ℓ−νˉν, H→ZZ→ℓ+ℓ−qˉq, and H→ZZ→νˉνqˉq 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−1. 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 1 TeV for all four decay modes and down to as low as 140 GeV, 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 Z boson pairs. 95 % confidence level upper limits range from 0.53 pb at mH=195 GeV to 0.008 pb at mH=950 GeV for the gluon-fusion production mode and from 0.31 pb at mH=195 GeV to 0.009 pb at mH=950 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 qZ in pp collision data collected with the ATLAS detector at √ s = 8 TeV

Fri 29 Sep 2017 16:42:03 KRAT

A search for the flavour-changing neutral-current decay t→qZ 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 s√=8 TeV, corresponding to an integrated luminosity of 20.3 fb−1, are analysed. Top-quark pair-production events with one top quark decaying through the t→qZ (q=u,c) channel and the other through the dominant Standard Model mode t→bW 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 t→qZ branching ratio of 7×10−4 (8×10−4) is set at the 95 % confidence level

Measurements of the Higgs boson production and decay rates and coupling strengths using pp collision data at √s = 7 and 8 TeV in the ATLAS experiment

Fri 29 Sep 2017 14:01:15 KRAT

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 H→γγ,ZZ∗,WW∗,Zγ,bˉb,ττ and μμ 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 fb−1 at √s=7 TeV and 20.3 fb−1 at √s=8 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 1.18+0.15−0.14. 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.

Measurement of the differential cross-sections of prompt and non-prompt production of J/ψ and ψ(2S) in pp collisions at √s = 7 and 8 TeV with the ATLAS detector

Fri 29 Sep 2017 14:01:13 KRAT

The production rates of prompt and non-prompt J/ψ and ψ(2S) mesons in their dimuon decay modes are measured using 2.1 and 11.4 fb−1 of data collected with the ATLAS experiment at the Large Hadron Collider, in proton–proton collisions at √s=7 and 8 respectively. Production cross-sections for prompt as well as non-prompt sources, ratios of ψ(2S) to J/ψ production, and the fractions of non-prompt production for J/ψ and ψ(2S) are measured as a function of meson transverse momentum and rapidity. The measurements are compared to theoretical predictions.

Search for direct top squark pair production in final states with two tau leptons in pp collisions at √ s = 8 TeV with the ATLAS detector

Fri 29 Sep 2017 14:01:13 KRAT

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−1 of proton–proton collision data at √s=8 TeV. 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 95 % confidence level are set as a function of the top squark and scalar tau masses. Depending on the scalar tau mass, ranging from the 87 GeV LEP limit to the top squark mass, lower limits between 490 and 650 GeV are placed on the top squark mass within the model considered.

Measurements of fiducial cross-sections for t t ¯ production with one or two additional b-jets in pp collisions at √s = 8 TeV using the ATLAS detector

Fri 29 Sep 2017 14:01:04 KRAT

Fiducial cross-sections for tˉt production with one or two additional b-jets are reported, using an integrated luminosity of 20.3 fb−1 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 tˉt events with at least one additional b-jet is measured to be 950 ± 70 (stat.) +240−190 (syst.) fb in the lepton-plus-jets channel and 50 ± 10 (stat.) +15−10 (syst.) fb in the eμ channel. The cross-section times branching ratio for events with at least two additional b-jets is measured to be 19.3 ± 3.5 (stat.) ± 5.7 (syst.) fb in the dilepton channel (eμ, μμ, and ee) using a method based on tight selection criteria, and 13.5 ± 3.3 (stat.) ± 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 ± 0.33 (stat.) ± 0.28 (syst.)% for the ratio of tˉt production with two additional b-jets to tˉt production with any two additional jets. All measurements are in good agreement with recent theory predictions.

Charged-particle distributions in pp interactions at √s = 8 TeV measured with the ATLAS detector

Fri 29 Sep 2017 14:01:02 KRAT

This paper presents measurements of distributions of charged particles which are produced in proton–proton collisions at a centre-of-mass energy of √s=8TeV and recorded by the ATLAS detector at the LHC. A special dataset recorded in 2012 with a small number of interactions per beam crossing (below 0.004) and corresponding to an integrated luminosity of 160 μb−1 was used. A minimum-bias trigger was utilised to select a data sample of more than 9 million collision events. The multiplicity, pseudorapidity, and transverse momentum distributions of charged particles are shown in different regions of kinematics and charged-particle multiplicity, including measurements of final states at high multiplicity. The results are corrected for detector effects and are compared to the predictions of various Monte Carlo event generator models which simulate the full hadronic final state.

Search for scalar leptoquarks in pp collisions at √s = 13 TeV with the ATLAS experiment

Fri 07 Dec 2018 11:36:41 KRAT

An inclusive search for a new-physics signature of lepton-jet resonances has been performed by the ATLAS experiment. Scalar leptoquarks, pair-produced in pp collisions at s√ = 13 TeV at the large hadron collider, have been considered. An integrated luminosity of 3.2 fb(−)(1), corresponding to the full 2015 dataset was used. First (second) generation leptoquarks were sought in events with two electrons (muons) and two or more jets. The observed event yield in each channel is consistent with Standard Model background expectations. The observed (expected) lower limits on the leptoquark mass at 95% confidence level are 1100 and 1050 GeV (1160 and 1040 GeV) for first and second generation leptoquarks, respectively, assuming a branching ratio into a charged lepton and a quark of 100%. Upper limits on the aforementioned branching ratio are also given as a function of leptoquark mass. Compared with the results of earlier ATLAS searches, the sensitivity is increased for leptoquark masses above 860 GeV, and the observed exclusion limits confirm and extend the published results.