Measurement of the inclusive cross-sections of single top-quark and top-antiquark t-channel production in pp collisions at √s=13 TeV with the ATLAS detector

Wed 29 Nov 2017 10:02:04 KRAT

A measurement of the t-channel single-top-quark and single-top-antiquark production cross-sections in the lepton+je ts channel is presented, using 3.2 fb−1 of proton--proton collision data at a centre-of-mass energy of 13 TeV, recorded with the ATLAS detector at the LHC in 2015. Events are selected by requiring one charged lepton (electron or muon), missing transverse momentum, and two jets with high transverse momentum, exactly one of which is required to be b-tagged. Using a binned maximum-likelihood fit to the discriminant distribution of a neural network, the cross-sections are determined to be σ(tq)=156±5(stat.)±27(syst.)±3(lumi.) pb for single top-quark production and σ(t¯q)=91±4(stat.)±18(syst.)±2(lumi.) pb for single top-antiquark production, assuming a top-quark mass of 172.5 GeV. The cross-section ratio is measured to be Rt=σ(tq)/σ(t¯q)=1.72±0.09(stat.)±0.18(syst.).

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.

Constraints on non-Standard Model Higgs boson interactions in an effective Lagrangian using differential cross sections measured in the H→γγ decay channel at √s = 8 TeV with the ATLAS detector

Wed 23 Aug 2017 09:11:53 KRAT

The strength and tensor structure of the Higgs boson's interactions are investigated using an effective Lagrangian, which introduces additional CP-even and CP-odd interactions that lead to changes in the kinematic properties of the Higgs boson and associated jet spectra with respect to the Standard Model. The parameters of the effective Lagrangian are probed using a fit to five differential cross sections previously measured by the ATLAS experiment in the H→γγH→γγ decay channel with an integrated luminosity of 20.3 fb−1 at View the MathML sources=8 TeV. In order to perform a simultaneous fit to the five distributions, the statistical correlations between them are determined by re-analysing the H→γγH→γγ candidate events in the proton–proton collision data. No significant deviations from the Standard Model predictions are observed and limits on the effective Lagrangian parameters are derived. The statistical correlations are made publicly available to allow for future analysis of theories with non-Standard Model interactions.

Search for invisible decays of a Higgs boson using vector-boson fusion in pp collisions at √s = 8 TeV with the ATLAS detector

Wed 22 Mar 2017 17:35:04 KRAT

A search for a Higgs boson produced via vector-boson fusion and decaying into invisible particles is presented, using 20.3 fb−1 of proton-proton collision data at a centre-of-mass energy of 8 TeV recorded by the ATLAS detector at the LHC. For a Higgs boson with a mass of 125 GeV, assuming the Standard Model production cross section, an upper bound of 0.28 is set on the branching fraction of H → invisible at 95% confidence level, where the expected upper limit is 0.31. The results are interpreted in models of Higgs-portal dark matter where the branching fraction limit is converted into upper bounds on the dark-matter-nucleon scattering cross section as a function of the dark-matter particle mass, and compared to results from the direct dark-matter detection experiments.

Search for resonances in diphoton events at √s = 13 TeV with the ATLAS detector

Wed 22 Mar 2017 17:35:03 KRAT

Searches for new resonances decaying into two photons in the ATLAS experiment at the CERN Large Hadron Collider are described. The analysis is based on proton-proton collision data corresponding to an integrated luminosity of 3.2 fb−1 at s√=13 TeV recorded in 2015. Two searches are performed, one targeted at a spin-2 particle of mass larger than 500 GeV, using Randall-Sundrum graviton states as a benchmark model, and one optimized for a spin-0 particle of mass larger than 200 GeV. Varying both the mass and the decay width, the most significant deviation from the background-only hypothesis is observed at a diphoton invariant mass around 750 GeV with local significances of 3.8 and 3.9 standard deviations in the searches optimized for a spin-2 and spin-0 particle, respectively. The global significances are estimated to be 2.1 standard deviations for both analyses. The consistency between the data collected at 13 TeV and 8 TeV is also evaluated. Limits on the production cross section times branching ratio to two photons for the two resonance types are reported.

Measurement of the inclusive isolated prompt photon cross section in pp collisions at √s = 8 TeV with the ATLAS detector

Wed 22 Mar 2017 17:35:02 KRAT

A measurement of the cross section for the inclusive production of isolated prompt photons in proton-proton collisions at a centre-of-mass energy of s√=8 TeV is presented. The measurement covers the pseudorapidity ranges |ηγ | < 1.37 and 1.56 ≤ |ηγ | < 2.37 in the transverse energy range 25 < ETγ < 1500 GeV. The results are based on an integrated luminosity of 20.2 fb−1, recorded by the ATLAS detector at the LHC. Photon candidates are identified by combining information from the calorimeters and the inner tracker. The background is subtracted using a data-driven technique, based on the observed calorimeter shower-shape variables and the deposition of hadronic energy in a narrow cone around the photon candidate. The measured cross sections are compared with leading-order and next-to-leading order perturbative QCD calculations and are found to be in a good agreement over ten orders of magnitude.

Observation of an excited Bc± meson state with the ATLAS detector

Wed 12 Dec 2018 18:20:24 KRAT

A search for excited states of the Bf meson is performed using 4.9 fb-1 of 7 TeVand 19.2 fb-1 of 8 TeV pp collision data collected by the ATLAS experiment at the LHC. A new state is observed through its hadronic transition to the ground state, with the latter detected in the decay Bf ^ J/ynf. The state appears in the m(Bfn+n—) — m(Bf) — 2m(nf) mass difference distribution with a significance of 5.2 standard deviations. The mass of the observed state is 6842 f 4 f 5 MeV, where the first error is statistical and the second is systematic. The mass and decay of this state are consistent with expectations for the second S-wave state of the Bf meson, Bf (2S).

Measurement of differential production cross-sections for a Z boson in association with b-jets in 7 TeV proton-proton collisions with the ATLAS detector

Wed 12 Dec 2018 16:45:25 KRAT

Measurements of differential production cross-sections of a Z boson in association with b-jets in pp collisions at ps = 7TeV are reported. The data analysed correspond to an integrated luminosity of 4.6 fb−1 recorded with the ATLAS detector at the Large Hadron Collider. Particle-level cross-sections are determined for events with a Z boson decaying into an electron or muon pair, and containing b-jets. For events with at least one b-jet, the cross-section is presented as a function of the Z boson transverse momentum and rapidity, together with the inclusive b-jet cross-section as a function of b-jet transverse momentum, rapidity and angular separations between the b-jet and the Z boson. For events with at least two b-jets, the cross-section is determined as a function of the invariant mass and angular separation of the two highest transverse momentum b-jets, and as a function of the Z boson transverse momentum and rapidity. Results are compared to leading-order and next-to-leading-order perturbative QCD calculations.

Measurement of three-jet production cross-sections in pp collisions at 7 TeV centre-of-mass energy using the ATLAS detector

Wed 05 Dec 2018 10:50:05 KRAT

Double-differential three-jet production cross-sections are measured in proton–proton collisions at a centre-of-mass energy of √s = 7 TeV using the ATLAS detector at the large hadron collider. The measurements are presented as a function of the three-jet mass (mjjj), in bins of the sum of the absolute rapidity separations between the three leading jets (Y ∗). Invariant masses extending up to 5 TeV are reached for 8 < Y ∗ < 10. These measurements use a sample of data recorded using the ATLAS detector in 2011, which corresponds to an integrated luminosity of 4.51 fb−1. Jets are identified using the anti-kt algorithm with two different jet radius parameters, R = 0.4 and R = 0.6. The dominant uncertainty in these measurements comes from the jet energy scale. Next-to-leading-order QCD calculations corrected to account for non-perturbative effects are compared to the measurements. Good agreement is found between the data and the theoretical predictions based on most of the available sets of parton distribution functions, over the full kinematic range, covering almost seven orders of magnitude in the measured cross-section values.

Measurement of the t¯t production cross-section as a function of jet multiplicity and jet transverse momentum in 7 TeV proton-proton collisions with the ATLAS detector

Wed 05 Dec 2018 10:15:26 KRAT

The t¯t production cross-section dependence on jet multiplicity and jet transverse momentum is reported for proton-proton collisions at a centre-of-mass energy of 7TeV in the single-lepton channel. The data were collected with the ATLAS detector at the CERN Large Hadron Collider and comprise the full 2011 data sample corresponding to an integrated luminosity of 4.6 fb−1. Differential cross-sections are presented as a function of the jet multiplicity for up to eight jets using jet transverse momentum thresholds of 25, 40, 60, and 80GeV, and as a function of jet transverse momentum up to the fifth jet. The results are shown after background subtraction and corrections for all known detector effects, within a kinematic range closely matched to the experimental acceptance. Several QCD-based Monte Carlo models are compared with the results. Sensitivity to the parton shower modelling is found at the higher jet multiplicities, at high transverse momentum of the leading jet and in the transverse momentum spectrum of the fifth leading jet. The MC@NLO+HERWIG MC is found to predict too few events at higher jet multiplicities.

Measurement of the branching ratio Γ(Λ0b → ψ(2S)Λ0)/Γ(Λ0b → J/ψΛ0) with the ATLAS detector

Wed 05 Dec 2018 10:15:10 KRAT

An observation of the View the MathML source decay and a comparison of its branching fraction with that of the View the MathML source decay has been made with the ATLAS detector in proton–proton collisions at View the MathML source 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 Λ0→pπ− decay is exploited for the Λ0 baryon reconstruction. The View the MathML source baryons are reconstructed with transverse momentum pT>10 GeV and pseudorapidity |η|<2.1. The measured branching ratio of the View the MathML source and View the MathML source decays is View the MathML source, lower than the expectation from the covariant quark model.

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

Wed 05 Dec 2018 10:15:06 KRAT

Charged-particle spectra obtained in 0.15 nb−1 of Pb+Pb interactions at sNN−−−√=2.76TeV 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

Measurement of the transverse polarization of Λ and Λ¯ hyperons produced in proton-proton collisions at √s=7 TeV using the ATLAS detector

Wed 05 Dec 2018 10:01:14 KRAT

The transverse polarization of Λ and Λ¯ hyperons produced in proton-proton collisions at a center-ofmass energy of 7 TeV is measured. The analysis uses 760 μb−1 of minimum bias data collected by the ATLAS detector at the LHC in the year 2010. The measured transverse polarization averaged over Feynman xF from 5 × 10−5 to 0.01 and transverse momentum pT from 0.8 to 15 GeV is −0.010 0.005ðstatÞ 0.004ðsystÞ for Λ and 0.002 0.006ðstatÞ 0.004ðsystÞ for Λ¯ . It is also measured as a function of xF and pT, but no significant dependence on these variables is observed. Prior to this measurement, the polarization was measured at fixed-target experiments with center-of-mass energies up to about 40 GeV. The ATLAS results are compatible with the extrapolation of a fit from previous

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

Wed 05 Dec 2018 10:01:07 KRAT

This Letter presents measurements of correlated production of nearby jets in Pb+Pb collisions at View the MathML source 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.

Measurement of the inclusive jet cross-section in proton-proton collisions at √s=7 TeV using 4.5 fb-1 of data with the ATLAS detector

Wed 05 Dec 2018 09:40:37 KRAT

The inclusive jet cross-section is measured in proton-proton collisions at acentre-of-mass energy of 7TeV using a data set corresponding to an integrated luminosity of 4.5 fb−1 collected with the ATLAS detector at the Large Hadron Collider in 2011. Jets are identified using the anti-kt algorithm with radius parameter values of 0.4 and 0.6. The double-differential cross-sections are presented as a function of the jet transverse momentum and the jet rapidity, covering jet transverse momenta from 100 GeV to 2TeV. Next-to-leading-order QCD calculations corrected for non-perturbative effects and electroweak effects, as well as Monte Carlo simulations with next-to-leading-order matrix elements interfaced to parton showering, are compared to the measured cross-sections. A quantitative comparison of the measured cross-sections to the QCD calculations using several sets of parton distribution functions is performed.

Measurement of the charge asymmetry in dileptonic decays of top quark pairs in pp collisions √s = 7 TeV using the ATLAS detector

Wed 05 Dec 2018 09:40:17 KRAT

A measurement of the top–antitop (tt¯) charge asymmetry is presented using data corresponding to an integrated luminosity of 4.6 fb−1 of LHC pp collisions at a centre of mass energy of 7 TeV collected by the ATLAS detector. Events with two charged leptons, at least two jets and large missing transverse momentum are selected. Two observables are studied: A``C based on the identified charged leptons, and Att¯C , based on the reconstructed tt¯ final state. The asymmetries are measured to be A``C = 0.024 ± 0.015 (stat.) ± 0.009 (syst.), A tt¯ C = 0.021 ± 0.025 (stat.) ± 0.017 (syst.).The measured values are in agreement with the Standard Model predictions.

Measurement of the CP-violating phase фs and the B0s meson decay width difference with B0s → J/ψф decays in ATLAS

Wed 05 Apr 2017 10:18:55 KRAT

A measurement of the Bs0 decay parameters in the Bs0 → J/ψϕ channel using an integrated luminosity of 14.3 fb−1 collected by the ATLAS detector from 8 TeV pp collisions at the LHC is presented. The measured parameters include the CP -violating phase ϕs, the decay width Γs and the width difference between the mass eigenstates ΔΓs. The values measured for the physical parameters are statistically combined with those from 4.9 fb−1 of 7 TeV data, leading to the following:

Measurements of ψ(2S) and X(3872) → J/ψπ + π − production in pp collisions at √s=8 TeV with the ATLAS detector

Tue 13 Mar 2018 10:41:36 KRAT

Differential cross sections are presented for the prompt and non-prompt production of the hidden-charm states X(3872) and ψ(2S), in the decay mode J/ψπ+π−, measured using 11.4 fb−1 of pp collisions at s√=8 TeV by the ATLAS detector at the LHC. The ratio of cross-sections X(3872)/ψ(2S) is also given, separately for prompt and non-prompt components, as well as the non-prompt fractions of X(3872) and ψ(2S). Assuming independent single effective lifetimes for non-prompt X(3872) and ψ(2S) production gives RB=B(B→X(3872)+any)B(X(3872)→J/ψπ+π−)B(B→ψ(2S)+any)B(ψ(2S)→J/ψπ+π−)=(3.95±0.32(stat)±0.08(sys))×10−2 separating short- and long-lived contributions, assuming that the short-lived component is due to Bc decays, gives RB = (3.57 ± 0.33(stat) ± 0.11(sys)) × 10−2, with the fraction of non-prompt X(3872) produced via Bc decays for pT(X(3872)) > 10 GeV being (25 ± 13(stat) ± 2(sys) ± 5(spin))%. The distributions of the dipion invariant mass in the X(3872) and ψ(2S) decays are also measured and compared to theoretical predictions.

Measurement of the relative width difference of the B0-B¯0 system with the ATLAS detector

Tue 13 Jun 2017 13:35:20 KRAT

This paper presents the measurement of the relative width difference ΔΓd/Γd of the B0-B¯¯¯¯0 system using the data collected by the ATLAS experiment at the LHC in pp collisions at s√=7 TeV and s√=8 TeV and corresponding to an integrated luminosity of 25.2 fb−1. The value of ΔΓd/Γd is obtained by comparing the decay-time distributions of B0 → J/ψKS and B0 → J/ψK*0(892) decays. The result is ΔΓd/Γd = (−0.1±1.1 (stat.)± 0.9 (syst.)) × 10−2. Currently, this is the most precise single measurement of ΔΓd/Γd. It agrees with the Standard Model prediction and the measurements by other experiments.

Measurements of top quark spin observables in tt¯ events using dilepton final states in √s= 8 TeV pp collisions with the ATLAS detector

Tue 07 Nov 2017 09:49:43 KRAT

Measurements of top quark spin observables in tt¯ events are presented based on 20.2 fb−1 of √s=8 TeV proton-proton collisions recorded with the ATLAS detector at the LHC. The analysis is performed in the dilepton final state, characterised by the presence of two isolated leptons (electrons or muons). There are 15 observables, each sensitive to a different coefficient of the spin density matrix of tt¯ production, which are measured independently. Ten of these observables are measured for the first time. All of them are corrected for detector resolution and acceptance effects back to the parton and stable-particle levels. The measured values of the observables at parton level are compared to Standard Model predictions at next-to-leading order in QCD. The corrected distributions at stable-particle level are presented and the means of the distributions are compared to Monte Carlo predictions. No significant deviation from the Standard Model is observed for any observable.

Measurement of charged-particle distributions sensitive to the underlying event in √s=13 TeV proton-proton collisions with the ATLAS detector at the LHC

Tue 07 Nov 2017 09:49:37 KRAT

We present charged-particle distributions sensitive to the underlying event, measured by the ATLAS detector in proton-proton collisions at a centre-of-mass energy of 13 TeV, in low-luminosity Large Hadron Collider fills corresponding to an integrated luminosity of 1.6 nb−1. The distributions were constructed using charged particles with absolute pseudorapidity less than 2.5 and with transverse momentum greater than 500 MeV, in events with at least one such charged particle with transverse momentum above 1 GeV. These distributions characterise the angular distribution of energy and particle flows with respect to the charged particle with highest transverse momentum, as a function of both that momentum and of charged-particle multiplicity. The results have been corrected for detector effects and are compared to the predictions of various Monte Carlo event generators, experimentally establishing the level of underlying-event activity at LHC Run 2 energies and providing inputs for the development of event generator modelling. The current models in use for UE modelling typically describe this data to 5% accuracy, compared with data uncertainties of less than 1%.

Search for pair production of vector-like top quarks in events with one lepton, jets, and missing transverse momentum in √s=13 TeV pp collisions with the ATLAS detector

Tue 06 Nov 2018 14:05:18 KRAT

The results of a search for vector-like top quarks using events with exactly one lepton, at least four jets, and large missing transverse momentum are reported. The search is optimised for pair production of vector-like top quarks in the Z(→νν) t + X decay channel. LHC pp collision data at a centre-of-mass energy of s√=13 TeV recorded by the ATLAS detector in 2015 and 2016 are used, corresponding to an integrated luminosity of 36.1 fb−1. No significant excess over the Standard Model expectation is seen and upper limits on the production cross-section of a vector-like T quark pair as a function of the T quark mass are derived. The observed (expected) 95% CL lower limits on the T mass are 870 GeV (890 GeV) for the weak-isospin singlet model, 1.05 TeV (1.06 TeV) for the weak-isospin doublet model and 1.16 TeV (1.17 TeV) for the pure Zt decay mode. Limits are also set on the mass as a function of the decay branching ratios, excluding large parts of the parameter space for masses below 1 TeV.

Evidence for the H → b¯b decay with the ATLAS detector

Tue 06 Nov 2018 14:05:01 KRAT

A search for the decay of the Standard Model Higgs boson into a bb¯¯ pair when produced in association with a W or Z boson is performed with the ATLAS detector. The analysed data, corresponding to an integrated luminosity of 36.1 fb−1, were collected in proton-proton collisions in Run 2 of the Large Hadron Collider at a centre-of-mass energy of 13 TeV. Final states containing zero, one and two charged leptons (electrons or muons) are considered, targeting the decays Z → νν, W → ℓν and Z → ℓℓ. For a Higgs boson mass of 125 GeV, an excess of events over the expected background from other Standard Model processes is found with an observed significance of 3.5 standard deviations, compared to an expectation of 3.0 standard deviations. This excess provides evidence for the Higgs boson decay into b-quarks and for its production in association with a vector boson. The combination of this result with that of the Run 1 analysis yields a ratio of the measured signal events to the Standard Model expectation equal to 0.90 ± 0.18(stat.) − 0.19 + 0.21 (syst.). Assuming the Standard Model production cross-section, the results are consistent with the value of the Yukawa coupling to b-quarks in the Standard Model.

Measurement of the kt splitting scales in Z → ℓℓ events in pp collisions at √s=8 TeV with the ATLAS detector

Tue 06 Nov 2018 14:04:57 KRAT

A measurement of the splitting scales occuring in the k t jet-clustering algorithm is presented for final states containing a Z boson. The measurement is done using 20.2 fb−1 of proton-proton collision data collected at a centre-of-mass energy of sqrt{s}=8 TeV by the ATLAS experiment at the LHC in 2012. The measurement is based on chargedparticle track information, which is measured with excellent precision in the p T region relevant for the transition between the perturbative and the non-perturbative regimes. The data distributions are corrected for detector effects, and are found to deviate from state-of-the-art predictions in various regions of the observables.

Studies of Zγ production in association with a high-mass dijet system in pp collisions at √s=8 TeV with the ATLAS detector

Tue 06 Nov 2018 14:04:52 KRAT

The production of a Z boson and a photon in association with a high-mass dijet system is studied using 20.2 fb−1 of proton-proton collision data at a centre-of-mass energy of s√=8 TeV recorded with the ATLAS detector in 2012 at the Large Hadron Collider. Final states with a photon and a Z boson decaying into a pair of either electrons, muons, or neutrinos are analysed. Electroweak and total pp → Zγjj cross-sections are extracted in two fiducial regions with different sensitivities to electroweak production processes. Quartic couplings of vector bosons are studied in regions of phase space with an enhanced contribution from pure electroweak production, sensitive to vector-boson scattering processes VV → Zγ. No deviations from Standard Model predictions are observed and constraints are placed on anomalous couplings parameterized by higher-dimensional operators using effective field theory.

Search for pair production of up-type vector-like quarks and for four-top-quark events in final states with multiple b-jets with the ATLAS detector

Tue 05 Nov 2019 15:32:21 KRAT

A search for pair production of up-type vector-like quarks (T) with a significant

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 tt¯W and tt¯Z production cross sections in pp collisions at √s = 8 TeV with the ATLAS detector

Tue 04 Dec 2018 10:20:08 KRAT

The production cross sections of top-quark pairs in association with massive vector bosons have been measured using data from pp collisions at s√=8s=8 TeV. The dataset corresponds to an integrated luminosity of 20.3 fb−1 collected by the ATLAS detector in 2012 at the LHC. Final states with two, three or four leptons are considered. A fit to the data considering the tt¯Wtt¯W and tt¯Ztt¯Z processes simultaneously yields a significance of 5.0σ (4.2σ) over the background-only hypothesis for tt¯Wtt¯W ( tt¯Ztt¯Z ) production. The measured cross sections are σtt¯W=369+100−91σtt¯W=369−91+100 fb and σtt¯Z=176+58−52σtt¯Z=176−52+58 fb. The background-only hypothesis with neither tt¯Wtt¯W nor tt¯Ztt¯Z production is excluded at 7.1σ. All measurements are consistent with next-to-leading-order calculations for the tt¯Wtt¯W and tt¯Ztt¯Z processes.

Measurement of colour flow with the jet pull angle in t t¯ events using the ATLAS detector at √s = 8 TeV

Tue 04 Dec 2018 10:12:45 KRAT

The distribution and orientation of energy inside jets is predicted to be an experimental handle on colour connections between the hard--scatter quarks and gluons initiating the jets. This Letter presents a measurement of the distribution of one such variable, the jet pull angle. The pull angle is measured for jets produced in tt¯ events with one W boson decaying leptonically and the other decaying to jets using 20.3 fb−1 of data recorded with the ATLAS detector at a centre-of-mass energy of s√=8 TeV at the LHC. The jet pull angle distribution is corrected for detector resolution and acceptance effects and is compared to various models.

Measurements of charge and CP asymmetries in b-hadron decays using top-quark events collected by the ATLAS detector in pp collisions at √s=8 TeV

Tue 03 Oct 2017 14:16:10 KRAT

Same- and opposite-sign charge asymmetries are measured in lepton+jets tt¯ events in which a b-hadron decays semileptonically to a soft muon, using data corresponding to an integrated luminosity of 20.3 fb−1 from proton-proton collisions at a centre-of-mass energy of s√=8 TeV collected with the ATLAS detector at the Large Hadron Collider at CERN. The charge asymmetries are based on the charge of the lepton from the top-quark decay and the charge of the soft muon from the semileptonic decay of a b-hadron and are measured in a fiducial region corresponding to the experimental acceptance. Four CP asymmetries (one mixing and three direct) are measured and are found to be compatible with zero and consistent with the Standard Model.

Measurement of the production cross-section of a single top quark in association with a W boson at 8 TeV with the ATLAS experiment

Tue 02 May 2017 12:56:23 KRAT

The cross-section for the production of a single top quark in association with a W boson in proton-proton collisions at s√=8TeV is measured. The dataset corresponds to an integrated luminosity of 20.3 fb−1, collected by the ATLAS detector in 2012 at the Large Hadron Collider at CERN. Events containing two leptons and one central b-jet are selected. The W t signal is separated from the backgrounds using boosted decision trees, each of which combines a number of discriminating variables into one classifier. Production of W t events is observed with a significance of 7.7σ. The cross-section is extracted in a profile likelihood fit to the classifier output distributions. The W t cross-section, inclusive of decay modes, is measured to be 23.0 ± 1.3(stat.)− 3.5+ 3.2(syst.)±1.1(lumi.) pb. The measured cross-section is used to extract a value for the CKM matrix element |Vtb| of 1.01 ± 0.10 and a lower limit of 0.80 at the 95% confidence level. The cross-section for the production of a top quark and a W boson is also measured in a fiducial acceptance requiring two leptons with pT> 25 GeV and |η| < 2.5, one jet with pT> 20 GeV and |η| < 2.5, and ETmiss > 20 GeV, including both W t and top-quark pair events as signal. The measured value of the fiducial cross-section is 0.85 ± 0.01(stat.)− 0.07+ 0.07(syst.)±0.03(lumi.) pb.

Search for top quark decays t → qH, with H → γγ, in √s=13 TeV pp collisions using the ATLAS detector

Thu 22 Nov 2018 10:01:42 KRAT

This article presents a search for flavour-changing neutral currents in the decay of a top quark into an up-type (q = c, u) quark and a Higgs boson, where the Higgs boson decays into two photons. The proton-proton collision data set analysed amounts to 36.1 fb−1 at s√=13 TeV collected by the ATLAS experiment at the LHC. Top quark pair events are searched for, where one top quark decays into qH and the other decays into bW . Both the hadronic and leptonic decay modes of the W boson are used. No significant excess is observed and an upper limit is set on the t → cH branching ratio of 2.2 × 10−3 at the 95% confidence level, while the expected limit in the absence of signal is 1.6 × 10−3. The corresponding limit on the tcH coupling is 0.090 at the 95% confidence level. The observed upper limit on the t → uH branching ratio is 2.4 × 10−3.

Search for pair production of heavy vector-like quarks decaying to high-p T W bosons and b quarks in the lepton-plus-jets final state in pp collisions at √s=13 TeV with the ATLAS detector

Thu 22 Nov 2018 09:54:23 KRAT

A search is presented for the pair production of heavy vector-like T quarks, primarily targeting the T quark decays to a W boson and a b-quark. The search is based on 36.1 fb−1 of pp collisions at s√=13 TeV recorded in 2015 and 2016 with the ATLAS detector at the CERN Large Hadron Collider. Data are analysed in the lepton-plus-jets final state, including at least one b-tagged jet and a large-radius jet identified as originating from the hadronic decay of a high-momentum W boson. No significant deviation from the Standard Model expectation is observed in the reconstructed T mass distribution. The observed 95% confidence level lower limit on the T mass are 1350 GeV assuming 100% branching ratio to Wb. In the SU(2) singlet scenario, the lower mass limit is 1170 GeV. This search is also sensitive to a heavy vector-like B quark decaying to Wt and other final states. The results are thus reinterpreted to provide a 95% confidence level lower limit on the B quark mass at 1250 GeV assuming 100% branching ratio to Wt; in the SU(2) singlet scenario, the limit is 1080 GeV. Mass limits on both T and B production are also set as a function of the decay branching ratios. The 100% branching ratio limits are found to be applicable to heavy vector-like Y and X production that decay to Wb and Wt, respectively.

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.

Search for the decay of the Higgs boson to charm quarks with the ATLAS experiment

Thu 20 Jun 2019 10:52:52 KRAT

Search for B−L R-parity-violating top squarks in √s=13 TeV pp collisions with the ATLAS experiment

Thu 20 Jun 2019 10:52:42 KRAT

A search is presented for the direct pair production of the stop, the supersymmetric partner of the top quark, that decays through an R-parity-violating coupling to a final state with two leptons and two jets, at least one of which is identified as a b-jet. The data set corresponds to an integrated luminosity of 36.1 fb−1 of proton-proton collisions at a center-of-mass energy of √s=13 TeV, collected in 2015 and 2016 by the ATLAS detector at the LHC. No significant excess is observed over the Standard Model background, and exclusion limits are set on stop pair production at a 95% confidence level. Lower limits on the stop mass are set between 600 GeV and 1.5 TeV for branching ratios above 10% for decays to an electron or muon and a b-quark.

Search for a structure in the B0sπ± invariant mass spectrum with the ATLAS experiment

Thu 20 Jun 2019 10:38:53 KRAT

Measurement of the soft-drop jet mass in pp collisions at √s=13 TeV with the ATLAS detector

Thu 20 Jun 2019 09:42:47 KRAT

Jet substructure observables have significantly extended the search program for physics beyond the standard model at the Large Hadron Collider. The state-of-the-art tools have been motivated by theoretical calculations, but there has never been a direct comparison between data and calculations of jet substructure observables that are accurate beyond leading-logarithm approximation. Such observables are significant not only for probing the collinear regime of QCD that is largely unexplored at a hadron collider, but also for improving the understanding of jet substructure properties that are used in many studies at the Large Hadron Collider. This Letter documents a measurement of the first jet substructure quantity at a hadron collider to be calculated at next-to-next-to-leading-logarithm accuracy. The normalized, differential cross section is measured as a function of log10ρ2, where ρ is the ratio of the soft-drop mass to the ungroomed jet transverse momentum. This quantity is measured in dijet events from 32.9 fb−1 of √s=13 TeV proton-proton collisions recorded by the ATLAS detector. The data are unfolded to correct for detector effects and compared to precise QCD calculations and leading-logarithm particle-level Monte Carlo simulations.

Search for photonic signatures of gauge-mediated supersymmetry in 13 TeV pp collisions with the ATLAS detector

Thu 20 Jun 2019 09:42:32 KRAT

A search is presented for photonic signatures, motivated by generalized models of gauge-mediated supersymmetry breaking. This search makes use of proton-proton collision data at √s=13 TeVcorresponding to an integrated luminosity of 36.1 fb−1 recorded by the ATLAS detector at the LHC, and it explores models dominated by both strong and electroweak production of supersymmetric partner states. Experimental signatures incorporating an isolated photon and significant missing transverse momentum are explored. These signatures include events with an additional photon or additional jet activity not associated with any specific underlying quark flavor. No significant excess of events is observed above the Standard Model prediction, and 95% confidence-level upper limits of between 0.083 and 0.32 fb are set on the visible cross section of contributions from physics beyond the Standard Model. These results are interpreted in terms of lower limits on the masses of gluinos, squarks, and gauginos in the context of generalized models of gauge-mediated supersymmetry, which reach as high as 2.3 TeV for strongly produced and 1.3 TeV for weakly produced supersymmetric partner pairs.

Measurement of dijet azimuthal decorrelations in pp collisions at √s=8 TeV with the ATLAS detector and determination of the strong coupling

Thu 20 Jun 2019 09:42:32 KRAT

A measurement of the rapidity and transverse momentum dependence of dijet azimuthal decorrelations is presented, using the quantity RΔϕ. The quantity RΔϕ specifies the fraction of the inclusive dijet events in which the azimuthal opening angle of the two jets with the highest transverse momenta is less than a given value of the parameter Δϕmax. The quantity RΔϕ is measured in proton-proton collisions at s=8 TeV as a function of the dijet rapidity interval, the event total scalar transverse momentum, and Δϕmax. The measurement uses an event sample corresponding to an integrated luminosity of 20.2 fb−1 collected with the ATLAS detector at the CERN Large Hadron Collider. Predictions of a perturbative QCD calculation at next-to-leading order in the strong coupling with corrections for nonperturbative effects are compared to the data. The theoretical predictions describe the data in the whole kinematic region. The data are used to determine the strong coupling αS and to study its running for momentum transfers from 260 GeV to above 1.6 TeV. Analysis that combines data at all momentum transfers results in αS(mZ)=0.1127−0.0027+0.0063.

Measurement of inclusive jet and dijet cross-sections in proton-proton collisions at √s=13 TeV with the ATLAS detector

Thu 20 Jun 2019 09:35:40 KRAT

Inclusive jet and dijet cross-sections are measured in proton-proton collisions at a centre-of-mass energy of 13 TeV. The measurement uses a dataset with an integrated luminosity of 3.2 fb−1 recorded in 2015 with the ATLAS detector at the Large Hadron Collider. Jets are identified using the anti-kt algorithm with a radius parameter value of R = 0.4. The inclusive jet cross-sections are measured double-differentially as a function of the jet transverse momentum, covering the range from 100 GeV to 3.5 TeV, and the absolute jet rapidity up to |y| = 3. The double-differential dijet production cross-sections are presented as a function of the dijet mass, covering the range from 300 GeV to 9 TeV, and the half absolute rapidity separation between the two leading jets within |y| < 3, y∗, up to y∗ = 3. Next-to-leading-order, and next-to-next-to-leading-order for the inclusive jet measurement, perturbative QCD calculations corrected for non-perturbative and electroweak effects are compared to the measured cross-sections.

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.

Top-quark mass measurement in the all-hadronic tt¯ decay channel at √s=8 TeV with the ATLAS detector

Thu 15 Feb 2018 09:35:33 KRAT

The top-quark mass is measured in the all-hadronic top-antitop quark decay channel using proton-proton collisions at a centre-of-mass energy of sqrt{s}=8 TeV with the ATLAS detector at the CERN Large Hadron Collider. The data set used in the analysis corresponds to an integrated luminosity of 20.2 fb−1. The large multi-jet background is modelled using a data-driven method. The top-quark mass is obtained from template fits to the ratio of the three-jet to the dijet mass. The three-jet mass is obtained from the three jets assigned to the top quark decay. From these three jets the dijet mass is obtained using the two jets assigned to the W boson decay. The top-quark mass is measured to be 173.72 ± 0.55 (stat.) ± 1.01 (syst.) GeV.

Search for Higgs bosons produced via vector-boson fusion and decaying into bottom quark pairs in √s=13 TeV pp collisions with the ATLAS detector

Thu 07 Nov 2019 09:25:20 KRAT

A search for the bb¯ decay of the Standard Model Higgs boson produced through vector-boson fusion is presented. Three mutually exclusive channels are considered: two all-hadronic channels and a photon-associated channel. Results are reported from the analysis of up to 30.6 fb−1 of pp data at s=13 TeV collected with the ATLAS detector at the LHC. The measured signal strength relative to the Standard Model prediction from the combined analysis is 2.5−1.3+1.4 for inclusive Higgs boson production and 3.0−1.6+1.7 for vector-boson fusion production only.

Search for resonances in the mass distribution of jet pairs with one or two jets identified as b-jets in proton-proton collisions at √s=13 TeV with the ATLAS detector

Thu 06 Jun 2019 18:27:48 KRAT

A search for new resonances decaying into jets containing b-hadrons in pp collisions with the ATLAS detector at the LHC is presented in the dijet mass range from 0.57 to 7 TeV. The data set corresponds to an integrated luminosity of up to 36.1 fb−1 collected in 2015 and 2016 at s=13 TeV. No evidence of a significant excess of events above the smooth background shape is found. Upper cross-section limits and lower limits on the corresponding signal mass parameters for several types of signal hypotheses are provided at 95% C.L. In addition, 95% C.L. upper limits are set on the cross sections for new processes that would produce Gaussian-shaped signals in the di-b-jet mass distributions.

Search for new phenomena in events with three or more charged leptons in pp collisions at √s = 8 TeV with the ATLAS detector

Sat 12 May 2018 11:45:34 KRAT

A generic search for anomalous production of events with at least three charged leptons is presented. The data sample consists of pp collisions at s√=8 TeV collected in 2012 by the ATLAS experiment at the CERN Large Hadron Collider, and corresponds to an integrated luminosity of 20.3 fb−1. Events are required to have at least three selected lepton candidates, at least two of which must be electrons or muons, while the third may be a hadronically decaying tau. Selected events are categorized based on their lepton flavour content and signal regions are constructed using several kinematic variables of interest. No significant deviations from Standard Model predictions are observed. Model-independent upper limits on contributions from beyond the Standard Model phenomena are provided for each signal region, along with prescription to re-interpret the limits for any model. Constraints are also placed on models predicting doubly charged Higgs bosons and excited leptons. For doubly charged Higgs bosons decaying to eτ or μτ, lower limits on the mass are set at 400 GeV at 95% confidence level. For excited leptons, constraints are provided as functions of both the mass of the excited state and the compositeness scale Λ, with the strongest mass constraints arising in regions where the mass equals Λ. In such scenarios, lower mass limits are set at 3.0 TeV for excited electrons and muons, 2.5 TeV for excited taus, and 1.6 TeV for every excited-neutrino flavour.

Measurements of top-quark pair differential cross-sections in the e μ channel in pp collisions at √ s = 13 TeV using the ATLAS detector

Mon 23 Oct 2017 16:48:33 KRAT

This article presents measurements of tt¯tt¯ differential cross-sections in a fiducial phase-space region, using an integrated luminosity of 3.2 fb−1−1 of proton–proton data at a centre-of-mass energy of s√=13s=13 TeV recorded by the ATLAS experiment at the LHC in 2015. Differential cross-sections are measured as a function of the transverse momentum and absolute rapidity of the top quark, and of the transverse momentum, absolute rapidity and invariant mass of the tt¯tt¯ system. The tt¯tt¯ events are selected by requiring one electron and one muon of opposite electric charge, and at least two jets, one of which must be tagged as containing a b-hadron. The measured differential cross-sections are compared to predictions of next-to-leading order generators matched to parton showers and the measurements are found to be consistent with all models within the experimental uncertainties with the exception of the Powheg-Box++ Herwig++ predictions, which differ significantly from the data in both the transverse momentum of the top quark and the mass of the tt¯tt¯ system.

ATLAS Run 1 searches for direct pair production of third-generation squarks at the Large Hadron Collider

Mon 21 Aug 2017 16:00:14 KRAT

This paper reviews and extends searches for the direct pair production of the scalar supersymmetric partners of the top and bottom quarks in proton–proton collisions collected by the ATLAS collaboration during the LHC Run 1. Most of the analyses use 20 fb−1 of collisions at a centre-of-mass energy of √s=8 TeV, although in some case an additional 4.7 fb−1 of collision data at √s=7 TeV are used. New analyses are introduced to improve the sensitivity to specific regions of the model parameter space. Since no evidence of third-generation squarks is found, exclusion limits are derived by combining several analyses and are presented in both a simplified model framework, assuming simple decay chains, as well as within the context of more elaborate phenomenological supersymmetric models.

Study of (W/Z)H production and Higgs boson couplings using H→WW* decays with the ATLAS detector

Mon 19 Nov 2018 12:50:29 KRAT

A search for Higgs boson production in association with a W or Z boson, in the H→ W W ∗ decay channel, is performed with a data sample collected with the ATLAS detector at the LHC in proton-proton collisions at centre-of-mass energies s√=7 TeV and 8 TeV, corresponding to integrated luminosities of 4.5 fb−1 and 20.3 fb−1, respectively. The WH production mode is studied in two-lepton and three-lepton final states, while two- lepton and four-lepton final states are used to search for the ZH production mode. The observed significance, for the combined W H and ZH production, is 2.5 standard deviations while a significance of 0.9 standard deviations is expected in the Standard Model Higgs boson hypothesis. The ratio of the combined W H and ZH signal yield to the Standard Model expectation, μ V H , is found to be μ V H = 3.0 − 1.1 + 1.3 (stat.) − 0.7 + 1.0 (sys.) for the Higgs boson mass of 125.36 GeV. The W H and ZH production modes are also combined with the gluon fusion and vector boson fusion production modes studied in the H → W W ∗ → ℓνℓν decay channel, resulting in an overall observed significance of 6.5 standard deviations and μ ggF + VBF + VH = 1. 16 − 0.15 + 0.16 (stat.) − 0.15 + 0.18 (sys.). The results are interpreted in terms of scaling factors of the Higgs boson couplings to vector bosons (κ V ) and fermions (κ F ); the combined results are: |κ V | = 1.06 − 0.10 + 0.10 , |κ F | = 0. 85 − 0.20 + 0.26 .

Search for new particles in events with one lepton and missing transverse momentum in pp collisions at √s = 8 TeV with the ATLAS detector

Mon 17 Dec 2018 10:03:21 KRAT

This paper presents a search for new particles in events with one lepton (electron or muon) and missing transverse momentum using 20.3 fb−1 of proton-proton collision data at ps = 8TeV recorded by the ATLAS experiment at the Large Hadron Collider. No significant excess beyond Standard Model expectations is observed. A W′ with Sequential Standard Model couplings is excluded at the 95% confidence level for masses up to 3.24TeV. Excited chiral bosons (W∗) with equivalent coupling strengths are excluded for masses up to 3.21TeV. In the framework of an effective field theory limits are also set on the dark matternucleon scattering cross-section as well as the mass scale M∗ of the unknown mediating interaction for dark matter pair production in association with a leptonically decaying W.

Erratum: Measurement of the inclusive jet cross-section in proton-proton collisions at √s = 7 TeV using 4.5 fb-1 of data with the ATLAS detector

Mon 16 Dec 2019 15:01:26 KRAT

Erratum to: JHEP02(2015)153. It was found that the non-perturbative corrections calculated using Pythia with the Perugia 2011 tune did not include the effect of the underlying event. The affected correction factors were recomputed using the Pythia 6.427 generator. These corrections are applied as baseline to the NLO pQCD calculations and thus the central values of the theoretical predictions have changed by a few percent with the new corrections. This has a minor impact on the agreement between the data and the theoretical predictions. Figures 2 and 6 to 13, and all the tables have been updated with the new values. A few sentences in the discussion in sections 5.2 and 9 were altered or removed.

Search for direct top squark pair production in events with a Higgs or Z boson, and missing transverse momentum in √s=13 TeV pp collisions with the ATLAS detector

Mon 14 May 2018 10:25:25 KRAT

A search for direct top squark pair production resulting in events with either a same-flavour opposite-sign dilepton pair with invariant mass compatible with a Z boson or a pair of jets compatible with a Standard Model (SM) Higgs boson (h) is presented. Requirements on the missing transverse momentum, together with additional selections on leptons, jets, jets identified as originating from b-quarks are imposed to target the other decay products of the top squark pair. The analysis is performed using proton-proton collision data at s√=13 TeV collected with the ATLAS detector at the LHC in 2015–2016, corresponding to an integrated luminosity of 36.1 fb−1. No excess is observed in the data with respect to the SM predictions. The results are interpreted in two sets of models. In the first set, direct production of pairs of lighter top squarks ( t~1) with long decay chains involving Z or Higgs bosons is considered. The second set includes direct pair production of the heavier top squark pairs ( t~2) decaying via t~2→Zt~1 or t~2→ht~1. The results exclude at 95% confidence level t~2 and t~1 masses up to about 800 GeV, extending the exclusion region of supersymmetric parameter space covered by previous LHC searches.

Measurement of differential cross sections and W+/W− cross-section ratios for W boson production in association with jets at √s=8 TeV with the ATLAS detector

Mon 10 Jun 2019 16:47:56 KRAT

This paper presents a measurement of the W boson production cross section and the W +/W − cross-section ratio, both in association with jets, in proton-proton collisions at sqrt{s}=8 TeV with the ATLAS experiment at the Large Hadron Collider. The measurement is performed in final states containing one electron and missing transverse momentum using data corresponding to an integrated luminosity of 20.2 fb−1. Differential cross sections for events with at least one or two jets are presented for a range of observables, including jet transverse momenta and rapidities, the scalar sum of transverse momenta of the visible particles and the missing transverse momentum in the event, and the transverse momentum of the W boson. For a subset of the observables, the differential cross sections of positively and negatively charged W bosons are measured separately. In the cross-section ratio of W +/W − the dominant systematic uncertainties cancel out, improving the measurement precision by up to a factor of nine. The observables and ratios selected for this paper provide valuable input for the up quark, down quark, and gluon parton distribution functions of the proton.

Search for flavour-changing neutral current top-quark decays t → qZ in proton-proton collisions at √s=13 TeV with the ATLAS detector

Mon 10 Jun 2019 16:33:20 KRAT

A search for flavour-changing neutral-current processes in top-quark decays is presented. Data collected with the ATLAS detector from proton-proton collisions at the Large Hadron Collider at a centre-of-mass energy of s√=13 TeV, corresponding to an integrated luminosity of 36.1 fb−1, are analysed. The search is performed using top-quark pair events, with one top quark decaying through the t → qZ (q = u, c) flavour-changing neutral-current channel, and the other through the dominant Standard Model mode t → bW. Only Z boson decays into charged leptons and leptonic W boson decays are considered as signal. Consequently, the final-state topology is characterized by the presence of three isolated charged leptons (electrons or muons), at least two jets, one of the jets originating from a b-quark, and missing transverse momentum from the undetected neutrino. The data are consistent with Standard Model background contributions, and at 95% confidence level the search sets observed (expected) upper limits of 1.7 × 10−4 (2.4 × 10−4) on the t → uZ branching ratio and 2.4 × 10−4 (3.2 × 10−4) on the t → cZ branching ratio, constituting the most stringent limits to date.

Search for long-lived charginos based on a disappearing-track signature in pp collisions at √s=13 TeV with the ATLAS detector

Mon 10 Jun 2019 16:33:16 KRAT

This paper presents a search for direct electroweak gaugino or gluino pair production with a chargino nearly mass-degenerate with a stable neutralino. It is based on an integrated luminosity of 36.1 fb−1 of pp collisions at s√=13 TeV collected by the ATLAS experiment at the LHC. The final state of interest is a disappearing track accompanied by at least one jet with high transverse momentum from initial-state radiation or by four jets from the gluino decay chain. The use of short track segments reconstructed from the innermost tracking layers significantly improves the sensitivity to short chargino lifetimes. The results are found to be consistent with Standard Model predictions. Exclusion limits are set at 95% confidence level on the mass of charginos and gluinos for different chargino lifetimes. For a pure wino with a lifetime of about 0.2 ns, chargino masses up to 460 GeV are excluded. For the strong production channel, gluino masses up to 1.65 TeV are excluded assuming a chargino mass of 460 GeV and lifetime of 0.2 ns.

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

Mon 10 Dec 2018 13:06:36 KRAT

Searches for heavy long-lived charged particles are performed using a data sample of 19.1 fb−1 from proton-proton collisions at a centre-of-mass energy of ps = 8TeV 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 between 10 and 50, with a 290 GeV limit in the case where only direct tau slepton production is considered.

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

Mon 10 Dec 2018 12:59:21 KRAT

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 8TeV. The analyzeddata set was recorded by the ATLAS detector and corresponds to an integrated luminosity of 20.3fb−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 14.6pb at the 95% confidence level. The fit gives a cross-section of σs=5.0 ±4.3pb, consistent with the Standard Model expectation.

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

Mon 10 Dec 2018 12:59:06 KRAT

Simultaneous measurements of the t¯t, WþW−, 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

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

Mon 10 Dec 2018 12:52:56 KRAT

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 20.3fb−1of 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 the Standard Model Higgs boson produced in association with top quarks and decaying into bb in pp collisions at √s = 8 TeV with the ATLAS detector

Mon 10 Dec 2018 12:44:01 KRAT

A search for the Standard Model Higgs boson produced in association with a top-quark pair, t ¯tH, is presented. The analysis uses 20.3 fb−1 of pp collision data at √s = 8 TeV, collected with the ATLAS detector at the Large Hadron Collider during 2012. The search is designed for the H b ¯b decay mode and uses events containing one or two electrons or muons. In order to improve the sensitivity of the search, events are categorised according to their jet and b-tagged jet multiplicities. A neural network is used to discriminate between signal and background events, the latter being dominated by t ¯t+jets production. In the single-lepton channel, variables calculated using a matrix element method are included as inputs to the neural network to improve discrimination of the irreducible t ¯t+b ¯b background. No significant excess of events above the background expectation is found and an observed (expected) limit of 3.4 (2.2) times the Standard Model cross section is obtained at 95% confidence level. The ratio of the measured t ¯tH signal cross section to the Standard Model expectation is found to be μ = 1.5±1.1 assuming a Higgs boson mass of 125 GeV .

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

Mon 10 Dec 2018 12:17:42 KRAT

A search for the b¯b 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 are 4.7 and 20.3 fb−1 from pp collisions at ps = 7 and 8TeV, respectively. The processes considered are associated (W/Z)H production, where W ! e/μ, Z ! ee/μμ and Z!. The observed (expected) deviation from the backgroundonly 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 ! b¯b 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

Mon 10 Dec 2018 12:17:39 KRAT

A search for a massive W gauge boson decaying to a top quark and a bottom quark is performed with theATLAS detector in pp collisions at the LHC. The dataset was taken at a centre-of-mass energy of √s = 8 TeV and corresponds to 20.3 fb−1 of integrated luminosity. This analysis is done in the hadronic decay mode of the top quark, where novel jet substructure techniques are used to identify jets from high-momentum top quarks. This allows for a search for high-mass W bosons in the range 1.5–3.0 TeV. b-tagging is used to identify jets originating from b-quarks. The data are consistent with Standard Model background-only expectations,

Search for W'->t¯b 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

Mon 10 Dec 2018 12:10:23 KRAT

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=8TeV, using a dataset corresponding to an integrated luminosity of 20.3fb−1. This analysis searches for Wbosons in the W→t¯bdecay 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 3.0TeV, for right-handed or left-handed Wbosons. No significant deviation from the Standard Model expectation is observed and limits are set on the W→t¯bcross-section times branching ratio and on the W-boson effective couplings as a function of the W-boson mass using the CLsprocedure. For a left-handed (right-handed) Wboson, masses below 1.70 (1.92)TeVare excluded at 95% confidence level.

Summary of the ATLAS experiment’s sensitivity to supersymmetry after LHC Run 1 — interpreted in the phenomenological MSSM

Mon 10 Dec 2018 12:10:10 KRAT

A summary of the constraints from the ATLAS experiment on R-parity conserving supersymmetry is presented. Results from 22 separate ATLAS searches are considered, each based on analysis of up to 20.3 fb−1 of proton-proton collision data at the centre-of-mass energy of s√ = 7 and 8 TeV at the Large Hadron Collider. The results are interpreted in the context of the 19-parameter phenomenological minimal supersymmetric standard model, in which the lightest supersymmetric particle is a neutralino, taking into account constraints from previous precision electroweak and flavour measurements as well as from dark matter related measurements. The results are presented in terms of constraints on supersymmetric particle masses and are compared to limits from simplified models. The impact of ATLAS searches on parameters such as the dark matter relic density, the couplings of the observed Higgs boson, and the degree of electroweak fine-tuning is also shown. Spectra for surviving supersymmetry model points with low fine-tunings are presented.

Search for Higgs and Z boson decays to J/ψγ and Υ(nS)γ with the ATLAS detector

Mon 10 Dec 2018 11:56:35 KRAT

A search for the decays of the Higgs and Z bosons to J=ψγ and ðnSÞγ (n ¼ 1; 2; 3) is performed with pp collision data samples corresponding to integrated luminosities of up to 20.3 fb−1 collected at pffisffiffi ¼ 8 TeV with the ATLAS detector at the CERN Large Hadron Collider. No significant excess of events is observed above expected backgrounds and 95% C.L. upper limits are placed on the branching fractions.

Search for low-scale gravity signatures in multi-jet final states with the ATLAS detector at √s = 8 TeV

Mon 10 Dec 2018 11:56:12 KRAT

A search for evidence of physics beyond the Standard Model in final states with multiple high-transverse-momentum jets is performed using 20.3 fb−1 of proton-proton collision data at ps = 8TeV recorded by the ATLAS detector at the LHC. No significant excess of events beyond Standard Model expectations is observed, and upper limits on the visible cross sections for non-Standard Model production of multi-jet final states are set. A wide variety of models for black hole and string ball production and decay are considered, and the upper limit on the cross section times acceptance is as low as 0.16 fb at the 95% confidence level. For these models, excluded regions are also given as function of the main model parameters.

Search for charged Higgs bosons decaying via H ± → τ ± ν in fully hadronic final states using pp collision data at √s=8 TeV with the ATLAS detector

Mon 10 Dec 2018 11:49:15 KRAT

The results of a search for charged Higgs bosons decaying to a τ lepton and a neutrino, H± ! τ±ν, are presented. The analysis is based on 19.5 fb−1 of proton-proton collision data at √s=8 TeV collected by the ATLAS xperiment at the Large Hadron Collider. Charged Higgs bosons are searched for in events consistent with top-quark pair production or in associated production with a top quark, depending on the considered H± mass. The final state is characterised by the presence of a hadronic τ decay, missing transverse momentum, b-tagged jets, a hadronically decaying W boson, and the absence of any isolated electrons or muons with high transverse momenta. The data are consistent with the expected background from Standard Model processes. A statistical analysis leads to 95% confidence-level upper limits on the product of branching ratios B(t ! bH±) × B(H± ! τ±ν), between 0.23% and 1.3% for charged Higgs boson masses in the range 80–160GeV. It also leads to 95% confidence-level upper limits on the production cross section times branching

Search for anomalous production of prompt same-sign lepton pairs and pair-produced doubly charged Higgs bosons with √s=8 TeV pp collisions using the ATLAS detector

Mon 10 Dec 2018 11:42:24 KRAT

A low-background inclusive search for new physics in events with same-sign dileptons is presented. The search uses proton-proton collisions corresponding to 20.3 fb−1 of integrated luminosity taken in 2012 at a centre-of-mass energy of 8TeV with the ATLAS detector at the LHC. Pairs of isolated leptons with the same electric charge and large transverse momenta of the type e±e±, e±μ±, and μ±μ± are selected and their invariant mass distribution is examined. No excess of events above the expected level of Standard Model background is found. The results are used to set upper limits on the cross-sections for processes beyond the Standard Model. Limits are placed as a function of the dilepton invariant mass within a fiducial region corresponding to the signal event selection criteria. Exclusion limits are also derived for a specific model of doubly charged Higgs boson production.

Search for new light gauge bosons in Higgs boson decays to four-lepton final states in pp collisions at √s = 8 TeV with the ATLAS detector at the LHC

Mon 10 Dec 2018 11:35:11 KRAT

This paper presents a search for Higgs bosons decaying to four leptons, either electrons or muons, via one or two light exotic gauge bosons Zd, H→ZZd→4ℓ or H→ZdZd→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→ZZd→4ℓ and on the kinetic mixing parameter between the Zd 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 Zd masses between 15 and 55 GeV. Upper bounds on the effective mass mixing parameter between the Z and the Zd are also set using the branching ratio limits in the H→ZZd→4ℓ search, and are in the range (1.5–8.7)×10−4 for 15

Evidence for the Higgs-boson Yukawa coupling to tau leptons with the ATLAS detector

Mon 03 Dec 2018 15:25:38 KRAT

Results of a search for H → τ τ decays are presented, based on the full set of proton-proton collision data recorded by the ATLAS experiment at the LHC during 2011 and 2012. The data correspond to integrated luminosities of 4.5 fb−1 and 20.3 fb−1 at centre-of-mass energies of √ s = 7 TeV and √ s = 8 TeV respectively. All combinations of leptonic (τ → `νν¯ with ` = e, µ) and hadronic (τ → hadrons ν) tau decays are considered. An excess of events over the expected background from other Standard Model processes is found with an observed (expected) significance of 4.5 (3.4) standard deviations. This excess provides evidence for the direct coupling of the recently discovered Higgs boson to fermions. The measured signal strength, normalised to the Standard Model expectation, of µ = 1.43 +0.43 −0.37 is consistent with the predicted Yukawa coupling strength in the Standard Model.

Search for Higgs and Z boson decays to фγ with the ATLAS detector

Mon 03 Dec 2018 11:27:51 KRAT

A search for the decays of the Higgs and Z bosons to a ϕ meson and a photon is performed with a pp collision data sample corresponding to an integrated luminosity of 2.7 fb−1 collected at s√=13 TeV with the ATLAS detector at the LHC. No significant excess of events is observed above the background, and 95% confidence level upper limits on the branching fractions of the Higgs and Z boson decays to ϕγ of 1.4×10−3 and 8.3×10−6, respectively, are obtained.

Search for strong gravity in multijet final states produced in pp collisions at √s = 13 TeV using the ATLAS detector at the LHC

Mon 03 Dec 2018 11:20:57 KRAT

A search is conducted for new physics in multijet final states using 3.6 inverse femtobarns of data from proton-proton collisions at s√=13 TeV taken at the CERN Large Hadron Collider with the ATLAS detector. Events are selected containing at least three jets with scalar sum of jet transverse momenta (HT) greater than 1 TeV. No excess is seen at large HT and limits are presented on new physics: models which produce final states containing at least three jets and having cross sections larger than 1.6 fb with HT> 5.8 TeV are excluded. Limits are also given in terms of new physics models of strong gravity that hypothesize additional space-time dimensions.

Identification of high transverse momentum top quarks in pp collisions at √s = 8 TeV with the ATLAS detector

Mon 03 Dec 2018 11:20:50 KRAT

This paper presents studies of the performance of several jet-substructure techniques, which are used to identify hadronically decaying top quarks with high transverse momentum contained in large-radius jets. The efficiency of identifying top quarks is measured using a sample of top-quark pairs and the rate of wrongly identifying jets from other quarks or gluons as top quarks is measured using multijet events collected with the ATLAS experiment in 20.3 fb−1 of 8 TeV proton-proton collisions at the Large Hadron Collider. Predictions from Monte Carlo simulations are found to provide an accurate description of the performance. The techniques are compared in terms of signal efficiency and background rejection using simulations, covering a larger range in jet transverse momenta than accessible in the dataset. Additionally, a novel technique is developed that is optimized to reconstruct top quarks in events with many jets.

Search for the production of single vector-like and excited quarks in the Wt final state in pp collisions at √s = 8 TeV with the ATLAS detector

Mon 03 Dec 2018 11:20:48 KRAT

A search for vector-like quarks and excited quarks in events containing a top quark and a W boson in the final state is reported here. The search is based on 20.3 fb−1 of proton-proton collision data taken at the LHC at a centre-of-mass energy of 8 TeV recorded by the ATLAS detector. Events with one or two leptons, and one, two or three jets are selected with the additional requirement that at least one jet contains a b-quark. Single-lepton events are also required to contain at least one large-radius jet from the hadronic decay of a high-pTW boson or a top quark. No significant excess over the expected background is observed and upper limits on the cross-section times branching ratio for different vector-like quark and excited-quark model masses are derived. For the excited-quark production and decay to Wt with unit couplings, quarks with masses below 1500 GeV are excluded and coupling-dependent limits are set.

Search for anomalous couplings in the W tb vertex from the measurement of double differential angular decay rates of single top quarks produced in the t-channel with the ATLAS detector

Mon 03 Dec 2018 11:13:46 KRAT

The electroweak production and subsequent decay of single top quarks is determined by the properties of the Wtb vertex. This vertex can be described by the complex parameters of an effective Lagrangian. An analysis of angular distributions of the decay products of single top quarks produced in the t -channel constrains these parameters simultaneously. The analysis described in this paper uses 4.6 fb−1 of proton-proton collision data at s√=7 TeV collected with the ATLAS detector at the LHC. Two parameters are measured simultaneously in this analysis. The fraction f1 of decays containing transversely polarised W bosons is measured to be 0.37 ± 0.07 (stat.⊕syst.). The phase δ− between amplitudes for transversely and longitudinally polarised W bosons recoiling against left-handed b-quarks is measured to be −0.014π ± 0.036π (stat.⊕syst.). The correlation in the measurement of these parameters is 0.15. These values result in two-dimensional limits at the 95% confidence level on the ratio of the complex coupling parameters gR and VL, yielding Re[gR/VL] ∈ [−0.36, 0.10] and Im[gR/VL] ∈ [−0.17, 0.23] with a correlation of 0.11. The results are in good agreement with the predictions of the Standard Model.

Search for the Standard Model Higgs boson decaying into bb¯ produced in association with top quarks decaying hadronically in pp collisions at √s = 8 TeV with the ATLAS detector

Mon 03 Dec 2018 11:13:45 KRAT

A search for Higgs boson production in association with a pair of top quarks (tt¯H) is performed, where the Higgs boson decays to bb¯¯, and both top quarks decay hadronically. The data used correspond to an integrated luminosity of 20.3 fb−1 of pp collisions at s√=8 TeV collected with the ATLAS detector at the Large Hadron Collider. The search selects events with at least six energetic jets and uses a boosted decision tree algorithm to discriminate between signal and Standard Model background. The dominant multijet background is estimated using a dedicated data-driven technique. For a Higgs boson mass of 125 GeV, an upper limit of 6.4 (5.4) times the Standard Model cross section is observed (expected) at 95% confidence level. The best-fit value for the signal strength is μ = 1.6 ± 2.6 times the Standard Model expectation for mH = 125 GeV. Combining all tt¯H searches carried out by ATLAS at s√=8 and 7 TeV, an observed (expected) upper limit of 3.1 (1.4) times the Standard Model expectation is obtained at 95% confidence level, with a signal strength μ = 1.7 ± 0.8.

Determination of the ratio of b-quark fragmentation fractions fs/fd in pp collisions at √s = 7 TeV with the ATLAS detector

Mon 03 Dec 2018 10:45:38 KRAT

With an integrated luminosity of 2.47 fb−1 recorded by the ATLAS experiment at the LHC, the exclusive decays B0s→J/ψϕ and B0d→J/ψK*0 of B mesons produced in pp collisions at s√=7 TeV are used to determine the ratio of fragmentation fractions fs/fd. From the observed B0s→J/ψϕ and B0d→J/ψK*0 yields, the quantity (fs/fd)[B(B0s→J/ψϕ)/B(B0d→J/ψK*0)] is measured to be 0.199±0.004(stat)±0.008(syst). Using a recent theory prediction for [B(B0s→J/ψϕ)/B(B0d→J/ψK*0)] yields (fs/fd)=0.240±0.004(stat)±0.010(syst)±0.017(th). This result is based on a new approach that provides a significant improvement of the world average.

Observation of long-range elliptic azimuthal anisotropies in √s = 13 TeV and 2.76 TeV pp collisions with the ATLAS detector

Mon 03 Dec 2018 09:50:10 KRAT

ATLAS has measured two-particle correlations as a function of the relative azimuthal angle, Δϕ, and pseudorapidity, Δη, in s√=13 and 2.76 TeV pp collisions at the LHC using charged particles measured in the pseudorapidity interval |η|<2.5. The correlation functions evaluated in different intervals of measured charged-particle multiplicity show a multiplicity-dependent enhancement at Δϕ∼0 that extends over a wide range of Δη, which has been referred to as the “ridge.” Per-trigger-particle yields, Y(Δϕ), are measured over 2<|Δη|<5. For both collision energies, the Y(Δϕ) distribution in all multiplicity intervals is found to be consistent with a linear combination of the per-trigger-particle yields measured in collisions with less than 20 reconstructed tracks, and a constant combinatoric contribution modulated by cos(2Δϕ). The fitted Fourier coefficient, v2,2, exhibits factorization, suggesting that the ridge results from per-event cos(2ϕ) modulation of the single-particle distribution with Fourier coefficients v2. The v2 values are presented as a function of multiplicity and transverse momentum. They are found to be approximately constant as a function of multiplicity and to have a pT dependence similar to that measured in p+Pb and Pb+Pb collisions. The v2 values in the 13 and 2.76 TeV data are consistent within uncertainties. These results suggest that the ridge in pp collisions arises from the same or similar underlying physics as observed in p+Pb collisions, and that the dynamics responsible for the ridge has no strong s√ dependence.

A search for top squarks with R-parity-violating decays to all-hadronic final states with the ATLAS detector in √s = 8 TeV proton-proton collisions

Mon 03 Dec 2018 09:35:48 KRAT

A search for the pair production of top squarks, each with R-parity-violating decays into two Standard Model quarks, is performed using 17.4 fb−1 of s√=8 TeV proton-proton collision data recorded by the ATLAS experiment at the LHC. Each top squark is assumed to decay to a b- and an s-quark, leading to four quarks in the final state. Background discrimination is achieved with the use of b-tagging and selections on the mass and substructure of large-radius jets, providing sensitivity to top squark masses as low as 100 GeV. No evidence of an excess beyond the Standard Model background prediction is observed and top squarks decaying to b¯¯s¯¯¯ are excluded for top squark masses in the range 100 ≤mt¯≤ 315 GeV at 95% confidence level.

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.

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.

Search for vectorlike B quarks in events with one isolated lepton, missing transverse momentum, and jets at √s = 8 TeV with the ATLAS detector

Mon 01 Jul 2019 12:51:35 KRAT

A search has been performed for pair production of heavy vectorlike 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

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.

Measurement of differential cross sections of isolated-photon plus heavy-flavour jet production in pp collisions at √s=8 TeV using the ATLAS detector

Fri 19 Oct 2018 15:39:47 KRAT

This Letter presents the measurement of differential cross sections of isolated prompt photons produced in association with a b-jet or a c-jet. These final states provide sensitivity to the heavy-flavour content of the proton and aspects related to the modelling of heavy-flavour quarks in perturbative QCD. The measurement uses proton–proton collision data at a centre-of-mass energy of 8 TeV recorded by the ATLAS detector at the LHC in 2012 corresponding to an integrated luminosity of up to 20.2 fb−1. The differential cross sections are measured for each jet flavour with respect to the transverse energy of the leading photon in two photon pseudorapidity regions: and . The measurement covers photon transverse energies GeV and GeV respectively for the two regions. For each jet flavour, the ratio of the cross sections in the two regions is also measured. The measurement is corrected for detector effects and compared to leading-order and next-to-leading-order perturbative QCD calculations, based on various treatments and assumptions about the heavy-flavour content of the proton. Overall, the predictions agree well with the measurement, but some deviations are observed at high photon transverse energies. The total uncertainty in the measurement ranges between 13% and 66%, while the central measurement exhibits the smallest uncertainty, ranging from 13% to 27%, which is comparable to the precision of the theoretical predictions.

Search for heavy Higgs bosons A/H decaying to a top quark pair in pp collisions at √s =8 TeV with the ATLAS detector

Fri 16 Nov 2018 14:15:22 KRAT

A search for heavy pseudoscalar (A) and scalar (H) Higgs bosons decaying into a top quark pair (t¯t) has been performed with 20.3 fb−1 of proton-proton collision data collected by the ATLAS experiment at the Large Hadron Collider at a center-of-mass energy √s=8 TeV. Interference effects between the signal process and standard model t¯t production, which are expected to distort the signal shape from a single peak to a peak-dip structure, are taken into account. No significant deviation from the standard model prediction is observed in the t¯t invariant mass spectrum in final states with an electron or muon, large missing transverse momentum, and at least four jets. The results are interpreted within the context of a type-II two-Higgs-doublet model. Exclusion limits on the signal strength are derived as a function of the mass mA/H and the ratio of the vacuum expectation values of the two Higgs fields, tan β, for mA/H >500 GeV.

Measurement of the tt¯γ production cross section in proton-proton collisions at √s=8 TeV with the ATLAS detector

Fri 16 Nov 2018 13:26:25 KRAT

The cross section of a top-quark pair produced in association with a photon is measured in proton-proton collisions at a centre-of-mass energy of s√=8 TeV with 20.2 fb−1 of data collected by the ATLAS detector at the Large Hadron Collider in 2012. The measurement is performed by selecting events that contain a photon with transverse momentum pT > 15 GeV, an isolated lepton with large transverse momentum, large missing transverse momentum, and at least four jets, where at least one is identified as originating from a b-quark. The production cross section is measured in a fiducial region close to the selection requirements. It is found to be 139 ± 7 (stat.) ± 17 (syst.) fb, in good agreement with the theoretical prediction at next-to-leading order of 151 ± 24 fb. In addition, differential cross sections in the fiducial region are measured as a function of the transverse momentum and pseudorapidity of the photon.

Measurements of top-quark pair differential cross-sections in the lepton+jets channel in pp collisions at √s=13 TeV using the ATLAS detector

Fri 16 Nov 2018 12:44:49 KRAT

Measurements of differential cross-sections of top-quark pair production in fiducial phase-spaces are presented as a function of top-quark and toverline{t} system kinematic observables in proton-proton collisions at a centre-of-mass energy of sqrt{s}=13 TeV. The data set corresponds to an integrated luminosity of 3.2 fb−1, recorded in 2015 with the ATLAS detector at the CERN Large Hadron Collider. Events with exactly one electron or muon and at least two jets in the final state are used for the measurement. Two separate selections are applied that each focus on different top-quark momentum regions, referred to as resolved and boosted topologies of the toverline{t} final state. The measured spectra are corrected for detector effects and are compared to several Monte Carlo simulations by means of calculated χ2 and p-values.

Search for supersymmetry in events with b-tagged jets and missing transverse momentum in pp collisions at √s=13 TeV with the ATLAS detector

Fri 16 Nov 2018 12:44:44 KRAT

A search for the supersymmetric partners of the Standard Model bottom and top quarks is presented. The search uses 36.1 fb−1 of pp collision data at s√=13 TeV collected by the ATLAS experiment at the Large Hadron Collider. Direct production of pairs of bottom and top squarks ( b¯1 and t¯1 ) is searched for in final states with b-tagged jets and missing transverse momentum. Distinctive selections are defined with either no charged leptons (electrons or muons) in the final state, or one charged lepton. The zero-lepton selection targets models in which the b¯1 is the lightest squark and decays via b¯1→bχ¯01 , where χ¯01 is the lightest neutralino. The one-lepton final state targets models where bottom or top squarks are produced and can decay into multiple channels, b¯1→bχ¯01 and b¯1→tχ¯±1 , or t¯1→tχ¯01 and t¯1→bχ¯±1 , where χ¯±1 is the lightest chargino and the mass difference mχ¯±1−mχ¯01 is set to 1 GeV. No excess above the expected Standard Model background is observed. Exclusion limits at 95% confidence level on the mass of third-generation squarks are derived in various supersymmetry-inspired simplified models.

Search for dark matter produced in association with a Higgs boson decaying to bb¯ using 36 fb-1 of pp collisions at √s =13 TeV with the ATLAS detector

Fri 16 Nov 2018 12:44:26 KRAT

Several extensions of the standard model predict associated production of dark-matter particles with a Higgs boson. Such processes are searched for in final states with missing transverse momentum and a Higgs boson decaying to a b¯b pair with the ATLAS detector using 36.1 fb−1 of pp collisions at a center-of-mass energy of 13 TeV at the LHC. The observed data are in agreement with the standard model predictions and limits are placed on the associated production of dark-matter particles and a Higgs boson.

Measurement of the Drell-Yan triple-differential cross section in pp collisions at √s=8 TeV

Fri 16 Nov 2018 12:37:24 KRAT

This paper presents a measurement of the triple-differential cross section for the Drell-Yan process Z/γ* → ℓ+ℓ− where ℓ is an electron or a muon. The measurement is performed for invariant masses of the lepton pairs, m ℓℓ , between 46 and 200 GeV using a sample of 20.2 fb−1 of pp collisions data at a centre-of-mass energy of sqrt{s}=8 TeV collected by the ATLAS detector at the LHC in 2012. The data are presented in bins of invariant mass, absolute dilepton rapidity, |yℓℓ|, and the angular variable cos θ* between the outgoing lepton and the incoming quark in the Collins-Soper frame. The measurements are performed in the range |y ℓℓ | < 2.4 in the muon channel, and extended to |y ℓℓ | < 3.6 in the electron channel. The cross sections are used to determine the Z boson forward-backward asymmetry as a function of |y ℓℓ | and m ℓℓ . The measurements achieve high-precision, below the percent level in the pole region, excluding the uncertainty in the integrated luminosity, and are in agreement with predictions. These precision data are sensitive to the parton distribution functions and the effective weak mixing angle.

Combination of searches for heavy resonances decaying into bosonic and leptonic final states using 36 fb−1 of proton-proton collision data at √s=13 TeV with the ATLAS detector

Fri 14 Jun 2019 09:37:39 KRAT

Searches for new heavy resonances decaying into different pairings of W, Z, or Higgs bosons, as well as directly into leptons, are presented using a data sample corresponding to 36.1 fb−1 of pp collisions at √s=13 TeV collected during 2015 and 2016 with the ATLAS detector at the CERN Large Hadron Collider. Analyses selecting bosonic decay modes in the qqqq, ννqq, ℓνqq, ℓℓqq, ℓνℓν, ℓℓνν, ℓνℓℓ, ℓℓℓℓ, qqbb, ννbb, ℓνbb, and ℓℓbb final states are combined, searching for a narrow-width resonance. Likewise, analyses selecting the leptonic ℓν and ℓℓ final states are also combined. These two sets of analyses are then further combined. No significant deviation from the Standard Model predictions is observed. Three benchmark models are tested: a model predicting the existence of a new heavy scalar singlet, a simplified model predicting a heavy vector-boson triplet, and a bulk Randall-Sundrum model with a heavy spin-2 Kaluza-Klein excitation of the graviton. Cross section limits are set at the 95% confidence level using an asymptotic approximation and are compared with predictions for the benchmark models. These limits are also expressed in terms of constraints on couplings of the heavy vector-boson triplet to quarks, leptons, and the Higgs boson. The data exclude a heavy vector-boson triplet with mass below 5.5 TeV in a weakly coupled scenario and 4.5 TeV in a strongly coupled scenario, as well as a Kaluza-Klein graviton with mass below 2.3 TeV.

Search for lepton-flavor-violating decays of the Z boson into a τ lepton and a light lepton with the ATLAS detector

Fri 14 Jun 2019 09:30:37 KRAT

Direct searches for lepton flavor violation in decays of the Z boson with the ATLAS detector at the LHC are presented. Decays of the Z boson into an electron or muon and a hadronically decaying τ lepton are considered. The searches are based on a data sample of proton-proton collisions collected by the ATLAS detector in 2015 and 2016, corresponding to an integrated luminosity of 36.1 fb−1 at a center-of-mass energy of √s=13 TeV. No statistically significant excess of events above the expected background is observed, and upper limits on the branching ratios of lepton-flavor-violating decays are set at the 95% confidence level: B(Z→eτ)<5.8×10−5 and B(Z→μτ)<2.4×10−5. This is the first limit on B(Z→eτ) with ATLAS data. The upper limit on B(Z→μτ) is combined with a previous ATLAS result based on 20.3 fb−1 of proton-proton collision data at a center-of-mass energy of √s=8 TeV and the combined upper limit at 95% confidence level is B(Z→μτ)<1.3×10−5.

Search for Higgs boson pair production in the γγb¯b final state with 13 TeV pp collision data collected by the ATLAS experiment

Fri 14 Jun 2019 09:30:32 KRAT

A search is performed for resonant and non-resonant Higgs boson pair production in the γγbb¯¯γγbb¯final state. The data set used corresponds to an integrated luminosity of 36.1 fb−1 of proton-proton collisions at a centre-of-mass energy of 13 TeV recorded by the ATLAS detector at the CERN Large Hadron Collider. No significant excess relative to the Standard Model expectation is observed. The observed limit on the non-resonant Higgs boson pair cross-section is 0.73 pb at 95% confidence level. This observed limit is equivalent to 22 times the predicted Standard Model cross-section. The Higgs boson self-coupling (κλ = λHHH/λ HHH SM ) is constrained at 95% confidence level to −8.2 < κλ < 13.2. For resonant Higgs boson pair production through X→HH→γγbb¯¯X→HH→γγbb¯, the limit is presented, using the narrow-width approximation, as a function of mX in the range 260 GeV < mX < 1000 GeV. The observed limits range from 1.1 pb to 0.12 pb over this mass range.

Search for long-lived neutral particles decaying into lepton jets in proton-proton collisions at √s=8 TeV with the ATLAS detector

Fri 14 Dec 2018 09:43:16 KRAT

Several models of physics beyond the Standard Model predict neutral particles that decay into final states consisting of collimated jets of light leptons and hadrons (socalled “lepton jets”). These particles can also be long-lived with decay length comparable to, or even larger than, the LHC detectors’ linear dimensions. This paper presents the results of a search for lepton jets in proton-proton collisions at the centre-of-mass energy of √ s = 8 TeV in a sample of 20.3 fb−1 collected during 2012 with the ATLAS detector at the LHC. Limits on models predicting Higgs boson decays to neutral long-lived lepton jets are derived as a function of the particle’s proper decay length.

Search for flavour-changing neutral current top quark decays t → Hq in pp collisions at √s = 8 TeV with the ATLAS detector

Fri 11 May 2018 15:41:27 KRAT

A search for flavour-changing neutral current decays of a top quark to an uptype quark (q = u, c) and the Standard Model Higgs boson, where the Higgs boson decays to bb¯¯ , is presented. The analysis searches for top quark pair events in which one top quark decays to Wb, with the W boson decaying leptonically, and the other top quark decays to Hq. 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 uses 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 and at least four jets. The search exploits the high multiplicity of b-quark jets characteristic of signal events, and employs a likelihood discriminant that uses the kinematic differences between the signal and the background, which is dominated by tt¯→WbWb decays. No significant excess of events above the background expectation is found, and observed (expected) 95% CL upper limits of 0.56% (0.42%) and 0.61% (0.64%) are derived for the t → Hc and t → Hu branching ratios respectively. The combination of this search with other ATLAS searches in the H → γγ and H → WW *, ττ decay modes significantly improves the sensitivity, yielding observed (expected) 95% CL upper limits on the t → Hc and t → Hu branching ratios of 0.46% (0.25%) and 0.45% (0.29%) respectively. The corresponding combined observed (expected) upper limits on the |λ tcH | and |λ tuH | couplings are 0.13 (0.10) and 0.13 (0.10) respectively. These are the most restrictive direct bounds on tqH interactions measured so far.

Search for lepton-flavour-violating H → μτ decays of the Higgs boson with the ATLAS detector

Fri 07 Dec 2018 10:40:45 KRAT

A direct search for lepton-flavour-violating H → μτ decays of the recently discovered Higgs boson with the ATLAS detector at the LHC is presented. The analysis is performed in the H → μτ had channel, where τ had is a hadronically decaying τ -lepton. The search is based on the data sample of proton-proton collisions collected by the ATLAS experiment corresponding to an integrated luminosity of 20.3 fb−1 at a centre-of-mass energy of s√=8s=8 TeV. No statistically significant excess of data over the predicted background is observed. The observed (expected) 95% confidence-level upper limit on the branching fraction, Br(H → μτ ), is 1.85% (1.24%).

Search for dark matter in events with heavy quarks and missing transverse momentum in pp collisions with the ATLAS detector

Fri 07 Dec 2018 10:33:13 KRAT

This article reports on a search for dark matter pair production in association with bottom or top quarks in 20.3 fb−1 of pp collisions collected at √s = 8 TeV by the ATLAS detector at the LHC. Eventswith largemissing transverse momentum are selected when produced in association with high-momentum jets of which one or more are identified as jets containing b-quarks. Final states with top quarks are selected by requiring a high jet multiplicity and in some cases a single lepton. The data are found to be consistent with the Standard Model expectations and limits are set on the mass scale of effective field theories that describe scalar and tensor interactions between dark matter and Standard Model particles. Limits on the dark-matter–nucleon cross-section for spin-independent and spin-dependent interactions are also provided. These limits are particularly strong for low-mass dark matter. Using a simplified model, constraints are set on the mass of dark matter and of a coloured mediator suitable to explain a possible signal of annihilating dark matter.

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

Fri 07 Dec 2018 10:19:05 KRAT

The final ATLAS Run 1 measurements of Higgs boson production and couplings in the decay channel H → ZZ → lþl−l0þl0−, where l, l0 ¼ e or μ, are presented. These measurements were performed using pp collision data corresponding to integrated luminosities of 4.5 and 20.3 fb−1 at center-of-mass energies of 7 and 8 TeV, respectively, recorded with the ATLAS detector at the LHC. The H → ZZ → 4l signal is