Measurement of the energy asymmetry in tt¯j production at 13 TeV with the ATLAS experiment and interpretation in the SMEFT framework

Wed 31 May 2023 15:31:20 KRAT

A measurement of the energy asymmetry in jet-associated top-quark pair production is presented using 139 fb(-1) of data collected by the ATLAS detector at the Large Hadron Collider during pp collisions at root s = 13 TeV. The observable measures the different probability of top and antitop quarks to have the higher energy as a function of the jet scattering angle with respect to the beam axis. The energy asymmetry is measured in the semileptonic t (t) over bar decay channel, and the hadronically decaying top quark must have transverse momentum above 350 GeV. The results are corrected for detector effects to particle level in three bins of the scattering angle of the associated jet. The measurement agrees with the SM prediction at next-to-leading-order accuracy in quantum chromodynamics in all three bins. In the bin with the largest expected asymmetry, where the jet is emitted perpendicular to the beam, the energy asymmetry is measured to be -0.043 +/- 0.020, in agreement with the SM prediction of -0.037 +/- 0.003. Interpreting this result in the framework of the Standard Model effective field theory (SMEFT), it is shown that the energy asymmetry is sensitive to the top-quark chirality in four-quark operators and is therefore a valuable new observable in global SMEFT fits.

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 jet activity produced in top-quark events with an electron, a muon and two b-tagged jets in the final state in pp collisions at √s=13 TeV with the ATLAS detector

Wed 29 Nov 2017 10:02:01 KRAT

Measurements of jet activity in top-quark pair events produced in proton–proton collisions are presented, using 3.2 fb−1 of pp collision data at a centre-of-mass energy of 13 TeV collected by the ATLAS experiment at the Large Hadron Collider. Events are chosen by requiring an opposite-charge eμ pair and two b-tagged jets in the final state. The normalised differential cross-sections of top-quark pair production are presented as functions of additional-jet multiplicity and transverse momentum, pT. The fraction of signal events that do not contain additional jet activity in a given rapidity region, the gap fraction, is measured as a function of the pT threshold for additional jets, and is also presented for different invariant mass regions of the eμbb¯ system. All measurements are corrected for detector effects and presented as particle-level distributions compared to predictions with different theoretical approaches for QCD radiation. While the kinematics of the jets from top-quark decays are described well, the generators show differing levels of agreement with the measurements of observables that depend on the production of additional jets.

Search for long-lived charginos based on a disappearing-track signature using 136 fb−1 of pp collisions at √s = 13 TeV with the ATLAS detector

Wed 10 May 2023 10:26:37 KRAT

A search for long-lived charginos produced either directly or in the cascade decay of heavy prompt gluino states is presented. The search is based on proton- proton collision data collected at a centre-of-mass energy of root s = 13 TeV between 2015 and 2018 with the ATLAS detector at the LHC, corresponding to an integrated luminosity of 136 fb(-1). Long-lived charginos are characterised by a distinct signature of a short and then disappearing track, and are reconstructed using at least four measurements in the ATLAS pixel detector, with no subsequent measurements in the silicon-microstrip tracking volume nor any associated energy deposits in the calorimeter. The final state is complemented by a large missing transverse-momentum requirement for triggering purposes and at least one high-transversemomentum jet. No excess above the expected backgrounds is observed. Exclusion limits are set at 95% confidence level on the masses of the chargino and gluino for different chargino lifetimes. Chargino masses up to 660 (210) GeV are excluded in scenarios where the chargino is a pure wino (higgsino). For charginos produced during the cascade decay of a heavy gluino, gluinos with masses below 2.1 TeV are excluded for a chargino mass of 300 GeV and a lifetime of 0.2 ns.

Two-particle Bose–Einstein correlations in pp collisions at s√=13 TeV measured with the ATLAS detector at the LHC

Wed 10 May 2023 10:26:33 KRAT

This paper presents studies of Bose-Einstein correlations (BEC) in proton-proton collisions at a centre-of-mass energy of 13 TeV, using data from the ATLAS detector at the CERN Large Hadron Collider. Data were collected in a special low-luminosity configuration with a minimum-bias trigger and a high-multiplicity track trigger, accumulating integrated luminosities of 151 mu b(-1) and 8.4 nb(-1), respectively. The BEC are measured for pairs of like-sign charged particles, each with vertical bar eta vertical bar < 2.5, for two kinematic ranges: the first with particle pr > 100 MeV and the second with particle pr > 500 MeV. The BEC parameters, characterizing the source radius and particle correlation strength, are investigated as functions of charged-particle multiplicity (up to 300) and average transverse momentum of the pair (up to 1.5 GeV). The double-differential dependence on charged-particle multiplicity and average transverse momentum of the pair is also studied. The BEC radius is found to be independent of the charged-particle multiplicity for high charged-particle multiplicity (above 100), confirming a previous observation at lower energy. This saturation occurs independent of the transverse momentum of the pair.

Measurement of the energy response of the ATLAS calorimeter to charged pions from W ± → τ ±(→ π±ντ)ντ events in Run 2 data

Wed 07 Jun 2023 15:15:32 KRAT

The energy response of the ATLAS calorimeter is measured for single charged pions with transverse momentum in the range 10

Measurement of the W boson polarisation in tt¯ events from pp collisions at √s = 8 TeV in the lepton + jets channel with ATLAS

Wed 04 Oct 2017 10:13:10 KRAT

This paper presents a measurement of the polarisation of W bosons from tar{t} decays, reconstructed in events with one high-p_{ ext{ T }} lepton and at least four jets. Data from pp collisions at the LHC were collected at sqrt{s} = 8 TeV and correspond to an integrated luminosity of 20.2 fb^{-1}. The angle heta ^{*} between the b-quark from the top quark decay and a direct W boson decay product in the W boson rest frame is sensitive to the W boson polarisation. Two different W decay products are used as polarisation analysers: the charged lepton and the down-type quark for the leptonically and hadronically decaying W boson, respectively. The most precise measurement of the W boson polarisation via the distribution of cos { heta ^{*}} is obtained using the leptonic analyser and events in which at least two of the jets are tagged as b-quark jets. The fitted fractions of longitudinal, left- and right-handed polarisation states are F_{mathrm {0}}=~0.709~pm ~{0.019}, F_{mathrm {L}}=~0.299~pm ~{0.015} and F_{mathrm {R}}=~-0.008~pm ~{0.014}, and are the most precisely measured W boson polarisation fractions to date. Limits on anomalous couplings of the Wtb vertex are set.

Measurement of jet fragmentation in Pb+Pb and pp collisions at √sNN=2.76 TeV with the ATLAS detector at the LHC

Wed 04 Oct 2017 10:13:05 KRAT

The distributions of transverse momentum and longitudinal momentum fraction of charged particles in jets are measured in Pb+Pb and pp collisions with the ATLAS detector at the LHC. The distributions are measured as a function of jet transverse momentum and rapidity. The analysis utilises an integrated luminosity of 0.14 nb^{-1} of Pb+Pb data and 4.0 pb^{-1} of pp data collected in 2011 and 2013, respectively, at the same centre-of-mass energy of 2.76 TeV per colliding nucleon pair. The distributions measured in pp collisions are used as a reference for those measured in Pb+Pb collisions in order to evaluate the impact on the internal structure of jets from the jet energy loss of fast partons propagating through the hot, dense medium created in heavy-ion collisions. Modest but significant centrality-dependent modifications of fragmentation functions in Pb+Pb collisions with respect to those in pp collisions are seen. No significant dependence of modifications on jet p_mathrm{T} and rapidity selections is observed except for the fragments with the highest transverse momenta for which some reduction of yields is observed for more forward jets.

Study of B+ c → J/ψD+ s and B+ c → J/ψD∗+ s decays in pp collisions at √s = 13 TeV with the ATLAS detector

Tue 25 Apr 2023 09:42:17 KRAT

A study of B-c(+) -> J/psi D-s(+) and B-c(+)-> J/psi D-s*(+) decays using 139 fb(-1) of in- tegrated luminosity collected with the ATLAS detector from root s = 13 TeV pp collisions at the LHC is presented. The ratios of the branching fractions of the two decays to the branching fraction of the B-c(+) -> J/psi pi(+) decay are measured: B(B-c(+) -> J/psi D-s(+))/B(B-c(+) -> J/psi pi(+)) = 2.76 +/- 0.47 and B(B-c(+)-> J/psi D-s*(+))/B(B-c(+) -> J/psi pi(+)) = 5.33 +/- 0.96. The ratio of the branching fractions of the two decays is found to be B(B-c(+)-> J/psi D-s*(+))/B(B-c(+) -> J/psi D-s(+)) = 1.93 +/- 0.26. For the B-c(+)-> J/psi D-s*(+) decay, the transverse polarization fraction, Gamma(+/-+/-)/Gamma, is measured to be 0.70 +/- 0.11. The reported uncertainties include both the statistical and systematic components added in quadrature. The precision of the measurements exceeds that in all previous studies of these decays. These results supersede those obtained in the earlier ATLAS study of the same decays with root s = 7 and 8 TeV pp collision data. A comparison with available theoretical predictions for the measured quantities is presented.

Measurements of Higgs boson production cross-sections in the H → τ +τ − decay channel in pp collisions at √s = 13 TeV with the ATLAS detector

Tue 25 Apr 2023 09:35:14 KRAT

Measurements of the production cross-sections of the Standard Model (SM) Higgs boson (H) decaying into a pair of tau -leptons are presented. The measurements use data collected with the ATLAS detector from pp collisions produced at the Large Hadron Collider at a centre-of-mass energy of p root s = 13TeV, corresponding to an integrated luminosity of 139 fb-1. Leptonic ( tau -> l upsilon(l)upsilon(tau)) and hadronic ( tau -> hadrons upsilon tau) decays of the tau -lepton are considered. All measurements account for the branching ratio of H -> tau tau and are performed with a requirement |yH| < 2.5, where yH is the true Higgs boson rapidity. The cross-section of the pp -> H -> tau tau process is measured to be 2.94 +/- 0.21(stat)+ 0.37 - 0.32(syst) pb, in agreement with the SM prediction of 3.17 +/- 0.09 pb. Inclusive cross-sections are determined separately for the four dominant production modes: 2.65 +/- 0.41(stat)+ 0.91 - 0.67(syst) pb for gluon-gluon fusion, 0.197 +/- 0.028(stat)+ 0.032 - 0.026(syst) pb for vectorboson fusion, 0.115 +/- 0.058(stat)+ 0.042 - 0.040(syst) pb for vector-boson associated production, and 0.033 +/- 0.031(stat)+ 0.022 - 0.017(syst) pb for top-quark pair associated production. Measurements in exclusive regions of the phase space, using the simplified template cross-section framework, are also performed. All results are in agreement with the SM predictions.

Search for new phenomena in three- or four-lepton events in pp collisions at √s = 13 TeV with the ATLAS detector

Tue 20 Jun 2023 23:18:36 KRAT

Combination of inclusive and differential t¯t charge asymmetry measurements using ATLAS and CMS data at √s = 7 and 8 TeV

Tue 11 Jun 2019 15:32:30 KRAT

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%.

Measurement of lepton differential distributions and the top quark mass in tt¯ production in pp collisions at √s=8 TeV with the ATLAS detector

Tue 06 Nov 2018 14:05:12 KRAT

This paper presents single lepton and dilepton kinematic distributions measured in dileptonic tar{t} events produced in 20.2 hbox {fb}^{-1} of sqrt{s}=8 TeV pp collisions recorded by the ATLAS experiment at the LHC. Both absolute and normalised differential cross-sections are measured, using events with an opposite-charge emu pair and one or two b-tagged jets. The cross-sections are measured in a fiducial region corresponding to the detector acceptance for leptons, and are compared to the predictions from a variety of Monte Carlo event generators, as well as fixed-order QCD calculations, exploring the sensitivity of the cross-sections to the gluon parton distribution function. Some of the distributions are also sensitive to the top quark pole mass; a combined fit of NLO fixed-order predictions to all the measured distributions yields a top quark mass value of {m_t^{mathrm {pole}}}=173.2pm 0.9pm 0.8pm 1.2 GeV, where the three uncertainties arise from data statistics, experimental systematics, and theoretical sources.

Determination of the strong coupling constant αs from transverse energy–energy correlations in multijet events at √s=8 TeV using the ATLAS detector

Tue 06 Nov 2018 14:05:08 KRAT

Measurements of transverse energy–energy correlations and their associated asymmetries in multi-jet events using the ATLAS detector at the LHC are presented. The data used correspond to sqrt{s} = 8~hbox {TeV} proton–proton collisions with an integrated luminosity of 20.2 ~hbox {fb}^{-1}. The results are presented in bins of the scalar sum of the transverse momenta of the two leading jets, unfolded to the particle level and compared to the predictions from Monte Carlo simulations. A comparison with next-to-leading-order perturbative QCD is also performed, showing excellent agreement within the uncertainties. From this comparison, the value of the strong coupling constant is extracted for different energy regimes, thus testing the running of alpha _{mathrm {s}}(mu ) predicted in QCD up to scales over 1~hbox {TeV}. A global fit to the transverse energy–energy correlation distributions yields alpha _{mathrm {s}}(m_Z) = 0.1162 pm 0.0011 ext{(exp.) }^{+0.0084}_{-0.0070} ext{(theo.) }, while a global fit to the asymmetry distributions yields a value of alpha _{mathrm {s}}(m_Z) = 0.1196 pm 0.0013 ext{(exp.) }^{+0.0075}_{-0.0045} ext{(theo.) }

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.

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

Measurement of the tt¯Z and tt¯W production cross sections in multilepton final states using 3.2 fb-1 of pp collisions at √s = 13 TeV with the ATLAS detector

Tue 03 Oct 2017 15:26:07 KRAT

A measurement of the tˉtZ and tˉtW production cross sections in final states with either two same-charge muons, or three or four leptons (electrons or muons) is presented. The analysis uses a data sample of proton–proton collisions at √s=13 TeV recorded with the ATLAS detector at the Large Hadron Collider in 2015, corresponding to a total integrated luminosity of 3.2 fb−1. The inclusive cross sections are extracted using likelihood fits to signal and control regions, resulting in σtˉtZ=0.9±0.3 pb and σtˉtW=1.5±0.8 pb, in agreement with the Standard Model predictions.

A measurement of the calorimeter response to single hadrons and determination of the jet energy scale uncertainty using LHC Run-1 pp-collision data with the ATLAS detector

Tue 03 Oct 2017 15:26:01 KRAT

A measurement of the calorimeter response to isolated charged hadrons in the ATLAS detector at the LHC is presented. This measurement is performed with 3.2 nb−1 of proton–proton collision data at s√=7 TeV from 2010 and 0.1 nb−1 of data at s√=8 TeV from 2012. A number of aspects of the calorimeter response to isolated hadrons are explored. After accounting for energy deposited by neutral particles, there is a 5% discrepancy in the modelling, using various sets of Geant4 hadronic physics models, of the calorimeter response to isolated charged hadrons in the central calorimeter region. The description of the response to anti-protons at low momenta is found to be improved with respect to previous analyses. The electromagnetic and hadronic calorimeters are also examined separately, and the detector simulation is found to describe the response in the hadronic calorimeter well. The jet energy scale uncertainty and correlations in scale between jets of different momenta and pseudorapidity are derived based on these studies. The uncertainty is 2–5% for jets with transverse momenta above 2 TeV, where this method provides the jet energy scale uncertainty for ATLAS.

Measurement of the prompt J/ψ pair production cross-section in pp collisions at √s=8 TeV with the ATLAS detector

Tue 03 Oct 2017 14:16:05 KRAT

The production of two prompt J/ψ mesons, each with transverse momenta pT>8.5 GeV and rapidity |y|<2.1, is studied using a sample of proton-proton collisions at √s=8 TeV, corresponding to an integrated luminosity of 11.4 fb−1 collected in 2012 with the ATLAS detector at the LHC. The differential cross-section, assuming unpolarised J/ψ production, is measured as a function of the transverse momentum of the lower-pTJ/ψ meson, di-J/ψpT and mass, the difference in rapidity between the two J/ψ mesons, and the azimuthal angle between the two J/ψ mesons. The fraction of prompt pair events due to double parton scattering is determined by studying kinematic correlations between the two J/ψ mesons. The total and double parton scattering cross-sections are compared with predictions. The effective cross-section of double parton scattering is measured to be σeff=6.3±1.6(stat)±1.0(syst) mb.

Measurement of inclusive and differential cross sections in the H → ZZ * → 4ℓ decay channel in pp collisions at √s=13 TeV with the ATLAS detector

Thu 22 Nov 2018 09:54:26 KRAT

Inclusive and differential fiducial cross sections of Higgs boson production in proton-proton collisions are measured in the H → ZZ * → 4ℓ decay channel. The proton-proton collision data were produced at the Large Hadron Collider at a centre-of-mass energy of 13 TeV and recorded by the ATLAS detector in 2015 and 2016, corresponding to an integrated luminosity of 36.1 fb−1. The inclusive fiducial cross section in the H → ZZ * → 4ℓ decay channel is measured to be 3.62 ± 0.50(stat) − 0.20 + 0.25 (sys) fb, in agreement with the Standard Model prediction of 2.91 ± 0.13 fb. The cross section is also extrapolated to the total phase space including all Standard Model Higgs boson decays. Several differential fiducial cross sections are measured for observables sensitive to the Higgs boson production and decay, including kinematic distributions of jets produced in association with the Higgs boson. Good agreement is found between data and Standard Model predictions. The results are used to put constraints on anomalous Higgs boson interactions with Standard Model particles, using the pseudo-observable extension to the kappa-framework.

Search for type-III seesaw heavy leptons in leptonic final states in pp collisions at √s = 13 TeV with the ATLAS detector

Thu 21 Dec 2023 15:16:29 KRAT

A search for the pair production of heavy leptons as predicted by the type-III seesaw mechanism is presented. The search uses proton-proton collision data at a centre-of-mass energy of 13 TeV, corresponding to 139 fb(-1) of integrated luminosity recorded by the ATLAS detector during Run 2 of the Large Hadron Collider. The analysis focuses on final states with three or four electrons or muons from the possible decays of new heavy leptons via intermediate electroweak bosons. No significant deviations above the Standard Model expectation are observed; upper and lower limits on the heavy lepton production cross-section and masses are derived respectively. These results are then combined for the first time with the ones already published by ATLAS using the channel with two leptons in the final state. The observed lower limit on the mass of the type-III seesaw heavy leptons combining two, three and four lepton channels together is 910 GeV at the 95% confidence level.

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

Thu 20 Jun 2019 10:52:52 KRAT

Measurements of differential cross sections of top quark pair production in association with jets in pp collisions at √s = 13 TeV using the ATLAS detector

Thu 20 Jun 2019 10:52:46 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 supersymmetry in final states with charm jets and missing transverse momentum in 13 TeV pp collisions with the ATLAS detector

Thu 20 Jun 2019 10:52:39 KRAT

Search for the Higgs boson produced in association with a vector boson and decaying into two spin-zero particles in the H → aa → 4b channel in pp collisions at √s=13 TeV with the ATLAS detector

Thu 20 Jun 2019 10:52:33 KRAT

Measurements of t¯t differential cross-sections of highly boosted top quarks decaying to all-hadronic final states in pp collisions at √s=13 TeV using the ATLAS detector

Thu 20 Jun 2019 10:52:32 KRAT

Measurements are made of differential cross-sections of highly boosted pair-produced top quarks as a function of top-quark and t¯t system kinematic observables using proton-proton collisions at a center-of-mass energy of √s=13 TeV. The data set corresponds to an integrated luminosity of 36.1 fb−1, recorded in 2015 and 2016 with the ATLAS detector at the CERN Large Hadron Collider. Events with two large-radius jets in the final state, one with transverse momentum pT>500 GeV and a second with pT>350 GeV, are used for the measurement. The top-quark candidates are separated from the multijet background using jet substructure information and association with a b-tagged jet. The measured spectra are corrected for detector effects to a particle-level fiducial phase space and a parton-level limited phase space, and are compared to several Monte Carlo simulations by means of calculated χ2 values. The cross-section for t¯t production in the fiducial phase-space region is 292±7(stat)±71(syst) fb, to be compared to the theoretical prediction of 384±36 fb.

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

Thu 20 Jun 2019 10:38:53 KRAT

Search for high-mass resonances decaying to τν in pp collisions at √s=13 TeV with the ATLAS detector

Thu 20 Jun 2019 10:38:40 KRAT

Probing the quantum interference between singly and doubly resonant top-quark production in pp collisions at √s=13 TeV with the ATLAS detector

Thu 20 Jun 2019 09:42:53 KRAT

This Letter presents a normalized differential cross-section measurement in a fiducial phase-space region where interference effects between top-quark pair production and associated production of a single top quark with a W boson and a b-quark are significant. Events with exactly two leptons (ee, μμ, or eμ) and two b-tagged jets that satisfy a multiparticle invariant mass requirement are selected from 36.1 fb−1 of proton-proton collision data taken at √s=13 TeV with the ATLAS detector at the LHC in 2015 and 2016. The results are compared with predictions from simulations using various strategies for the interference. The standard prescriptions for interference modeling are significantly different from each other but are within 2σ of the data. State-of-the-art predictions that naturally incorporate interference effects provide the best description of the data in the measured region of phase space most sensitive to these effects. These results provide an important constraint on interference models and will guide future model development and tuning.

ZZ → ℓ+ℓ-ℓ'+ℓ' − cross-section measurements and search for anomalous triple gauge couplings in 13 TeV pp collisions with the ATLAS detector

Thu 20 Jun 2019 09:42:50 KRAT

Measurements of ZZ production in the ℓ+ℓ−ℓ'+ℓ'− channel in proton–proton collisions at 13 TeV center-of-mass energy at the Large Hadron Collider are presented. The data correspond to 36.1 fb−1of collisions collected by the ATLAS experiment in 2015 and 2016. Here ℓ and ℓ′ stand for electrons or muons. Integrated and differential ZZ→ℓ+ℓ−ℓ'+ℓ'− cross sections with Z→ℓ+ℓ− candidate masses in the range of 66 GeV to 116 GeV are measured in a fiducial phase space corresponding to the detector acceptance and corrected for detector effects. The differential cross sections are presented in bins of twenty observables, including several that describe the jet activity. The integrated cross section is also extrapolated to a total phase space and to all standard model decays of Z bosons with mass between 66 GeV and 116 GeV, resulting in a value of 17.3±0.9[±0.6(stat)±0.5(syst)±0.6(lumi)] pb. The measurements are found to be in good agreement with the standard model. A search for neutral triple gauge couplings is performed using the transverse momentum distribution of the leading Z boson candidate. No evidence for such couplings is found and exclusion limits are set on their parameters.

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 charged Higgs bosons decaying into top and bottom quarks at √s=13 TeV with the ATLAS detector

Thu 20 Jun 2019 09:42:41 KRAT

A search for charged Higgs bosons heavier than the top quark and decaying via H± → tb is presented. The data analysed corresponds to 36.1 fb−1 of pp collisions at s√=13 TeV and was recorded with the ATLAS detector at the LHC in 2015 and 2016. The production of a charged Higgs boson in association with a top quark and a bottom quark, pp → tbH±, is explored in the mass range from mH± = 200 to 2000 GeV using multi-jet final states with one or two electrons or muons. Events are categorised according to the multiplicity of jets and how likely these are to have originated from hadronisation of a bottom quark. Multivariate techniques are used to discriminate between signal and background events. No significant excess above the background-only hypothesis is observed and exclusion limits are derived for the production cross-section times branching ratio of a charged Higgs boson as a function of its mass, which range from 2.9 pb at mH± = 200 GeV to 0.070 pb at mH± = 2000 GeV. The results are interpreted in two benchmark scenarios of the Minimal Supersymmetric Standard Model.

Search for dark matter in events with a hadronically decaying vector boson and missing transverse momentum in pp collisions at √s = 13 TeV with the ATLAS detector

Thu 20 Jun 2019 09:42:36 KRAT

A search for dark matter (DM) particles produced in association with a hadronically decaying vector boson is performed using pp collision data at a centre-of-mass energy of s√=13 TeV corresponding to an integrated luminosity of 36.1 fb−1, recorded by the ATLAS detector at the Large Hadron Collider. This analysis improves on previous searches for processes with hadronic decays of W and Z bosons in association with large missing transverse momentum (mono-W/Z searches) due to the larger dataset and further optimization of the event selection and signal region definitions. In addition to the mono-W/Z search, the as yet unexplored hypothesis of a new vector boson Z′ produced in association with dark matter is considered (mono-Z′ search). No significant excess over the Standard Model prediction is observed. The results of the mono-W/Z search are interpreted in terms of limits on invisible Higgs boson decays into dark matter particles, constraints on the parameter space of the simplified vector-mediator model and generic upper limits on the visible cross sections for W/Z+DM production. The results of the mono-Z′ search are shown in the framework of several simplified-model scenarios involving DM production in association with the Z′ boson.

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.

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 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 top-squark pair production in final states with one lepton, jets, and missing transverse momentum using 36 fb−1 of √s=13 TeV pp collision data with the ATLAS detector

Thu 20 Jun 2019 09:35:37 KRAT

The results of a search for the direct pair production of top squarks, the supersymmetric partner of the top quark, in final states with one isolated electron or muon, several energetic jets, and missing transverse momentum are reported. The analysis also targets spin-0 mediator models, where the mediator decays into a pair of dark-matter particles and is produced in association with a pair of top quarks. The search uses data from proton-proton collisions delivered by the Large Hadron Collider in 2015 and 2016 at a centre-of-mass energy of s√=13 TeV and recorded by the ATLAS detector, corresponding to an integrated luminosity of 36 fb−1. A wide range of signal scenarios with different mass-splittings between the top squark, the lightest neutralino and possible intermediate supersymmetric particles are considered, including cases where the W bosons or the top quarks produced in the decay chain are off-shell. No significant excess over the Standard Model prediction is observed. The null results are used to set exclusion limits at 95% confidence level in several supersymmetry benchmark models. For pair-produced top-squarks decaying into top quarks, top-squark masses up to 940 GeV are excluded. Stringent exclusion limits are also derived for all other considered top-squark decay scenarios. For the spin-0 mediator models, upper limits are set on the visible cross-section.

Search for charged Higgs bosons decaying via H± → τ±ντ in the τ+jets and τ+lepton final states with 36 fb−1 of pp collision data recorded at √s=13 TeV with the ATLAS experiment

Thu 20 Jun 2019 09:35:33 KRAT

Charged Higgs bosons produced either in top-quark decays or in association with a top-quark, subsequently decaying via H± → τ±ντ, are searched for in 36.1 fb−1 of proton-proton collision data at s√=13s=13 TeV recorded with the ATLAS detector. Depending on whether the top-quark produced together with H± decays hadronically or leptonically, the search targets τ+jets and τ+lepton final states, in both cases with a hadronically decaying τ-lepton. No evidence of a charged Higgs boson is found. For the mass range of mH±mH± = 90–2000 GeV, upper limits at the 95% confidence level are set on the production cross-section of the charged Higgs boson times the branching fraction B(H±→τ±ντ)B(H±→τ±ντ) in the range 4.2–0.0025 pb. In the mass range 90–160 GeV, assuming the Standard Model cross-section for tt¯tt¯ production, this corresponds to upper limits between 0.25% and 0.031% for the branching fraction B(t→bH±)×B(H±→τ±ντ)B(t→bH±)×B(H±→τ±ντ).

Search for events with a pair of displaced vertices from long-lived neutral particles decaying into hadronic jets in the ATLAS muon spectrometer in pp collisions at √s=13 TeV

Thu 20 Apr 2023 10:24:31 KRAT

Measurements of jet observables sensitive to b-quark fragmentation in tt events at the LHC with the ATLAS detector

Thu 20 Apr 2023 10:24:28 KRAT

Observation of WWW production in pp collisions at √s=13 TeV with the ATLAS detector

Thu 20 Apr 2023 09:49:24 KRAT

This Letter reports the observation of WWW production and a measurement of its cross section using detector at the Large Hadron Collider. Events with two same-sign leptons (electrons or muons) and at least two jets, as well as events with three charged leptons, are selected. A multivariate technique is then used to discriminate between signal and background events. Events from WWW production are observed with a significance of 8.0 standard deviations, where the expectation is 5.4 standard deviations. The inclusive WWW production cross section is measured to be 820 ??? 100 ??stat?? ??? 80 ??syst?? fb, approximately 2.6 standard deviations from the predicted cross section of 511 ??? 18 fb calculated at next-to-leading-order QCD and leading-order electroweak accuracy.

Measurement of the inclusive jet cross-sections in proton-proton collisions at √s=8 TeV with the ATLAS detector

Thu 15 Feb 2018 09:35:56 KRAT

Inclusive jet production cross-sections are measured in proton-proton collisions at a centre-of-mass energy of s√=8s=8 TeV recorded by the ATLAS experiment at the Large Hadron Collider at CERN. The total integrated luminosity of the analysed data set amounts to 20.2 fb−1. Double-differential cross-sections are measured for jets defined by the anti-kt jet clustering algorithm with radius parameters of R = 0.4 and R = 0.6 and are presented as a function of the jet transverse momentum, in the range between 70 GeV and 2.5 TeV and in six bins of the absolute jet rapidity, between 0 and 3.0. The measured cross-sections are compared to predictions of quantum chromodynamics, calculated at next-to-leading order in perturbation theory, and corrected for non-perturbative and electroweak effects. The level of agreement with predictions, using a selection of different parton distribution functions for the proton, is quantified. Tensions between the data and the theory predictions are observed.

Identification and rejection of pile-up jets at high pseudorapidity with the ATLAS detector

Thu 15 Feb 2018 09:35:43 KRAT

The rejection of forward jets originating from additional proton–proton interactions (pile-up) is crucial for a variety of physics analyses at the LHC, including Standard Model measurements and searches for physics beyond the Standard Model. The identification of such jets is challenging due to the lack of track and vertex information in the pseudorapidity range |η|>2.5|η|>2.5 . This paper presents a novel strategy for forward pile-up jet tagging that exploits jet shapes and topological jet correlations in pile-up interactions. Measurements of the per-jet tagging efficiency are presented using a data set of 3.2 fb −1−1 of proton–proton collisions at a centre-of-mass energy of 13 TeVTeV collected with the ATLAS detector. The fraction of pile-up jets rejected in the range 2.5<|η|<4.52.5<|η|<4.5 is estimated in simulated events with an average of 22 interactions per bunch-crossing. It increases with jet transverse momentum and, for jets with transverse momentum between 20 and 50 GeV, it ranges between 49% and 67% with an efficiency of 85% for selecting hard-scatter jets. A case study is performed in Higgs boson production via the vector-boson fusion process, showing that these techniques mitigate the background growth due to additional proton–proton interactions, thus enhancing the reach for such signatures.

Search for Higgs boson pair production in the two bottom quarks plus two photons final state in pp collisions at √s=13 TeV with the ATLAS detector

Thu 13 Apr 2023 09:42:29 KRAT

The ATLAS inner detector trigger performance in pp collisions at 13 TeV during LHC Run 2

Thu 08 Jun 2023 15:10:33 KRAT

The design and performance of the inner detector trigger for the high level trigger of the ATLAS experiment at the Large Hadron Collider during the 2016-2018 data taking period is discussed. In 2016, 2017, and 2018 the ATLAS detector recorded 35.6 fb(-1), 46.9 fb(-1), and 60.6 fb(-1) respectively of proton-proton collision data at a centre-of-mass energy of 13TeV. In order to deal with the very high interaction multiplicities per bunch crossing expected with the 13TeV collisions the inner detector trigger was redesigned during the long shutdown of the Large Hadron Collider from 2013 until 2015. An overview of these developments is provided and the performance of the tracking in the trigger for the muon, electron, tau and b-jet signatures is discussed. The high performance of the inner detector trigger with these extreme interaction multiplicities demonstrates how the inner detector tracking continues to lie at the heart of the trigger performance and is essential in enabling the ATLAS physics programme.

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 low-mass dijet resonances using trigger-level jets with the Atlas detector in pp collisions at √s=13 TeV

Thu 06 Jun 2019 18:34:51 KRAT

Searches for dijet resonances with sub-TeV masses using the ATLAS detector at the Large Hadron Collider can be statistically limited by the bandwidth available to inclusive single-jet triggers, whose data-collection rates at low transverse momentum are much lower than the rate from standard model multijet production. This Letter describes a new search for dijet resonances where this limitation is overcome by recording only the event information calculated by the jet trigger algorithms, thereby allowing much higher event rates with reduced storage needs. The search targets low-mass dijet resonances in the range 450–1800 GeV. The analyzed data set has an integrated luminosity of up to 29.3 fb−1 and was recorded at a center-of-mass energy of 13 TeV. No excesses are found; limits are set on Gaussian-shaped contributions to the dijet mass distribution from new particles and on a model of dark-matter particles with axial-vector couplings to quarks.

Search for flavor-changing neutral currents in top quark decays t→Hc and t→Hu in multilepton final states in proton-proton collisions at √s=13 TeV with the ATLAS detector

Thu 06 Jun 2019 18:34:47 KRAT

Flavor-changing neutral currents are not present in the Standard Model at tree level and are suppressed in loop processes by the unitarity of the Cabibbo-Kobayashi-Maskawa matrix; the corresponding rates for top quark decay processes are experimentally unobservable. Extensions of the Standard Model can generate new flavor-changing neutral current processes, leading to signals which, if observed, would be unambiguous evidence of new interactions. A data set corresponding to an integrated luminosity of 36.1 fb-1 of pp collisions at a center-of-mass energy of s=13 TeV recorded with the ATLAS detector at the Large Hadron Collider is used to search for top quarks decaying to up or charm quarks with the emission of a Higgs boson, with subsequent Higgs boson decay to final states with at least one electron or muon. No signal is observed and limits on the branching fractions B(t→Hc)<0.16% and B(t→Hu)<0.19% at 95% confidence level are obtained (with expected limits of 0.15% in both cases).

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.

Electron efficiency measurements with the ATLAS detector using 2012 LHC proton–proton collision data

Thu 05 Oct 2017 14:48:39 KRAT

This paper describes the algorithms for the reconstruction and identification of electrons in the central region of the ATLAS detector at the Large Hadron Collider (LHC). These algorithms were used for all ATLAS results with electrons in the final state that are based on the 2012 pp collision data produced by the LHC at s√ = 8 TeV. The efficiency of these algorithms, together with the charge misidentification rate, is measured in data and evaluated in simulated samples using electrons from Z→ee, Z→eeγ and J/ψ→ee decays. For these efficiency measurements, the full recorded data set, corresponding to an integrated luminosity of 20.3 fb−1, is used. Based on a new reconstruction algorithm used in 2012, the electron reconstruction efficiency is 97% for electrons with ET=15 GeV and 99% at ET=50 GeV. Combining this with the efficiency of additional selection criteria to reject electrons from background processes or misidentified hadrons, the efficiency to reconstruct and identify electrons at the ATLAS experiment varies from 65 to 95%, depending on the transverse momentum of the electron and background rejection.

Constraints on Higgs boson properties using WW∗(→eνμν)jj production in 36.1fb−1 of s√=13 TeV pp collisions with the ATLAS detector

Thu 04 May 2023 16:12:10 KRAT

This article presents the results of two studies of Higgs boson properties using the WW*(-> e nu mu nu)jj final state, based on a dataset corresponding to 36.1fb(-1) of root s = 13 TeV proton-proton collisions recorded by the ATLAS experiment at the Large Hadron Collider. The first study targets Higgs boson production via gluon-gluon fusion and constrains the CP properties of the effective Higgs-gluon interaction. Using angular distributions and the overall rate, a value of tan (alpha) = 0.0 +/- 0.4(stat.) +/- 0.3(syst.) is obtained for the tangent of the mixing angle for CP-even and CP-odd contributions. The second study exploits the vector-boson fusion production mechanism to probe the Higgs boson couplings to longitudinally and transversely polarised W and Z bosons in both the production and the decay of the Higgs boson; these couplings have not been directly constrained previously. The polarisation-dependent coupling-strength scale factors are defined as the ratios of the measured polarisation-dependent coupling strengths to those predicted by the Standard Model, and are determined using rate and kinematic information to be a(L) = 0.91(-0.18)(+0.10)(stat.)(-0.17)(+0.09)(syst.) and a(T) = 1.2 +/- 0.4(stat.)(-0.3)(+0.2)(syst.). These coupling strengths are translated into pseudo-observables, resulting in kappa(VV) = 0.91(-0.18)(+0.10)(stat.)(-0.17)(+0.09)(syst.) and epsilon(VV) = 0.13(-0.20)(+0.28)(stat.)(-0.10)(+0.08)(syst.). All results are consistent with the Standard Model predictions.

Operation and performance of the ATLAS semiconductor tracker in LHC Run 2

Sat 24 Jun 2023 12:13:01 KRAT

The semiconductor tracker (SCT) is one of the tracking systems for charged particles in the ATLAS detector. It consists of 4088 silicon strip sensor modules. During Run 2 (2015-2018) the Large Hadron Collider delivered an integrated luminosity of 156 fb(-1) to the ATLAS experiment at a centre-of-mass proton-proton collision energy of 13 TeV. The instantaneous luminosity and pile-up conditions were far in excess of those assumed in the original design of the SCT detector. Due to improvements to the data acquisition system, the SCT operated stably throughout Run 2. It was available for 99.9% of the integrated luminosity and achieved a data-quality efficiency of 99.85%. Detailed studies have been made of the leakage current in SCT modules and the evolution of the full depletion voltage, which are used to study the impact of radiation damage to the modules. '

Erratum to: Search for single top-quark production via flavour-changing neutral currents at 8 TeV with the ATLAS detector

Mon 26 Jun 2023 04:05:46 KRAT

Measurements of the Higgs boson inclusive and differential fiducial cross-sections in the diphoton decay channel with pp collisions at √s = 13 TeV with the ATLAS detector

Mon 24 Apr 2023 14:20:16 KRAT

A measurement of inclusive and differential fiducial cross-sections for the production of the Higgs boson decaying into two photons is performed using 139 fb(-1) of proton-proton collision data recorded at root s = 13 TeV by the ATLAS experiment at the Large Hadron Collider. The inclusive cross-section times branching ratio, in a fiducial region closely matching the experimental selection, is measured to be 67 +/- 6 fb, which is in agreement with the state-of-the-art Standard Model prediction of 64 +/- 4 fb. Extrapolating this result to the full phase space and correcting for the branching ratio, the total cross-section for Higgs boson production is estimated to be 58 +/- 6 pb. In addition, the cross-sections in four fiducial regions sensitive to various Higgs boson production modes and differential cross-sections as a function of either one or two of several observables are measured. All the measurements are found to be in agreement with the Standard Model predictions. The measured transverse momentum distribution of the Higgs boson is used as an indirect probe of the Yukawa coupling of the Higgs boson to the bottom and charm quarks. In addition, five differential cross-section measurements are used to constrain anomalous Higgs boson couplings to vector bosons in the Standard Model effective field theory framework.

Modelling and computational improvements to the simulation of single vector-boson plus jet processes for the ATLAS experiment

Mon 24 Apr 2023 14:13:14 KRAT

This paper presents updated Monte Carlo configurations used to model the production of single electroweak vector bosons (W, Z/gamma*) in association with jets in proton-proton collisions for the ATLAS experiment at the Large Hadron Collider. Improvements pertaining to the electroweak input scheme, parton-shower splitting kernels and scale-setting scheme are shown for multi-jet merged configurations accurate to next-to-leading order in the strong and electroweak couplings. The computational resources required for these set-ups are assessed, and approximations are introduced resulting in a factor three reduction of the per-event CPU time without affecting the physics modelling performance. Continuous statistical enhancement techniques are introduced by ATLAS in order to populate low cross-section regions of phase space and are shown to match or exceed the generated effective luminosity. This, together with the lower per-event CPU time, results in a 50% reduction in the required computing resources compared to a legacy set-up previously used by the ATLAS collaboration. The set-ups described in this paper will be used for future ATLAS analyses and lay the foundation for the next generation of Monte Carlo predictions for single vector-boson plus jets production.

Search for invisible Higgs-boson decays in events with vector-boson fusion signatures using 139 fb−1 of proton-proton data recorded by the ATLAS experiment

Mon 24 Apr 2023 13:52:18 KRAT

A direct search for Higgs bosons produced via vector-boson fusion and subsequently decaying into invisible particles is reported. The analysis uses 139 fb(-1) of pp collision data at a centre-of-mass energy of root s =13 TeV recorded by the ATLAS detector at the LHC. The observed numbers of events are found to be in agreement with the background expectation from Standard Model processes. For a scalar Higgs boson with a mass of 125 GeV and a Standard Model production cross section, an observed upper limit of 0.145 is placed on the branching fraction of its decay into invisible particles at 95% confidence level, with an expected limit of 0.103. These results are interpreted in the context of models where the Higgs boson acts as a portal to dark matter, and limits are set on the scattering cross section of weakly interacting massive particles and nucleons. Invisible decays of additional scalar bosons with masses from 50 GeV to 2 TeV are also studied, and the derived upper limits on the cross section times branching fraction decrease with increasing mass from 1.0 pb for a scalar boson mass of 50 GeV to 0.1 pb at a mass of 2 TeV.

Direct constraint on the Higgs–charm coupling from a search for Higgs boson decays into charm quarks with the ATLAS detector

Mon 24 Apr 2023 13:52:14 KRAT

A search for the Higgs boson decaying into a pair of charm quarks is presented. The analysis uses proton- proton collisions to target the production of a Higgs boson in association with a leptonically decaying W or Z boson. The dataset delivered by the LHC at a centre-of-mass energy of root s = 13 TeV and recorded by the ATLAS detector corresponds to an integrated luminosity of 139 fb(-1). Flavour-tagging algorithms are used to identify jets originating from the hadronisation of charm quarks. The analysis method is validated with the simultaneous measurement of WW, WZ and ZZ production, with observed (expected) significances of 2.6 (2.2) standard deviations above the background-only prediction for the (W/Z)Z(-> c (c) over bar) process and 3.8 (4.6) standard deviations for the (W/Z)W(-> cq) process. The (WIZ)H(-> c (c) over bar) search yields an observed (expected) upper limit of 26 (31) times the predicted Standard Model crosssection times branching fraction for a Higgs boson with a mass of 125 GeV, corresponding to an observed (expected) constraint on the charm Yukawa coupling modifier vertical bar k(c)vertical bar < 8.5 (12.4), at the 95% confidence level. A combination with the ATLAS (W/Z)H, H -> b<(b)over bar> analysis is performed, allowing the ratio k(c)/k(b) to be constrained to less than 4.5 at the 95% confidence level, smaller than the ratio of the b- and c-quark masses, and therefore determines the Higgs-charm coupling to be weaker than the Higgs-bottom coupling at the 95% confidence level.

Emulating the impact of additional proton–proton interactions in the ATLAS simulation by presampling sets of inelastic Monte Carlo events

Mon 20 Mar 2023 15:48:23 KRAT

AtlFast3: The next generation of fast simulation in ATLAS

Mon 20 Mar 2023 15:34:15 KRAT

Measurement of detector-corrected observables sensitive to the anomalous production of events with jets and large missing transverse momentum in pp collisions at √s=13 TeV using the ATLAS detector

Mon 19 Nov 2018 12:43:33 KRAT

Observables sensitive to the anomalous production of events containing hadronic jets and missing momentum in the plane transverse to the proton beams at the Large Hadron Collider are presented. The observables are defined as a ratio of cross sections, for events containing jets and large missing transverse momentum to events containing jets and a pair of charged leptons from the decay of a Z/γ∗ boson. This definition minimises experimental and theoretical systematic uncertainties in the measurements. This ratio is measured differentially with respect to a number of kinematic properties of the hadronic system in two phase-space regions; one inclusive single-jet region and one region sensitive to vector-boson-fusion topologies. The data are found to be in agreement with the Standard Model predictions and used to constrain a variety of theoretical models for dark-matter production, including simplified models, effective field theory models, and invisible decays of the Higgs boson. The measurements use 3.2 fb −1 of proton–proton collision data recorded by the ATLAS experiment at a centre-of-mass energy of 13 TeV and are fully corrected for detector effects, meaning that the data can be used to constrain new-physics models beyond those shown in this paper.

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 supersymmetry in final states with missing transverse momentum and multiple b-jets in proton-proton collisions at √s=13 TeV with the ATLAS detector

Mon 10 Jun 2019 16:33:28 KRAT

A search for supersymmetry involving the pair production of gluinos decaying via third-generation squarks into the lightest neutralino (χ~01) is reported. It uses LHC proton-proton collision data at a centre-of-mass energy s√=13 TeV with an integrated luminosity of 36.1 fb−1 collected with the ATLAS detector in 2015 and 2016. The search is performed in events containing large missing transverse momentum and several energetic jets, at least three of which must be identified as originating from b-quarks. To increase the sensitivity, the sample is divided into subsamples based on the presence or absence of electrons or muons. No excess is found above the predicted background. For χ~01 masses below approximately 300 GeV, gluino masses of less than 1.97 (1.92) TeV are excluded at 95% confidence level in simplified models involving the pair production of gluinos that decay via top (bottom) squarks. An interpretation of the limits in terms of the branching ratios of the gluinos into third-generation squarks is also provided. These results improve upon the exclusion limits obtained with the 3.2 fb−1 of data collected in 2015.

Search for Higgs boson decays to beyond-the-Standard-Model light bosons in four-lepton events with the ATLAS detector at √s=13 TeV

Mon 10 Jun 2019 16:33:26 KRAT

A search is conducted for a beyond-the-Standard-Model boson using events where a Higgs boson with mass 125 GeV decays to four leptons (ℓ = e or μ). This decay is presumed to occur via an intermediate state which contains one or two on-shell, promptly decaying bosons: H → ZX/XX → 4ℓ, where X is a new vector boson Zd or pseudoscalar a with mass between 1 and 60 GeV. The search uses pp collision data collected with the ATLAS detector at the LHC with an integrated luminosity of 36.1 fb−1 at a centre-of-mass energy s√=13 TeV. No significant excess of events above Standard Model background predictions is observed; therefore, upper limits at 95% confidence level are set on modelindependent fiducial cross-sections, and on the Higgs boson decay branching ratios to vector and pseudoscalar bosons in two benchmark models.

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.

Search for high-mass dilepton resonances using 139 fb−1 of pp collision data collected at √s=13 TeV with the ATLAS detector

Mon 10 Feb 2020 11:33:37 KRAT

A search for high-mass dielectron and dimuon resonances in the mass range of 250 GeV to 6 TeV is presented. The data were recorded by the ATLAS experiment in proton–proton collisions at a centre-ofmass energy of √s = 13 TeV during Run 2 of the Large Hadron Collider and correspond to an integrated luminosity of 139 fb−1. A functional form is fitted to the dilepton invariant-mass distribution to model the contribution from background processes, and a generic signal shape is used to determine the significance of observed deviations from this background estimate. No significant deviation is observed and upper limits are placed at the 95% confidence level on the fiducial cross-section times branching ratio for various resonance width hypotheses. The derived limits are shown to be applicable to spin-0, spin-1 and spin-2 signal hypotheses. For a set of benchmark models, the limits are converted into lower limits on the resonance mass and reach 4.5 TeV for the E6-motivated Z ψ boson. Also presented are limits on Heavy Vector Triplet model couplings.

Search for pair production of Higgs bosons in the bb¯ bb¯ final state using proton-proton collisions at √s=13 TeV with the ATLAS detector

Mon 10 Feb 2020 11:33:26 KRAT

A search for Higgs boson pair production in the b ¯bb¯b final state is carried out with up to 36.1 fb−1 of LHC proton-proton collision data collected at √ s = 13 TeV with the ATLAS detector in 2015 and 2016. Three benchmark signals are studied: a spin-2 graviton decaying into a Higgs boson pair, a scalar resonance decaying into a Higgs boson pair, and Standard Model non-resonant Higgs boson pair production. Two analyses are carried out, each implementing a particular technique for the event reconstruction that targets Higgs bosons reconstructed as pairs of jets or single boosted jets. The resonance mass range covered is 260–3000 GeV. The analyses are statistically combined and upper limits on the production cross section of Higgs boson pairs times branching ratio to b ¯bb¯b are set in each model. No significant excess is observed; the largest deviation of data over prediction is found at a mass of 280 GeV, corresponding to 2.3 standard deviations globally. The observed 95% confidence level upper limit on the non-resonant production is 13 times the Standard Model prediction.

Search for light resonances decaying to boosted quark pairs and produced in association with a photon or a jet in proton-proton collisions at √s=13 TeV with the ATLAS detector

Mon 10 Feb 2020 11:33:25 KRAT

This Letter presents a search for new light resonances decaying to pairs of quarks and produced in association with a high-pT photon or jet. The dataset consists of proton–proton collisions with an integrated luminosity of 36.1 fb−1 at a centre-of-mass energy of √s = 13 TeV recorded by the ATLAS detector at the Large Hadron Collider. Resonance candidates are identified as massive large-radius jets with substructure consistent with a particle decaying into a quark pair. The mass spectrum of the candidates is examined for local excesses above background. No evidence of a new resonance is observed in the data, which are used to exclude the production of a lepto-phobic axial-vector Z boson.

Search for invisible Higgs boson decays in vector boson fusion at √s=13 TeV with the ATLAS detector

Mon 10 Feb 2020 11:26:25 KRAT

We report a search for Higgs bosons that are produced via vector boson fusion and subsequently decay into invisible particles. The experimental signature is an energetic jet pair with invariant mass of O(1) TeV and O(100) GeV missing transverse momentum. The analysis uses 36.1 fb−1 of pp collision data at √s=13 TeV recorded by the ATLAS detector at the LHC. In the signal region the 2252 observed events are consistent with the background estimation. Assuming a 125 GeV scalar particle with Standard Model cross sections, the upper limit on the branching fraction of the Higgs boson decay into invisible particles is 0.37 at 95% confidence level where 0.28 was expected. This limit is interpreted in Higgs portal models to set bounds on the wimp–nucleon scattering cross section. We also consider invisible decays of additional scalar bosons with masses up to 3 TeV for which the upper limits on the cross section times branching fraction are in the range of 0.3–1.7 pb.

Measurement of the nuclear modification factor for inclusive jets in Pb+Pb collisions at √sNN=5.02TeVwith the ATLAS detector

Mon 10 Feb 2020 09:41:23 KRAT

Measurements of the yield and nuclear modification factor, RAA, for inclusive jet production are performed using 0.49 nb−1 of Pb+Pb data at √sNN = 5.02 TeV and 25 pb−1 of pp data at √s = 5.02 TeV with the ATLAS detector at the LHC. Jets are reconstructed with the anti-kt algorithm with radius parameter R = 0.4 and are measured over the transverse momentum range of 40–1000 GeV in six rapidity intervals covering |y| < 2.8. The magnitude of RAA increases with increasing jet transverse momentum, reaching a value of approximately 0.6 at 1 TeV in the most central collisions. The magnitude of RAA also increases towards peripheral collisions. The value of RAA is independent of rapidity at low jet transverse momenta, but it is observed to decrease with increasing rapidity at high transverse momenta

Fiducial, total and differential cross-section measurements of t-channel single top-quark production in pp collisions at 8 TeV using data collected by the ATLAS detector

Mon 09 Apr 2018 09:49:52 KRAT

Detailed measurements of t-channel single top-quark production are presented. They use 20.2 fb^{-1} of data collected by the ATLAS experiment in proton–proton collisions at a centre-of-mass energy of 8 TeV at the LHC. Total, fiducial and differential cross-sections are measured for both top-quark and top-antiquark production. The fiducial cross-section is measured with a precision of 5.8% (top quark) and 7.8% (top antiquark), respectively. The total cross-sections are measured to be sigma _{ ext {tot}} (tq) = 56.7^{+4.3}_{-3.8};mathrm{pb} for top-quark production and sigma _{ ext {tot}} (ar{t} q) = 32.9^{+3.0}_{-2.7};mathrm{pb} for top-antiquark production, in agreement with the Standard Model prediction. In addition, the ratio of top-quark to top-antiquark production cross-sections is determined to be R_t=1.72 pm 0.09. The differential cross-sections as a function of the transverse momentum and rapidity of both the top quark and the top antiquark are measured at both the parton and particle levels. The transverse momentum and rapidity differential cross-sections of the accompanying jet from the t-channel scattering are measured at particle level. All measurements are compared to various Monte Carlo predictions as well as to fixed-order QCD calculations where available.

Constraints on off-shell Higgs boson production and the Higgs boson total width in ZZ→4ℓ and ZZ→2ℓ2ν final states with the ATLAS detector

Fri 24 May 2019 17:54:28 KRAT

A measurement of off-shell Higgs boson production in the and decay channels, where ℓ stands for either an electron or a muon, is performed using data from proton–proton collisions at a centre-of-mass energy of TeV. The data were collected by the ATLAS experiment in 2015 and 2016 at the Large Hadron Collider, and they correspond to an integrated luminosity of . An observed (expected) upper limit on the off-shell Higgs signal strength, defined as the event yield normalised to the Standard Model prediction, of 3.8 (3.4) is obtained at 95% confidence level (CL). Assuming the ratio of the Higgs boson couplings to the Standard Model predictions is independent of the momentum transfer of the Higgs production mechanism considered in the analysis, a combination with the on-shell signal-strength measurements yields an observed (expected) 95% CL upper limit on the Higgs boson total width of 14.4 (15.2) MeV.

Search for pair production of Higgsinos in final states with at least three b-tagged jets in √s=13 TeV pp collisions using the ATLAS detector

Fri 24 May 2019 17:47:25 KRAT

A search for pair production of the supersymmetric partners of the Higgs boson (higgsinos H˜) in gauge-mediated scenarios is reported. Each higgsino is assumed to decay to a Higgs boson and a gravitino. Two complementary analyses, targeting high- and low-mass signals, are performed to maximize sensitivity. The two analyses utilize LHC pp collision data at a center-of-mass energy s=13 TeV, the former with an integrated luminosity of 36.1 fb-1 and the latter with 24.3 fb-1, collected with the ATLAS detector in 2015 and 2016. The search is performed in events containing missing transverse momentum and several energetic jets, at least three of which must be identified as b-quark jets. No significant excess is found above the predicted background. Limits on the cross section are set as a function of the mass of the H˜ in simplified models assuming production via mass-degenerate higgsinos decaying to a Higgs boson and a gravitino. Higgsinos with masses between 130 and 230 GeV and between 290 and 880 GeV are excluded at the 95% confidence level. Interpretations of the limits in terms of the branching ratio of the higgsino to a Z boson or a Higgs boson are also presented, and a 45% branching ratio to a Higgs boson is excluded for mH˜≈400 GeV.

Evidence for light-by-light scattering in heavy-ion collisions with the ATLAS detector at the LHC

Fri 23 Nov 2018 10:38:52 KRAT

Light-by-light scattering (γγ right arrow γγ) is a quantum-mechanical process that is forbidden in the classical theory of electrodynamics. This reaction is accessible at the Large Hadron Collider thanks to the large electromagnetic field strengths generated by ultra-relativistic colliding lead ions. Using 480 μb−1 of lead–lead collision data recorded at a centre-of-mass energy per nucleon pair of 5.02 TeV by the ATLAS detector, here we report evidence for light-by-light scattering. A total of 13 candidate events were observed with an expected background of 2.6 ± 0.7 events. After background subtraction and analysis corrections, the fiducial cross-section of the process Pb + Pb (γγ) right arrow Pb(∗) + Pb(∗)γγ, for photon transverse energy ET > 3 GeV, photon absolute pseudorapidity |η| < 2.4, diphoton invariant mass greater than 6 GeV, diphoton transverse momentum lower than 2 GeV and diphoton acoplanarity below 0.01, is measured to be 70 ± 24 (stat.) ± 17 (syst.) nb, which is in agreement with the standard model predictions.

Measurements of b-jet tagging efficiency with the ATLAS detector using tt¯ events at √s=13 TeV

Fri 23 Aug 2019 11:14:18 KRAT

The efficiency to identify jets containing b-hadrons (b-jets) is measured using a high purity sample of dileptonic top quark-antiquark pairs ( tt¯ ) selected from the 36.1 fb−1 of data collected by the ATLAS detector in 2015 and 2016 from proton-proton collisions produced by the Large Hadron Collider at a centre-of-mass energy s√=13 TeV. Two methods are used to extract the efficiency from tt¯ events, a combinatorial likelihood approach and a tag-and-probe method. A boosted decision tree, not using b-tagging information, is used to select events in which two b-jets are present, which reduces the dominant uncertainty in the modelling of the flavour of the jets. The efficiency is extracted for jets in a transverse momentum range from 20 to 300 GeV, with data-to-simulation scale factors calculated by comparing the efficiency measured using collision data to that predicted by the simulation. The two methods give compatible results, and achieve a similar level of precision, measuring data-to-simulation scale factors close to unity with uncertainties ranging from 2% to 12% depending on the jet transverse momentum.

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

Fri 23 Aug 2019 11:14:17 KRAT

A search is presented for the pair production of heavy vector-like B quarks, primarily targeting B quark decays into a W boson and a top 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, characterised by a high-transverse-momentum isolated electron or muon, large missing transverse momentum, and multiple jets, of which at least one is b-tagged. No significant deviation from the Standard Model expectation is observed. The 95% confidence level lower limit on the B mass is 1350 GeV assuming a 100% branching ratio to Wt. In the SU(2) singlet scenario, the lower mass limit is 1170 GeV. The 100% branching ratio limits are found to be also applicable to heavy vector-like X production, with charge +5/3, that decay into Wt. This search is also sensitive to a heavy vector-like B quark decaying into other final states (Zb and Hb) and thus mass limits on B production are set as a function of the decay branching ratios.

Measurement of jet fragmentation in 5.02 TeV proton-lead and proton-proton collisions with the ATLAS detector

Fri 23 Aug 2019 11:14:12 KRAT

A measurement of the fragmentation functions of jets into charged particles in collisions and pp collisions is presented. The analysis utilizes 28 nb−1 of data and 26 pb−1 of pp data, both at = 5.02 TeV, collected in 2013 and 2015, respectively, with the ATLAS detector at the LHC. The measurement is reported in the centre-of-mass frame of the nucleon–nucleon system for jets in the rapidity range ⁎1.6 and with transverse momentum 45 260 GeV. Results are presented both as a function of the charged-particle transverse momentum and as a function of the longitudinal momentum fraction of the particle with respect to the jet. The pp fragmentation functions are compared with results from Monte Carlo event generators and two theoretical models. The ratios of the to pp fragmentation functions are found to be consistent with unity.

Measurement of the tt¯ production cross section in the τ+jets final state in pp collisions at √s =8 TeV using the ATLAS detector

Fri 20 Oct 2017 10:38:24 KRAT

The top-quark pair production cross section is measured in final states with one electron or muon and one hadronically decaying τ lepton from the process tt → (`ν`)(τντ)bb, where ` = e, µ. The data sample corresponds to an integrated luminosity

Search for resonant pair production of Higgs bosons in the b¯bb¯b final state using pp collisions at √s=13 TeV with the ATLAS detector

Fri 19 May 2023 14:48:00 KRAT

A search for resonant Higgs boson pair production in the b (b) over barb (b) over bar final state is presented. The analysis uses 126 fb(-1)- 139 fb(-1) of pp collision data at root s = 13 TeV collected with the ATLAS detector at the Large Hadron Collider. The analysis is divided into two channels, targeting Higgs boson decays which are reconstructed as pairs of small-radius jets or as individual large-radius jets. Spin-0 and spin2 benchmark signal models are considered, both of which correspond to resonant HH production via gluon-gluon fusion. The data are consistent with Standard Model predictions. Upper limits are set on the production cross section times branching ratio to Higgs boson pairs of a new resonance in the mass range from 251 GeV to 5 TeV.

Search for single production of a vectorlike T quark decaying into a Higgs boson and top quark with fully hadronic final states using the ATLAS detector

Fri 19 May 2023 11:39:06 KRAT

A search is made for a vectorlike T quark decaying into a Higgs boson and a top quark in 13 TeV protonproton collisions using the ATLAS detector at the Large Hadron Collider with a data sample corresponding to an integrated luminosity of 139 fb???1. The Higgs-boson and top-quark candidates are identified in the all-hadronic decay mode, where H - bb ?? and t - bW - bqq??0 are reconstructed as large-radius jets. The candidate Higgs boson, top quark, and associated B hadrons are identified using tagging algorithms. No significant excess is observed above the background, so limits are set on the production cross section of a singlet T quark at 95% confidence level, depending on the mass mT and coupling ??T of the vectorlike T quark to Standard Model particles. In the considered mass range between 1.0 and 2.3 TeV, the upper limit on the allowed coupling values increases with mT from a minimum value of 0.35 for 1.07 < mT < 1.4 TeV to 1.6 for mT 1/4 2.3 TeV.

Constraints on Higgs boson production with large transverse momentum using H→b¯b decays in the ATLAS detector

Fri 19 May 2023 11:39:05 KRAT

This paper reports constraints on Higgs boson production with transverse momentum above 1 TeV. The analyzed data from proton-proton collisions at a center-of-mass energy of 13 TeV were recorded with the ATLAS detector at the Large Hadron Collider from 2015 to 2018 and correspond to an integrated luminosity of 136 fb(-1.) Higgs bosons decaying into b (b) over bar are reconstructed as single large-radius jets recoiling against a hadronic system and are identified by the experimental signature of two b-hadron decays. The experimental techniques are validated in the same kinematic regime using the Z -> b (b) over bar process. The 95% confidence-level upper limit on the cross section for Higgs boson production with transverse momentum above 450 GeV is 115 fb, and above 1 TeV it is 9.6 fb. The Standard Model cross section predictions for a Higgs boson with a mass of 125 GeV in the same kinematic regions are 18.4 fb and 0.13 fb, respectively.

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.

Measurement of b-hadron pair production with the ATLAS detector in proton-proton collisions at √s=8 TeV

Fri 16 Nov 2018 13:26:24 KRAT

A measurement of b-hadron pair production is presented, based on a data set corresponding to an integrated luminosity of 11.4 fb−1 of proton-proton collisions recorded at s√=8 TeV with the ATLAS detector at the LHC. Events are selected in which a b-hadron is reconstructed in a decay channel containing J/ψ → μμ, and a second b-hadron is reconstructed in a decay channel containing a muon. Results are presented in a fiducial volume defined by kinematic requirements on three muons based on those used in the analysis. The fiducial cross section is measured to be 17.7 ± 0.1(stat.) ± 2.0(syst.) nb. A number of normalised differential cross sections are also measured, and compared to predictions from the Pythia8, Herwig++, MadGraph5_aMC@NLO+Pythia8 and Sherpa event generators, providing new constraints on heavy flavour production.

Analysis of the Wtb vertex from the measurement of triple-differential angular decay rates of single top quarks produced in the t-channel at √s=8 TeV with the ATLAS detector

Fri 16 Nov 2018 12:44:57 KRAT

The electroweak production and subsequent decay of single top quarks in the t-channel is determined by the properties of the Wtb vertex, which can be described by the complex parameters of an effective Lagrangian. An analysis of a triple-differential decay rate in t-channel production is used to simultaneously determine five generalised helicity fractions and phases, as well as the polarisation of the produced top quark. The complex parameters are then constrained. This analysis is based on 20.2 fb−1 of proton-proton collision data at a centre-of-mass energy of 8 TeV collected with the ATLAS detector at the LHC. The fraction of decays containing transversely polarised W bosons is measured to be f1 = 0.30 ± 0.05. The phase between amplitudes for transversely and longitudinally polarised W bosons recoiling against left-handed b-quarks is measured to be δ− = 0.002π + 0.017π + 0.016π , giving no indication of CP violation. The fractions of longitudinal or transverse W bosons accompanied by right-handed b-quarks are also constrained. Based on these measurements, limits are placed at 95% CL on the ratio of the complex coupling parameters Re [gR/V L ∈ [−0.12, 0.17] and Im [gR/VL ∈ [−0.07, 0.06]. Constraints are also placed on the ratios |VR/VL| and |gL/VL|. In addition, the polarisation of single top quarks in the t-channel is constrained to be P > 0.72 (95% CL). None of the above measurements make assumptions about the value of any of the other parameters or couplings and all of them are in agreement with the Standard Model.

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.

Search for resonant and nonresonant Higgs boson pair production in the b¯ bτ + τ − decay channel in pp collisions at √s = 13 TeV with the ATLAS detector

Fri 14 Jun 2019 09:51:33 KRAT

A search for resonant and nonresonant pair production of Higgs bosons in the b¯bτ+τ− final state is presented. The search uses 36.1 fb−1 of pp collision data with √s=13 TeV recorded by the ATLAS experiment at the LHC in 2015 and 2016. Decays of the τ-lepton pairs with at least one τ lepton decaying to final states with hadrons and a neutrino are considered. No significant excess above the expected background is observed in the data. The cross-section times branching ratio for nonresonant Higgs boson pair production is constrained to be less than 30.9 fb, 12.7 times the standard model expectation, at 95% confidence level. The data are also analyzed to probe resonant Higgs boson pair production, constraining a model with an extended Higgs sector based on two doublets and a Randall-Sundrum bulk graviton model. Upper limits are placed on the resonant Higgs boson pair production cross-section times branching ratio, excluding resonances X in the mass range 305 GeV

Observation of H → b ¯b decays and VH production with the ATLAS detector

Fri 14 Jun 2019 09:44:40 KRAT

A search for the decay of the Standard Model Higgs boson into a pair when produced in association with a W or Z boson is performed with the ATLAS detector. The data, corresponding to an integrated luminosity of were collected in proton–proton collisions during Run 2 of the Large Hadron Collider at a centre-of-mass energy of . For a Higgs boson mass of , an excess of events over the expected background from other Standard Model processes is found with an observed (expected) significance of 4.9 (4.3) standard deviations. A combination with the results from other searches in Run 1 and in Run 2 for the Higgs boson in the decay mode is performed, which yields an observed (expected) significance of 5.4 (5.5) standard deviations, thus providing direct observation of the Higgs boson decay into b-quarks. The ratio of the measured event yield for a Higgs boson decaying into to the Standard Model expectation is . Additionally, a combination of Run 2 results searching for the Higgs boson produced in association with a vector boson yields an observed (expected) significance of 5.3 (4.8) standard deviations.

Measurement of the suppression and azimuthal anisotropy of muons from heavy-flavor decays in Pb+Pb collisions at √sNN = 2.76 TeV with the ATLAS detector

Fri 14 Jun 2019 09:44:37 KRAT

ATLAS measurements of the production of muons from heavy-flavor decays in √sNN=2.76 TeV Pb+Pb collisions and √s=2.76 TeV pp collisions at the LHC are presented. Integrated luminosities of 0.14 nb−1 and 570 nb−1 are used for the Pb+Pb and pp measurements, respectively, which are performed over the muon transverse momentum range 4

Prompt and non-prompt J/ψ elliptic flow in Pb+Pb collisions at √sNN=5.02 Tev with the ATLAS detector

Fri 14 Jun 2019 09:44:36 KRAT

The elliptic flow of prompt and non-prompt J/ψJ/ψ was measured in the dimuon decay channel in Pb+Pb collisions at sNN−−−√=5.02sNN=5.02 TeVTeV with an integrated luminosity of 0.42 nb−10.42 nb−1 with the ATLAS detector at the LHC. The prompt and non-prompt signals are separated using a two-dimensional simultaneous fit of the invariant mass and pseudo-proper decay time of the dimuon system from the J/ψJ/ψ decay. The measurement is performed in the kinematic range of dimuon transverse momentum and rapidity 9

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.

Observation of Higgs boson production in association with a top quark pair at the LHC with the ATLAS detector

Fri 14 Jun 2019 09:37:33 KRAT

The observation of Higgs boson production in association with a top quark pair (tt¯H), based on the analysis of proton–proton collision data at a centre-of-massenergy of 13 TeV recorded with the ATLAS detector at the Large Hadron Collider, is presented. Using data corresponding to integrated luminosities of up to 79.8 fb−1, and considering Higgs boson decays into bb¯, ⁎WW⁎, τ+τ−, γγ, and ⁎ZZ⁎, the observed significance is 5.8 standard deviations, compared to an expectation of 4.9 standard deviations. Combined with the tt¯H searches using a dataset corresponding to integrated luminosities of 4.5 fb−1 at 7 TeV and 20.3 fb−1 at 8 TeV, the observed (expected) significance is 6.3 (5.1) standard deviations. Assuming Standard Model branching fractions, the total tt¯H production cross section at 13 TeV is measured to be 670 ± 90 (stat.) −100+110 (syst.) fb, in agreement with the Standard Model prediction.

Observation of centrality-dependent acoplanarity for muon pairs produced via two-photon scattering in Pb + Pb collisions at √sNN=5.02 TeV with the ATLAS detector

Fri 14 Jun 2019 09:30:42 KRAT

This Letter presents a measurement of γγ→μ+μ− production in Pb+Pb collisions recorded by the ATLAS detector at the Large Hadron Collider at √sNN=5.02 TeV with an integrated luminosity of 0.49 nb−1. The azimuthal angle and transverse momentum correlations between the muons are measured as a function of collision centrality. The muon pairs are produced from γγ through the interaction of the large electromagnetic fields of the nuclei. The contribution from background sources of muon pairs is removed using a template fit method. In peripheral collisions, the muons exhibit a strong back-to-back correlation consistent with previous measurements of muon pair production in ultraperipheral collisions. The angular correlations are observed to broaden significantly in central collisions. The modifications are qualitatively consistent with rescattering of the muons while passing through the hot matter produced in the collision.

Search for heavy resonances decaying into a W or Z boson and a Higgs boson in final states with leptons and b-jets in 36 fb-1 of √s = 13 TeV pp collisions with the ATLAS detector

Fri 14 Jun 2019 09:30:38 KRAT

A search is conducted for new resonances decaying into a W or Z boson and a 125 GeV Higgs boson in the

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.

Measurement of forward-backward multiplicity correlations in lead-lead, proton-lead, and proton-proton collisions with the ATLAS detector

Fri 11 May 2018 13:00:41 KRAT

Two-particle pseudorapidity correlations are measured in √sNN = 2.76 TeV Pb + Pb, √sNN = 5.02 TeV p+Pb, and √s = 13 TeV pp collisions at the Large Hadron Collider (LHC), with total integrated luminosities of approximately 7μb−1, 28 nb−1, and 65 nb−1, respectively. The correlation function CN(η1,η2) is measured as a function of event multiplicity using charged particles in the pseudorapidity range |η| < 2.4. The correlation function contains a significant short-range component, which is estimated and subtracted. After removal of the short-range component, the shape of the correlation function is described approximately by 1 + a21 1/2η1η2 in all collision systems over the full multiplicity range. The values of a21 1/2 are consistent forthe opposite-charge pairs and same-charge pairs, and for the three collision systems at similar multiplicity. Thevalues of a21 1/2 and the magnitude of the short-range component both follow a power-law dependence on the event multiplicity. The short-range component in p + Pb collisions, after symmetrizing the proton and lead directions, is found to be smaller at a given η than in pp collisions with comparable multiplicity.

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

Fri 07 Jun 2019 10:02:08 KRAT

A search for the exclusive decays of the Higgs and Z bosons to a ϕ or ρ meson and a photon is performed with a pp collision data sample corresponding to an integrated luminosity of up to 35.6 fb−1 collected at sqrt{s}=13 TeV with the ATLAS detector at the CERN Large Hadron Collider. These decays have been suggested as a probe of the Higgs boson couplings to light quarks. No significant excess of events is observed above the background, as expected from the Standard Model. Upper limits at 95% confidence level were obtained on the branching fractions of the Higgs boson decays to ϕγ and ργ of 4.8 × 10−4 and 8.8 × 10−4, respectively. The corresponding 95% confidence level upper limits for the Z boson decays are 0.9 × 10−6 and 25 × 10−6 for ϕγ and ργ, respectively.