56 research outputs found

    Search for flavour-changing neutral currents in processes with one top quark and a photon using 81 fb⁻Âč of pp collisions at \sqrts = 13 TeV with the ATLAS experiment

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    A search for flavour-changing neutral current (FCNC) events via the coupling of a top quark, a photon, and an up or charm quark is presented using 81 fb−1 of proton–proton collision data taken at a centre-of-mass energy of 13 TeV with the ATLAS detector at the LHC. Events with a photon, an electron or muon, a b-tagged jet, and missing transverse momentum are selected. A neural network based on kinematic variables differentiates between events from signal and background processes. The data are consistent with the background-only hypothesis, and limits are set on the strength of the tqÎł coupling in an effective field theory. These are also interpreted as 95% CL upper limits on the cross section for FCNC tÎł production via a left-handed (right-handed) tuÎł coupling of 36 fb (78 fb) and on the branching ratio for t→γu of 2.8×10−5 (6.1×10−5). In addition, they are interpreted as 95% CL upper limits on the cross section for FCNC tÎł production via a left-handed (right-handed) tcÎł coupling of 40 fb (33 fb) and on the branching ratio for t→γc of 22×10−5 (18×10−5). © 2019 The Author(s

    Measurement of the nuclear modification factor of b -jets in 5.02 TeV Pb+Pb collisions with the ATLAS detector

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    This paper presents a measurement of b-jet production in Pb+Pb and pp collisions at √sNN=5.02 TeV with the ATLAS detector at the LHC. The measurement uses 260 pb-1 of pp collisions collected in 2017 and 1.4 nb-1 of Pb+Pb collisions collected in 2018. In both collision systems, jets are reconstructed via the anti-kt algorithm. The b-jets are identified from a sample of jets containing muons from the semileptonic decay of b-quarks using template fits of the muon momentum relative to the jet axis. In pp collisions, b-jets are reconstructed for radius parameters R=0.2 and R=0.4, and only R=0.2 jets are used in Pb+Pb collisions. For comparison, inclusive R=0.2 jets are also measured using 1.7 nb-1 of Pb+Pb collisions collected in 2018 and the same pp collision data as the b-jet measurement. The nuclear modification factor, RAA, is calculated for both b-jets and inclusive jets with R=0.2 over the transverse momentum range of 80–290 GeV. The nuclear modification factor for b-jets decreases from peripheral to central collisions. The ratio of the b-jet RAA to inclusive jet RAA is also presented and suggests that the RAA for b-jets is larger than that for inclusive jets in central Pb+Pb collisions. The measurements are compared with theoretical calculations and suggest a role for mass and colour-charge effects in partonic energy loss in heavy-ion collisions

    Jet energy scale and resolution measured in proton–proton collisions at √s = 13 TeV with the ATLAS detector

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    Jet energy scale and resolution measurements with their associated uncertainties are reported for jets using 36–81 fb−1 of proton–proton collision data with a centreof-mass energy of √s = 13 TeV collected by the ATLAS detector at the LHC. Jets are reconstructed using two different input types: topo-clusters formed from energy deposits in calorimeter cells, as well as an algorithmic combination of charged-particle tracks with those topo-clusters, referred to as the ATLAS particle-flow reconstruction method. The antikt jet algorithm with radius parameter R = 0.4 is the primary jet definition used for both jet types. This result presents new jet energy scale and resolution measurements in the high pileup conditions of late LHC Run 2 as well as a full calibration of particle-flow jets in ATLAS. Jets are initially calibrated using a sequence of simulation-based corrections. Next, several in situ techniques are employed to correct for differences between data and simulation and to measure the resolution of jets. The systematic uncertainties in the jet energy scale for central jets (|η| < 1.2) vary from 1% for a wide range of high-pT jets (250 < pT < 2000 GeV), to 5% at very low pT (20 GeV) and 3.5% at very high pT (> 2.5 TeV). The relative jet energy resolution is measured and ranges from (24 ± 1.5)% at 20 GeV to (6 ± 0.5)% at 300 GeV
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