54 research outputs found
Top-quark pair production in heavy-ion collisions with the ATLAS experiment
No full textMeasurements of top quarks in heavy-ion collisions are expected to provide novel probes of nuclear modifications to parton distribution functions as well as to bring unique information about the evolution of strongly interacting matter. We report the observation of the top-quark pair production in proton-lead collisions at the centre-of-mass energy of 8.16 TeV in the ATLAS experiment at the LHC. Top-quark pair production is measured in the lepton+jets and the dilepton channels, with a significance well above 5 standard deviations in each channel separately. The results from the measurement of the nuclear modification factor are also presented
11th Large Hadron Collider Physics Conference 2023
No full textJets are commonly produced in heavy-ion collisions at the LHC energies. Their calibration is essential for precise measurements of various processes, such as top-quark pair production. The report presents the measurement of jet energy scale and resolution in proton-lead collisions collected at 8.16 TeV in 2016 by the ATLAS experiment. The method involving the balance between boson and jet transverse momenta is used for jet and to estimate jet performance in both data and Monte Carlo simulation. The performance of two jet definitions, referred to as PFlow and HI jets, is evaluated and results are compared including systematic uncertainties. The results of these studies are a key input to the ongoing analysis of top-quark pair production in p+Pb collisions
Measurement of jet performance in proton-lead collisions in the ATLAS experiment
No full textJets can be copiously produced in heavy-ion collisions at the LHC energies. Their calibration is crucial for precise measurements of various processes, such as top-quark pair production. The poster presents the measurement of jet energy scale and resolution in proton-lead collisions collected at 8.16 TeV in 2016. The balance between Z boson and jet transverse momenta is explored for jet and to estimate jet performance in both data and simulation. The performance of two jet definitions, referred to as EMPFlow and HIJets, is studied and results are compared including systematic uncertainties. The presented results are a key input to the ongoing analysis of top-quark pair production in p+Pb collisions
Measurement of high-pT electron performance in proton-lead collisions in the ATLAS experiment
No full textElectrons constitute an essential component of final states from the leptonic decay channels of W and Z bosons. Their reconstruction and identification are especially challenging in heavy-ion collisions due to high detector occupancy. Therefore, the evaluation of electron performance is crucial for precision measurements of properties of quark-gluon plasma produced in heavy-ion collisions at the LHC energies. The measurements focus on electron reconstruction, identification, isolation, and trigger performance in p+Pb collisions at TeV collected by the ATLAS experiment in 2016. The data correspond to a total integrated luminosity of 165 nb. The Tag-and-Probe method is applied, which allows for the estimation of electron efficiency independently in data and Monte Carlo simulation
Comparison of two approaches to jet reconstruction in proton-lead collisions in ATLAS
No full textJets are commonly produced in heavy-ion collisions at the LHC energies. Their calibration is essential for precise measurements of various processes, such as top-quark pair production. The poster presents the measurement of jet energy scale and resolution in proton-lead collisions collected at 8.16 TeV in 2016. The method involving the balance between Z boson and jet transverse momenta is used for jet and to estimate jet performance in both data and Monte Carlo simulation. The performance of two jet definitions, referred to as EMPFlow and HIJets, is evaluated and results are compared including systematic uncertainties. The results of these studies are a key input to the ongoing analysis of top-quark pair production in p+Pb collisions
Charged-hadron production in pp, p plus Pb, Pb plus Pb, and Xe plus Xe collisions at √sNN=5 TeV with the ATLAS detector at the LHC
No full textThis paper presents measurements of charged-hadron spectra obtained in pp, p+Pb, and Pb+Pb collisions at s or sNN = 5.02 TeV, and in Xe+Xe collisions at sNN = 5.44 TeV. The data recorded by the ATLAS detector at the LHC have total integrated luminosities of 25 pb −1, 28 nb −1, 0.50 nb −1, and 3 μb −1, respectively. The nuclear modification factors RpPb and R AA are obtained by comparing the spectra in heavy-ion and pp collisions in a wide range of charged-particle transverse momenta and pseudorapidity. The nuclear modification factor RpPb shows a moderate enhancement above unity with a maximum at p T ≈ 3 GeV; the enhancement is stronger in the Pb-going direction. The nuclear modification factors in both Pb+Pb and Xe+Xe collisions feature a significant, centrality-dependent suppression. They show a similar distinct p T-dependence with a local maximum at p T ≈ 2 GeV and a local minimum at p T ≈ 7 GeV. This dependence is more distinguishable in more central collisions. No significant |η|-dependence is found. A comprehensive comparison with several theoretical predictions is also provided. They typically describe R AA better in central collisions and in the p T range from about 10 to 100 GeV. [Figure not available: see fulltext.
Measurement of the production of a W boson in association with a charmed hadron in pp collisions at p s=13 TeV with the ATLAS detector
No full textThe production of a W boson in association with a single charm quark is studied using 140 fb - 1 of s = 13 TeV proton-proton collision data collected with the ATLAS detector at the Large Hadron Collider. The charm quark is tagged by the presence of a charmed hadron reconstructed with a secondary-vertex fit. The W boson is reconstructed from the decay to either an electron or a muon and the missing transverse momentum present in the event. The charmed mesons reconstructed are D + → K - π + π + and D * + → D 0 π + → ( K - π + ) π + and the charge conjugate decays in the fiducial regions where p T ( e , μ ) > 30 GeV , | η ( e , μ ) | 8 GeV , and | η ( D ( * ) ) | < 2.2 . The integrated and normalized differential cross sections as a function of the pseudorapidity of the lepton from the W boson decay, and of the transverse momentum of the charmed hadron, are extracted from the data using a profile likelihood fit. The measured total fiducial cross sections are σ fid OS - SS ( W - + D + ) = 50.2 ± 0.2 ( stat ) - 2.3 + 2.4 ( syst ) pb , σ fid OS - SS ( W + + D - ) = 48.5 ± 0.2 ( stat ) - 2.2 + 2.3 ( syst ) pb , σ fid OS - SS ( W - + D * + ) = 51.1 ± 0.4 ( stat ) - 1.8 + 1.9 ( syst ) pb , and σ fid OS - SS ( W + + D * - ) = 50.0 ± 0.4 ( stat ) - 1.8 + 1.9 ( syst ) pb . Results are compared with the predictions of next-to-leading-order quantum chromodynamics calculations performed using state-of-the-art parton distribution functions. Additionally, the ratio of charm to anticharm production cross sections is studied to probe the s - s ̄ quark asymmetry. The ratio is found to be R c ± = 0.971 ± 0.006 ( stat ) ± 0.011 ( syst ) . The ratio and cross-section measurements are consistent with the predictions obtained with parton distribution function sets that have a symmetric s - s ̄ sea, indicating that any s - s ̄ asymmetry in the Bjorken- x region relevant for this measurement is small
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