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The first measurement of the dijet transverse momentum balance xj in proton-lead (pPb) collisions at a nucleon-nucleon center-of-mass energy of √s̅_̅(̅N̅N̅)̅ = 8.16 TeV is presented. The xj observable, defined as the ratio of the subleading over leading jet transverse momentum in a dijet pair, is used to search for jet quenching effects. The data, corresponding to an integrated luminosity of 174.6 nb−1, were collected with the CMS detector in 2016. The xj distributions and their average values are studied as functions of the charged-particle multiplicity of the events and for various dijet rapidity selections. The latter enables probing hard scattering of partons carrying distinct nucleon momentum fractions x in the proton- and lead-going directions. The former, aided by the high-multiplicity triggers, allows probing for potential jet quenching effects in high-multiplicity events (with up to 400 charged particles), for which collective phenomena consistent with quark-gluon plasma (QGP) droplet formation were previously observed. The ratios of xj distributions for high- to low-multiplicity events are used to quantify the possible medium effects. These ratios are consistent with simulations of the hard-scattering process that do not include QGP production. These measurements set an upper limit on medium-induced energy loss of the subleading jet of 1.26% of its transverse momentum at the 90% confidence level in high multiplicity pPb events.

This paper presents measurements of charged-hadron spectra obtained in  pp ,  p +Pb, and Pb+Pb collisions at √s or √s NN = 5 . 02 TeV, and in Xe+Xe collisions at √s NN = 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  μ b − 1 , respectively. The nuclear modification factors  R p Pb  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  R p Pb  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.

Measurements of elliptic (v2) and triangular (v3) flow coefficients of π±, K±, p+p̄, K⁰_(S), and Λ+Λ̄ obtained with the scalar product method in Xe-Xe collisions at √s̅_̅(̅N̅N̅)̅ = 5.44 TeV are presented. The results are obtained in the rapidity range |y| < 0.5 and reported as a function of transverse momentum, pT, for several collision centrality classes. The flow coefficients exhibit a particle mass dependence for pT < 3 GeV/c, while a grouping according to particle type (i.e., meson and baryon) is found at intermediate transverse momenta (3 < pT < 8 GeV/c). The magnitude of the baryon v2 is larger than that of mesons up to pT = 6 GeV/c. The centrality dependence of the shape evolution of the pT-differential v2 is studied for the various hadron species. The v2 coefficients of π±, K±, and p+p̄ are reproduced by MUSIC hydrodynamic calculations coupled to a hadronic cascade model (UrQMD) for pT < 1 GeV/c. A comparison with vn measurements in the corresponding centrality intervals in Pb-Pb collisions at √s̅_̅(̅N̅N̅)̅ = 5.02 TeV yields an enhanced v2 in central collisions and diminished value in semicentral collisions.

The ALICE Collaboration reports the first fully-corrected measurements of the N-subjettiness observable for track-based jets in heavy-ion collisions. This study is performed using data recorded in pp and Pb-Pb collisions at centre-of-mass energies of √s̅= 7 TeV and √s̅N̅N̅ = 2.76 TeV, respectively. In particular the ratio of 2-subjettiness to 1-subjettiness, τ2/τ1, which is sensitive to the rate of two-pronged jet substructure, is presented. Energy loss of jets traversing the strongly interacting medium in heavy-ion collisions is expected to change the rate of two-pronged substructure relative to vacuum. The results are presented for jets with a resolution parameter of R = 0.4 and charged jet transverse momentum of 40 ≤ pτ,jet ≤ 60 GeV/c, which constitute a larger jet resolution and lower jet transverse momentum interval than previous measurements in heavy-ion collisions. This has been achieved by utilising a semi-inclusive hadron-jet coincidence technique to suppress the larger jet combinatorial background in this kinematic region. No significant modification of the τ2/τ1 observable for track-based jets in Pb-Pb collisions is observed relative to vacuum PYTHIA6 and PYTHIA8 references at the same collision energy. The measurements of τ2/τ1, together with the splitting aperture angle ΔR, are also performed in pp collisions at √s̅ = 7 TeV for inclusive jets. These results are compared with PYTHIA calculations at √s̅ = 7 TeV, in order to validate the model as a vacuum reference for the Pb-Pb centre-of-mass energy. The PYTHIA references for τ2/τ1 are shifted to larger values compared to the measurement in pp collisions. This hints at a reduction in the rate of two-pronged jets in Pb-Pb collisions compared to pp collisions.

This paper reports the observation of top-quark pair production in proton-lead collisions in the ATLAS experiment at the Large Hadron Collider. The measurement is performed using 165 nb − 1 of p +Pb data collected at √sNN = 8 . 16 TeV in 2016. Events are categorised in two analysis channels, consisting of either events with exactly one lepton (electron or muon) and at least four jets, or events with two opposite-charge leptons and at least two jets. In both channels at least one b -tagged jet is also required. Top-quark pair production is observed with a significance over five standard deviations in each channel. The top-quark pair production cross-section is measured to be σ t¯t =58.1±2.0(stat.) +4.8 −4.4 (syst.) nb, with a total uncertainty of 9%. In addition, the nuclear modification factor is measured to be R pA =1.090±0.039(stat.) +0.094 −0.087 (syst.). The measurements are found to be in good agreement with theory predictions involving nuclear parton distribution functions.

This paper reports the observation of top-quark pair production in proton-lead collisions in the ATLAS experiment at the Large Hadron Collider. The measurement is performed using 165 nb − 1  of  p +Pb data collected at √s NN  = 8 . 16 TeV in 2016. Events are categorised in two analysis channels, consisting of either events with exactly one lepton (electron or muon) and at least four jets, or events with two opposite-charge leptons and at least two jets. In both channels at least one  b -tagged jet is also required. Top-quark pair production is observed with a significance over five standard deviations in each channel. The top-quark pair production cross-section is measured to be nb, with a total uncertainty of 9%. In addition, the nuclear modification factor is measured to be . The measurements are found to be in good agreement with theory predictions involving nuclear parton distribution functions.

The production ratio of psi(2S) to J/psi charmonium states is presented as a function of multiplicity in proton-lead collisions at a centre-of-mass energy of root s(NN) = 8.16TeV, for both prompt and nonprompt sources. The total luminosity recorded by the LHCb experiment corresponds to 13.6 nb(-1) for pPb collisions and 20.8 nb(-1) for Pbp collisions, where the first particle corresponds to the particle traveling towards the detector. Measurements are performed in the dimuon final state at forward (backward) centre-of-mass rapidity, with respect to the proton direction, 1.5 < y* < 4.0 (-5.0 < y* < -2.5) for pPb (Pbp) collisions. A multiplicity dependence of the prompt production ratio is observed in pPb collisions, whereas no dependence is found in nonprompt production, nor in either prompt or nonprompt production in Pbp collisions. These results suggest that in the Pb-going direction additional suppression mechanisms beyond comover effects may be present, possibly related to the formation of quark-gluon plasma. This highlights a transition from small to large collision systems and provides important insight into the suppression of charmonia in proton-nucleus collisions.

The measurement of the production of prompt D0, D+, D*+, and $D_S^+$ mesons in proton–lead (p–Pb) collisions at the centre-of-mass energy per nucleon pair of \\mathrm{\\sqrt{s_{NN}}}$ = 5.02 TeV, with an integrated luminosity of 292 ± 11 μb-1, are reported. Differential production cross sections are measured at mid-rapidity (-96 < ycms< 0.04) as a function of transverse momentum (pT) in the intervals 0 < pT< 36 GeV/c for D0, 1 < pT< 36 GeV/c for D+ and D*+, and 2 < pT< 24 GeV/c for D+ mesons. For each species, the nuclear modification factor RpPb is calculated as a function of pT using a proton-proton (pp) reference measured at the same collision energy. The results are compatible with unity in the whole pT range. The average of the non-strange D mesons RpPb is compared with theoretical model predictions that include initial-state effects and parton transport model predictions. The pT dependence of the D0, D+, and D*+ nuclear modification factors is also reported in the interval 1 < pT< 36 GeV/c as a function of the collision centrality, and the central-to-peripheral ratios are computed from the D-meson yields measured in different centrality classes. The results are further compared with charged-particle measurements and a similar trend is observed in all the centrality classes. The ratios of the pT-differential cross sections of D0, D+, D*+, and $D_S^+$ mesons are also reported. The $D_S^+$ and D+ yields are compared as a function of the charged-particle multiplicity for several pT intervals. No modification in the relative abundances of the four species is observed with respect to pp collisions within the statistical and systematic uncertainties.

Exclusive production of dielectron pairs, γγ → e + e − , is studied using = 1.72 nb −1 of data from ultraperipheral collisions of lead nuclei at √s NN = 5.02 TeV recorded by the ATLAS detector at the LHC. The process of interest proceeds via photon–photon interactions in the strong electromagnetic fields of relativistic lead nuclei. Dielectron production is measured in the fiducial region defined by following requirements: electron transverse momentum > 2.5 GeV, absolute electron pseudorapidity | η e | < 2.5, dielectron invariant mass m ee > 5 GeV, and dielectron transverse momentum < 2 GeV. Differential cross-sections are measured as a function of m ee , average , absolute dielectron rapidity | y ee |, and scattering angle in the dielectron rest frame, |cos θ * |, in the inclusive sample, and also with a requirement of no activity in the forward direction. The total integrated fiducial cross-section is measured to be 215±1(stat.)(syst.)±4(lumi.) μb. Within experimental uncertainties the measured integrated cross-section is in good agreement with the QED predictions from the Monte Carlo programs STARLIGHT and SUPERCHIC, confirming the broad features of the initial photon fluxes. The differential cross-sections show systematic differences from these predictions which are more pronounced at high | y ee | and |cos θ * | values.