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Item type:Publication, Underlying event properties in pp collisions at √s = 13 TeV(Springer, 2020-04-01)Abstract This article reports measurements characterizing the Underlying Event (UE) associated with hard scatterings at midrapidity (|η| < 0.8) in pp collisions at $$ \sqrt{s} $$ s = 13 TeV. The hard scatterings are identified by the leading particle, the charged particle with the highest transverse momentum ( $$ {p}_{\mathrm{T}}^{\mathrm{leading}} $$ p T leading ) in the event. Charged-particle number-densities and summed transverse-momentum densities are measured in different azimuthal regions defined with respect to the leading particle direction: Toward, Transverse, and Away. The Toward and Away regions contain the fragmentation products of the hard scatterings in addition to the UE contribution, whereas particles in the Transverse region are expected to originate predominantly from the UE. The study is performed as a function of $$ {p}_{\mathrm{T}}^{\mathrm{leading}} $$ p T leading with three different p T thresholds for the associated particles, $$ {p}_{\mathrm{T}}^{\mathrm{track}} $$ p T track > 0.15, 0.5, and 1.0 GeV/c. The charged-particle density in the Transverse region rises steeply for low values of $$ {p}_{\mathrm{T}}^{\mathrm{leading}} $$ p T leading and reaches a plateau. The results confirm the trend that the charged-particle density in the Transverse region shows a stronger increase with $$ \sqrt{s} $$ s than the inclusive charged-particle density at midrapidity. The UE activity is increased by approximately 20% when going from 7 TeV to 13 TeV pp collisions. The plateau in the Transverse region (5 < $$ {p}_{\mathrm{T}}^{\mathrm{leading}} $$ p T leading < 40 GeV/c) is further characterized by the probability distribution of its charged-particle multiplicity normalized to its average value (relative transverse activity, R T) and the mean transverse momentum as a function of R T. Experimental results are compared to model calculations using PYTHIA 8 and EPOS LHC. The overall agreement between models and data is within 30%. These measurements provide new insights on the interplay between hard scatterings and the associated UE in pp collisions. - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Jet-hadron correlations measured relative to the second order event plane in Pb-Pb collisions at sNN =2.76 TeV(American Physical Society, 2020-06-01)The quark gluon plasma produced in ultrarelativistic heavy-ion collisions at the Large Hadron Collider (LHC) can be studied by measuring the modifications of jets formed by hard scattered partons which interact with the medium. We studied these modifications via angular correlations of jets with charged hadrons for jets with momenta $20<{p}_{\mathrm{T}}^{\mathrm{jet}}<40$ GeV/$c$ as a function of the associated particle momentum. The reaction plane fit method is used in this analysis to remove the flow modulated background. The analysis of angular correlations for different orientations of the jet relative to the second order event plane allows for the study of the path length dependence of medium modifications to jets. We present the dependence of azimuthal angular correlations of charged hadrons with respect to the angle of the axis of a reconstructed jet relative to the event plane in Pb-Pb collisions at $\sqrt{{s}_{NN}}$ = 2.76 TeV. The dependence of particle yields associated with jets on the angle of the jet with respect to the event plane is presented. Correlations at different angles relative to the event plane are compared through ratios and differences of the yield. No dependence of the results on the angle of the jet with respect to the event plane is observed within uncertainties, which is consistent with no significant path length dependence of the medium modifications for this observable. - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Multiharmonic correlations of different flow amplitudes in Pb-Pb collisions at sNN = 2.76 TeV(American Physical Society, 2021-08-27)The event-by-event correlations between three flow amplitudes are measured for the first time in Pb-Pb collisions, using higher-order symmetric cumulants. We find that different three-harmonic correlations develop during the collective evolution of the medium when compared to correlations that exist in the initial state. These new results cannot be interpreted in terms of previous lower-order flow measurements since contributions from two-harmonic correlations are explicitly removed in the new observables. A comparison to Monte Carlo simulations provides new and independent constraints for the initial conditions and system properties of nuclear matter created in heavy-ion collisions.