Transverse Momentum Distributions and Nuclear Modification Factors in Heavy-Ion Collisions with ALICE at the Large Hadron Collider

In this work, Pb-Pb collisions measured in 2010 during the first data taking period (Run 1) at a centre-of-mass energy of √snn = 2.76 TeV and data taken in 2015 during Run 2 at √snn = 5.02 TeV are analysed. In November 2017, the LHC brought xenon ions to collision for the first time; this data set that was taken at √snn = 5.44 TeV is also analysed. Transverse momentum pt distributions at high pt of charged particles have shown that particle yields in heavy-ion (AA) collisions are suppressed compared to a superposition of independent nucleon-nucleon collisions (binary collision scaling). This observation is related to parton energy loss in the Quark Gluon Plasma (QGP). To obtain the charged-particle yield as a function of pt, corrections are made for tracking efficiency and acceptance, for contamination by secondary particles from weak decays or secondary interactions and for the pt resolution. To circumvent differences in the particle composition of event generators and data, the charged-particle reconstruction efficiency is calculated from the particle-dependent efficiencies weighted by the relative abundances of each particle measured during Run 1. The correction for contamination with secondary particles is usually obtained from Monte-Carlo (MC) simulations. The abundances of secondary particles in data and MC is estimated by analysing the distance of closest approach of tracks to the event vertex. It is found that the contamination correction from MC has to be scaled up by ~50% to match the data. The improvement of the analysis methods resulted in a reduction of the total relative systematic uncertainties by about 50% compared to previous analyses due to an improved reconstruction and calibration procedure in Run 2, as well as to improved track selection methods. The transverse momentum distribution of charged particles from Pb-Pb and Xe-Xe collisions were measured for nine classes of centrality. The measurement was performed for particles within -0.8 to 0.8 in pseudo-rapidity and for a transverse-momentum range of 0.15<pt<50 GeV. The nuclear modification factor (RAA) is defined as the pt-differential yield in a AA collision divided by the cross section in pp collisions, scaled by the nuclear overlap function calculated in a Monte-Carlo Glauber approach. Any suppression of particle yields in AA compared to a superposition of individual pp collisions results in a nuclear modification factor below unity. All measurements exhibit a moderate suppression for peripheral collisions. With increasing collision centrality, a pronounced suppression with RAA ~ 0.13 at intermediate pt develops. At higher pt, a significant rise of the nuclear modification factor is observed. The comparison of RAA as a function of the charged particle density per unit of rapidity dN_ch/dη shows a remarkable agreement of the observed suppression at high pt in Xe-Xe and Pb-Pb collisions at both energies scales for dN_ch/dη>400. This scaling does not hold for collisions with lower particle multiplicities. This observation is consistent with a dependence of the partonic energy loss on the square of the path length in the medium. At lower transverse momenta a dependence of RAA on the collision energy is observed, which might be due to the collision energy dependence of the bulk particle production.