Measurements of Charged-Particle Transverse Momentum Spectra in PbPb Collisions at a Center-of-Mass Energy of 2.76 TeV per Nucleon Pair and in pPb Collisions at a Center-of-Mass Energy of 5.02 TeV per Nucleon Pair with the CMS Detector

Abstract

The transverse momentum spectra of charged particles at both midrapidity and forward rapidity in PbPb collisions at a center-of-mass energy of 2.76 TeV per nucleon pair and pPb collisions at a center-of-mass energy of 5.02 TeV per nucleon pair have been measured by the CMS detector at the LHC. At low transverse momentum the charged-particle spectra reflect the dynamics of the hot and dense medium produced in PbPb collisions, while at high transverse momentum they are sensitive to partonic energy loss and the initial state of the collision system. The PbPb spectra are presented for a transverse momentum range of 0.3 to 5.0 GeV/c in 12 classes of centrality ranging from 0 to 80%. These PbPb spectra measurements are extrapolated to zero transverse momentum to determine the mean transverse momentum. The values of the mean transverse momentum increase from the most peripheral collisions to reach an approximately constant value for collisions in the centrality range of 0 to 35%, indicating similar dynamical evolution in the more central collisions. When compared to lower collision energy measurements, the values of the mean transverse momentum are found to be 20% higher, indicating an increase in the initial energy density and in the expansion velocity of the produced system. The pPb spectra are presented for a transverse momentum range of 0.4 to 100 GeV/c. The nuclear modification factor is determined by normalizing the measured pPb spectrum to an interpolated pp reference spectrum constructed from previous measurements, and is significantly enhanced for particles with a value of transverse momentum above 30 GeV/c. This enhancement is stronger than predicted by next-to-leading order theoretical calculations.