Charged-particle nuclear modification factors in PbPb and pPb collisions at sqrt(s[NN]) = 5.02 TeV

TL;DR

This paper measures charged-particle spectra and nuclear modification factors in PbPb and pPb collisions at 5.02 TeV, revealing significant suppression at intermediate transverse momenta and moderate enhancement at high momenta, compared to theoretical models.

Contribution

It provides new measurements of R[AA] and R[pA] at 5.02 TeV, extending previous results and comparing them with theoretical predictions and lower-energy data.

Findings

Maximal suppression of R[AA] by a factor of 7-8 at 6-9 GeV in central collisions

R[AA] approaches unity near 200 GeV, indicating reduced suppression at high pt

R[pA] shows weak momentum dependence and moderate enhancement above unity for pt > 20 GeV

Abstract

The spectra of charged particles produced within the pseudorapidity window abs(eta) < 1 at sqrt(s[NN]) = 5.02 TeV are measured using 404 inverse microbarns of PbPb and 27.4 inverse picobarns of pp data collected by the CMS detector at the LHC in 2015. The spectra are presented over the transverse momentum ranges spanning 0.5 < pt < 400 GeV in pp and 0.7 < pt < 400 GeV in PbPb collisions. The corresponding nuclear modification factor, R[AA], is measured in bins of collision centrality. The R[AA] in the 5% most central collisions shows a maximal suppression by a factor of 7-8 in the pt region of 6-9 GeV. This dip is followed by an increase, which continues up to the highest pt measured, and approaches unity in the vicinity of pt = 200 GeV. The R[AA] is compared to theoretical predictions and earlier experimental results at lower collision energies. The newly measured pp spectrum is combined with the pPb spectrum previously published by the CMS Collaboration to construct the pPb nuclear modification factor, R[pA], up to 120 GeV. For pt > 20 GeV, R[pA] exhibits weak momentum dependence and shows a moderate enhancement above unity.