Charged-particle nuclear modification factors in PbPb and pPb collisions at s N N = 5.02 $$ \sqrt{s_{\mathrm{N}\;\mathrm{N}}}=5.02 $$ TeV

TLDR

Charged‑particle spectra in pp and PbPb collisions at √sNN=5.02 TeV are measured over 0.5–400 GeV (pp) and 0.7–400 GeV (PbPb) in |η|<1, and compared to lower‑energy results and theory. Using 404 μb⁻¹ of PbPb and 27.4 pb⁻¹ of pp data, CMS measured charged‑particle spectra and derived the nuclear modification factor RAA in centrality bins, and combined the new pp spectrum with a prior pPb spectrum to obtain RpA up to 120 GeV. In the most central PbPb collisions, RAA is suppressed by a factor of 7–8 at 6–9 GeV, rises toward unity near 200 GeV, while RpA above 20 GeV shows only a modest, weakly pT‑dependent enhancement above unity.

Abstract

The spectra of charged particles produced within the pseudorapidity window |η| < 1 at $$ \sqrt{s_{\mathrm{N}\;\mathrm{N}}}=5.02 $$ TeV are measured using 404 μb−1 of PbPb and 27.4 pb−1 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 < p T < 400 GeV in pp and 0.7 < p T < 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 p T region of 6-9 GeV. This dip is followed by an increase, which continues up to the highest p T measured, and approaches unity in the vicinity of p T = 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 p T > 20 GeV, R pA exhibits weak momentum dependence and shows a moderate enhancement above unity.