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

Abstract

The spectra of charged particles produced within the pseudorapidity window |η| < 1 at √ s NN = 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. The CMS collaboration 241 IntroductionThe charged-particle transverse momentum (p T ) spectrum is an important tool for studying parton energy loss in the dense QCD medium, known as the quark gluon plasma (QGP), that is produced in high energy nucleus-nucleus (AA) collisions [1,2]. In such collisions, high-p T particles, which originate from parton fragmentation, are sensitive to the amount of energy loss that the partons experience traversing the medium. By comparing highp T particle yields in AA collisions to predictions of theoretical models, insight into the fundamental properties of the QGP can be gained. Over the years, a number of results have been made available by experiments at SPS [3,4], at RHIC [5][6][7][8], and at the CERN LHC [9][10][11]. The modification of high-p T particle production is typically quantified using the ratio of the charged-particle p T spectrum in AA collisions to that of pp collisions, scaled by the average number of binary nucleon-nucleon collisions, N coll . This quantity is known as the nuclear modification factor, R AA , and can also be formulated as function of p T as R AA (p T ) = dN AA /dp T N coll dN pp /dp T = dN AA /dp T T AA dσ pp /dp T , ( 1) where N AA and N pp are the charged-particle yields in AA collisions and pp collisions, and σ pp is the charged-particle cross section in pp collisions. The ratio of N coll with the total inelastic pp cross section, defined as T AA = N coll /σ pp inel , is known as the nuclear overlap function and can be calculated from a Glauber model of the nuclear collision geometry [12]. In this work we adopt natural units, such that c = 1.-1 - JHEP04(2017)039The factor of 5 suppression observed in the R AA of charged hadrons and neutral pions at RHIC [5][6][7][8] was an indication of strong medium effects on particle production in the final state. However, the RHIC measurements were limited to a p T range below 25 GeV and a collision energy...

Transverse momentum spectra of hadrons in high energy pp and heavy ion collisions

“…The bahavior of the parameter T is similar to what is shown in figure 5. Figure 8 shows the invariant yields of the charged particles as a function of p T for pPb collisions and for many centralities of PbPb collisions at s 5.02 TeV NN = measured by the CMS experiment [47]. The solid curves are the fitted Tsallis distributions (equation (2)).…”

“…The solid curves are the fitted Tsallis distribution functions (equation (2)). Figure 10 shows the invariant yields of the charged particles as a function of p T for pPb and PbPb collisions at s 5.02 TeV NN = measured by the CMS experiment [47,56]. The solid curves are the modified Tsallis distributions given by equation (3).…”

“…The power law Figure 10. The invariant yields of the charged particles as a function of the transverse momentum p T for pPb and PbPb collisions at s 5.02 TeV NN = measured by the CMS [47,56]. The solid curves are the modified Tsallis distributions (equation (3)).…”

“…The Tsallis distribution function gives good description of the data for collision energies which can be inferred from the values of χ 2 /NDF given in the table 1. Figure 3 shows the invariant yields of the charged particles as a function of p T for pp collisions at s 0.9, = 2.36, 2.76, 5.02 and 7 TeV measured by the CMS experiment [45][46][47][48]. The solid curves are the Tsallis distributions fitted to the spectra.…”

“…The invariant yields of the charged particles as a function of transverse momentum p T for pp collision at s 0.9, 2.36, = 2.76, 5.02 and 7 TeV measured by the CMS experiment[46][47][48]. The solid curves are the fitted Tsallis distribution functions.…”