Abstract
Transverse momentum spectra of charged particles are measured by the CMS experiment at the CERN LHC in pPb collisions at sqrt{s_{_mathrm {NN}}} =5.02 ,text {TeV}, in the range 0.4 < p_{mathrm {T}} < 120 {,text {GeV/}c} and pseudorapidity |eta _textsc {cm} | < 1.8 in the proton–nucleon center-of-mass frame. For p_{mathrm {T}} <10 {,text {GeV/}c}, the charged-particle production is asymmetric about eta _textsc {cm} = 0, with smaller yield observed in the direction of the proton beam, qualitatively consistent with expectations from shadowing in nuclear parton distribution functions (nPDF). A pp reference spectrum at sqrt{s}=5.02 ,text {TeV} is obtained by interpolation from previous measurements at higher and lower center-of-mass energies. The p_{mathrm {T}} distribution measured in pPb collisions shows an enhancement of charged particles with p_{mathrm {T}} >20 {,text {GeV/}c} compared to expectations from the pp reference. The enhancement is larger than predicted by perturbative quantum chromodynamics calculations that include antishadowing modifications of nPDFs.
Highlights
RAB( pT, η) = d2 N d2σAB/d pT dη pp/d pT dη (1)where N AB is the particle yield in a collision between nuclear species A and B, σ pp is the corresponding cross section in pp collisions, and TAB is the average nuclear overlap function [3] in the AB collision
The nuclear parton distribution functions (nPDF) are constrained by measurements in lepton–nucleus deep-inelastic scattering (DIS) and Drell–Yan (DY) production of dilepton pairs from qq annihilation in proton–nucleus collisions [5]
The measurement of the charged-particle nuclear modification factor of Eq (1) requires a rescaling of the pp cross section by the average nuclear thickness function in minimum-bias pPb collisions. This factor amounts to TpPb = (0.0983 ± 0.0044) mb−1 for inelastic pPb collisions and is obtained from a Glauber Monte Carlo (MC) simulation [3,42], where the Pb nucleus is described using a Woods-Saxon distribution with nuclear radius 6.62 ± 0.13 fm and skin depth of 0.546 ± 0.055 fm [3,43]
Summary
Where N AB is the particle yield in a collision between nuclear species A and B, σ pp is the corresponding cross section in pp collisions, and TAB is the average nuclear overlap function [3] in the AB collision (in the case of proton–nucleus collisions, the quantity TAB = TpA is called average nuclear thickness function). Departures from unity are indicative of final-state effects such as parton energy loss, and/or initialstate effects such as modifications of the nuclear parton distribution functions (nPDF) [4]. The nPDFs are constrained by measurements in lepton–nucleus deep-inelastic scattering (DIS) and Drell–Yan (DY) production of dilepton pairs from qq annihilation in proton–nucleus collisions [5]. In the small parton fractional momentum regime (x 0.01), the nPDFs are found to be suppressed relative to the proton PDFs, a phenomenon commonly referred to as “shadowing” [6]. For the x regime 0.02 x 0.2, the nPDFs are enhanced (“antishadowing”) relative to the free-nucleon PDFs [5]
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