Transverse momentum distribution and nuclear modification factor of charged particles in p-Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV

TL;DR

This paper investigates whether the strong high-$p_T$ hadron suppression observed in Pb--Pb collisions at the LHC originates from initial-state effects by measuring charged-particle production in p--Pb at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV with ALICE. It measures the $p_T$ distributions of charged particles in minimum-bias p--Pb (NSD) events and constructs a pp reference by interpolation between $\sqrt{s}=2.76$ TeV and 7 TeV, with an additional next-to-leading-order (NLO) pQCD scaling for $p_T>5$ GeV/$c$. The nuclear modification factor $R_{\mathrm{pPb}}(p_T)$ is found to be consistent with unity for $p_T \gtrsim 2$ GeV/$c$, indicating no significant initial-state suppression. Comparisons with CGC-based and shadowing models show reasonable agreement in some implementations, while others diverge, supporting limited cold-nuclear-matter effects; overall, the results reinforce that Pb--Pb jet quenching at the LHC reflects hot QCD matter rather than initial-state effects.

Abstract

The transverse momentum ($p_{\mathrm T}$) distribution of primary charged particles is measured in minimum bias (non-single-diffractive) p-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV with the ALICE detector at the LHC. The $p_{\mathrm T}$ spectra measured near central rapidity in the range $0.5<p_{\mathrm T}<20$ GeV/$c$ exhibit a weak pseudorapidity dependence. The nuclear modification factor $R_{\mathrm{pPb}}$ is consistent with unity for $p_{\mathrm T}$ above 2 GeV/$c$. This measurement indicates that the strong suppression of hadron production at high $p_{\mathrm T}$ observed in Pb-Pb collisions at the LHC is not due to an initial-state effect. The measurement is compared to theoretical calculations.

Figures (3)

• Figure 1: Transverse momentum distributions of charged particles in minimum bias (NSD) p--Pb collisions for different pseudorapidity ranges (upper panel). The spectra are scaled by the factors indicated. The histogram represents the reference spectrum in inelastic (INEL) pp collisions (see text). The lower panel shows the ratio of the spectra at forward pseudorapidities to that at $|\eta_{\mathrm{cms}}|<0.3$. The vertical bars (boxes) represent the statistical (systematic) errors.
• Figure 2: The nuclear modification factor of charged particles as a function of transverse momentum in minimum bias (NSD) p--Pb collisions at $\sqrt{s_{\mathrm{NN}}} = 5.02$ TeV. The data for $|\eta_{\mathrm{cms}}|<0.3$ are compared to measurements Abelev:2012eq in central (0--5% centrality) and peripheral (70--80%) Pb--Pb collisions at $\sqrt{s_{\mathrm{NN}}} = 2.76$ TeV. The statistical errors are represented by vertical bars, the systematic errors by (filled) boxes around data points. The relative systematic uncertainties on the normalization are shown as boxes around unity near $p_{\mathrm{T}}=0$ for p--Pb (left box), peripheral Pb--Pb (middle box) and central Pb-Pb (right box).
• Figure 3: Transverse momentum dependence of the nuclear modification factor $R_{\mathrm{pPb}}$ of charged particles measured in minimum bias (NSD) p--Pb collisions at $\sqrt{s_{\mathrm{NN}}} = 5.02$ TeV. The ALICE data in $|\eta_{\mathrm{cms}}|<0.3$ (symbols) are compared to model calculations (bands or lines, see text for details). The vertical bars (boxes) show the statistical (systematic) errors. The relative systematic uncertainty on the normalization is shown as a box around unity near $p_{\mathrm{T}}=0$.