Transverse momentum dependence of inclusive primary charged-particle production in p-Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV

TL;DR

<3-5 sentence high-level summary> This work probes whether initial-state effects modify inclusive charged-particle production in proton–lead collisions at the LHC by measuring the midrapidity $p_T$ spectra and comparing to a pp baseline scaled by the nuclear overlap function. Using a substantially larger minimum-bias data set, ALICE extends the coverage to $0.15<p_T<50$ GeV/$c$ and derives the $R_{\mathrm{pPb}}$ with a Glauber-based reference, finding $R_{\mathrm{pPb}}\approx1$ for $p_T\gtrsim 2$ GeV/$c$ and a possible Cronin-like enhancement near $p_T\approx4$ GeV/$c$, and is compared to EPS09s shadowing and other model predictions; overall they indicate negligible initial-state effects at high $p_T$ and support binary-collision scaling as the baseline for interpreting Pb$-$Pb suppression. This establishes a robust p$-$Pb baseline and reinforces that the strong hadron suppression in Pb$-$Pb arises from final-state interactions in hot QCD matter.

Abstract

The transverse momentum ($p_{\mathrm T}$) distribution of primary charged particles is measured at midrapidity in minimum-bias p-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV with the ALICE detector at the LHC in the range $0.15<p_{\mathrm T}<50$ GeV/$c$. The spectra are compared to the expectation based on binary collision scaling of particle production in pp collisions, leading to a nuclear modification factor consistent with unity for $p_{\mathrm T}$ larger than 2 GeV/$c$, with a weak indication of a Cronin-like enhancement for $p_{\rm T}$ around 4 GeV/$c$. The measurement is compared to theoretical calculations and to data in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=2.76$ TeV.

Figures (4)

• 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 (cross section scaled by the nuclear overlap function, $T_{\mathrm{pPb}}$) in inelastic pp collisions, determined in $|\eta|<0.8$. The lower panel shows the ratio of spectra in p--Pb at backward pseudorapidities to that at $|\eta_{\mathrm{cms}}|<0.3$. The vertical bars (boxes) represent the statistical (systematic) uncertainties.
• Figure 2: The nuclear modification factor of charged particles as a function of transverse momentum, measured in minimum-bias (NSD) p--Pb collisions at $\sqrt{s_{\mathrm{NN}}} = 5.02$ TeV in two pseudorapidity ranges, $|\eta_{\mathrm{cms}}|<0.3$ and $-1.3<\eta_{\mathrm{cms}}<0.3$. The statistical errors are represented by vertical bars, the systematic errors by boxes around data points. The relative systematic uncertainties on the normalization are shown as boxes around unity near $p_{\mathrm{T}}=0$.
• 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 Helenius:2012wdKang:2012aaXu:2012au (bands, see text for details). The vertical bars (boxes) show the statistical (systematic) uncertainties. The relative systematic uncertainty on the normalization is shown as a box around unity near $p_{\mathrm{T}}=0$.
• Figure 4: Transverse momentum dependence of the nuclear modification factor $R_{\mathrm{pPb}}$ of charged particles (h$^\pm$) measured in minimum-bias (NSD) p--Pb collisions at $\sqrt{s_{\mathrm{NN}}} = 5.02$ TeV in comparison to data on the nuclear modification factor $R_{\mathrm{PbPb}}$ in central Pb--Pb collisions at $\sqrt{s_{\mathrm{NN}}} = 2.76$ TeV. The Pb--Pb data are for charged particle CMS:2012aaAbelev:2012eq, direct photon Chatrchyan:2012vq, Z$^0$Chatrchyan:2011ua and W$^{\pm}$Chatrchyan:2012nt production. All data are for midrapidity.