Pseudorapidity density of charged particles in p-Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV

TL;DR

This study measures the charged-particle pseudorapidity density in NSD p–Pb collisions at 5.02 TeV with the ALICE SPD, translating the tracklet-based yield into dN_ch/deta_cmS and per-participant normalization via a Glauber model. The result, dN_ch/deta_cmS = 16.81 ± 0.71 (syst) at midrapidity and 2.14 ± 0.17 per participant, reveals a forward–backward asymmetry between the proton and lead sides. Comparisons to gluon-saturation and two-component QCD-inspired models show broad agreement within about 20%, while some saturation scenarios predict steeper eta dependencies. The measurements provide new constraints on initial-state particle production in high-energy nuclear collisions and inform small-x QCD dynamics relevant for future collider studies.

Abstract

The charged-particle pseudorapidity density measured over 4 units of pseudorapidity in non-single-diffractive (NSD) p-Pb collisions at a centre-of-mass energy per nucleon pair $\sqrt{s_{\rm NN}} = 5.02$ TeV is presented. The average value at midrapidity is measured to be $16.81 \pm 0.71$ (syst.), which corresponds to $2.14 \pm 0.17$ (syst.) per participating nucleon. This is 16% lower than in NSD pp collisions interpolated to the same collision energy, and 84% higher than in d-Au collisions at $\sqrt{s_{\rm NN}} = 0.2$ TeV. The measured pseudorapidity density in p-Pb collisions is compared to model predictions, and provides new constraints on the description of particle production in high-energy nuclear collisions.

Figures (2)

• Figure 1: Pseudorapidity density of charged particles measured in NSD p--Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV compared to theoretical predictions Dumitru:2011wqBarnafoldi:2011pxTribedy:2011aaXu:2012auAlbacete:2012xq. The calculations Barnafoldi:2011pxTribedy:2011aa have been shifted to the laboratory system.
• Figure 2: Charged-particle pseudorapidity density at midrapidity normalized to the number of participants, calculated with the Glauber model, for p--Pb, p--Au and d--Au Alber:1997snBack:2003hx collisions as a function of $\sqrt{s_{\rm NN}}$, compared to NSD ua1ua51ua52starppcdfalicepp2Khachatryan:2010xs, and inelastic Thome:1977kyAlner:1986xuAamodt:2010ftAlver:2010ck pp ($\mathrm {p\overline{p}}$) collisions, as well as central heavy-ion Abreu:2002fwAdler:2001yqBearden:2001xwBearden:2001qqAdcox:2000spBack:2000gwBack:2001bqBack:2002wbAlver:2010ckAamodt:2010pbATLAS:2011agChatrchyan:2011pb collisions. The curves $\propto s_{\rm NN}^{0.11}$ and $s_{\rm NN}^{0.15}$ (from Aamodt:2010pb) are superimposed on the NSD pp ($\mathrm {p\overline{p}}$) and central heavy-ion data, respectively, while $\propto s_{\rm NN}^{0.10}$ (from Aamodt:2010ft) on the inelastic pp ($\mathrm {p\overline{p}}$) data.