Centrality dependence of charged particle production at large transverse momentum in Pb-Pb collisions at $\sqrt{s_{\rm{NN}}} = 2.76$ TeV

TL;DR

The paper investigates parton energy loss in hot QCD matter created in Pb-Pb collisions at the LHC by measuring inclusive charged-particle $p_T$ spectra in $|\eta|<0.8$ over $0.15<p_T<50$ GeV/$c$ and computing the nuclear modification factor $R_{AA}(p_T)$ with a pp reference at the same energy. Using a large Pb-Pb data sample and Glauber-model inputs, the authors show a strong centrality-dependent suppression: in the most central (0–5%) events, $R_{AA}$ peaks at about $0.13$ around $p_T\approx6$–$7$ GeV/$c$ and rises to roughly $0.4$ for $p_T>30$ GeV/$c$, while in peripheral (70–80%) events $R_{AA}$ is about $0.6$–$0.7$ with little $p_T$ dependence. The results, which are consistent with CMS and in tension with some energy-loss models, indicate a large medium density and substantial parton energy loss at the LHC; comparisons with RHIC data and model calculations illuminate the interplay between initial spectra, quark–gluon composition, and medium effects. The work lays groundwork for more detailed jet-quenching studies, including correlations and full jet reconstruction, to further constrain the properties of the produced medium.

Abstract

The inclusive transverse momentum ($p_{\rm T}$) distributions of primary charged particles are measured in the pseudo-rapidity range $|η|<0.8$ as a function of event centrality in Pb-Pb collisions at $\sqrt{s_{\rm{NN}}}=2.76$ TeV with ALICE at the LHC. The data are presented in the $p_{\rm T}$ range $0.15<p_{\rm T}<50$ GeV/$c$ for nine centrality intervals from 70-80% to 0-5%. The Pb-Pb spectra are presented in terms of the nuclear modification factor $R_{\rm{AA}}$ using a pp reference spectrum measured at the same collision energy. We observe that the suppression of high-$p_{\rm T}$ particles strongly depends on event centrality. In central collisions (0-5%) the yield is most suppressed with $R_{\rm{AA}}\approx0.13$ at $p_{\rm T}=6$-7 GeV/$c$. Above $p_{\rm T}=7$ GeV/$c$, there is a significant rise in the nuclear modification factor, which reaches $R_{\rm{AA}} \approx0.4$ for $p_{\rm T}>30$ GeV/$c$. In peripheral collisions (70-80%), the suppression is weaker with $R_{\rm{AA}} \approx 0.7$ almost independently of $p_{\rm T}$. The measured nuclear modification factors are compared to other measurements and model calculations.

Figures (4)

• Figure 1: Charged particle $p_{\rm T}$ distribution measured in Pb--Pb collisions in different centrality intervals. The spectra are scaled for better visibility. The dashed lines show the pp reference PPREFERENCE spectra scaled by the nuclear overlap function determined for each centrality interval (Table \ref{['CENTR']}) and by the Pb-Pb spectra scaling factors. The systematic and statistical uncertainties for Pb--Pb are added quadratically. The uncertainties on the pp reference are not shown.
• Figure 2: Nuclear modification factor $R_{\rm{AA}}$ of charged particles measured in Pb--Pb collisions in nine centrality intervals. The boxes around data points denote $p_{{\rm T}}$-dependent systematic uncertainties. The systematic uncertainties on the normalization which are related to $\langle T_{\rm{AA}} \rangle$ and the normalization of the pp data are added in quadrature and shown as boxes at $R_{\rm{AA}} =1$.
• Figure 3: Nuclear modification factor $R_{\rm{AA}}$ of charged particles as a function of $\langle N_{{\rm{part}}} \rangle$ (top panel) and ${\rm{d}}{N_{\rm{ch}}}/\rm{d}\eta$ (bottom panel) measured by ALICE in Pb--Pb collisions in different $p_\mathrm T$-intervals, compared to PHENIX results in $5<p_\mathrm T<7$ GeV/$c$PHENIXRAA. The boxes around the data represent the $p_{\rm T}$-dependent uncertainties on the Pb--Pb $p_{\rm T}$ spectra. The boxes at $R_{\rm{AA}} =1$ represent the systematic uncertainties on the pp reference in different $p_\mathrm T$-intervals ($p_{\rm{T}}$-interval increases from left to right, the left-most is for PHENIX). The systematic uncertainties on the overall normalization for ALICE and PHENIX are not shown.
• Figure 4: Nuclear modification factor $R_{\rm{AA}}$ of charged particles measured by ALICE in the most central Pb--Pb collisions (0--5%) in comparison to results from CMS CMSRAA and model calculations HOROWITZ2011ASWCHEN2011MAJUMDER2011RENK2011RENK2011YAJEM. The boxes around the data denote $p_{{\rm T}}$-dependent systematic uncertainties. For CMS statistical and systematic uncertainties on $R_{\rm {AA}}$ are added in quadrature. The systematic uncertainties on the normalization which are related to $\langle T_{\rm {AA}} \rangle$ and the normalization of the pp data are added in quadrature and shown as boxes at $R_{\rm{AA}} =1$ (the right-most is for CMS).