Charged particle production in Pb--Pb collisions at the LHC with the ALICE detector

TL;DR

This paper reports on measurements of charged particle production in lead-lead collisions at the LHC, revealing high energy density, centrality dependence, and parton energy loss effects in the quark-gluon plasma.

Contribution

First detailed measurement of charged particle multiplicity and spectra in Pb--Pb collisions at 2.76 TeV at the LHC, with analysis of centrality dependence and energy loss effects.

Findings

Charged particle multiplicity in central collisions is 1601 ± 60.

Energy density is about three times higher than at RHIC.

Intermediate transverse momentum particles are significantly suppressed.

Abstract

The ALICE collaboration measured charged particle production in $\sqrt{s_{NN}}=2.76$ TeV Pb--Pb collisions at the LHC. We report on results on charged particle multiplicity and transverse momentum spectra. All the results are presented as a function of the centrality of the collision, estimated with a Glauber Monte Carlo fit to multiplicity distributions reconstructed in various detectors. The applicability of the Glauber model at LHC energies, the precision of the centrality determination and the related systematic uncertainties are discussed in detail. Particles are tracked in the pseudorapidity window $|\eta| \lesssim 0.9$\ with the silicon Inner Tracking System (ITS) and the Time Projection Chamber (TPC), over the range $0.15 < \pt \lesssim 50$ GeV/$c$. The low-$p_t$ cut-off is further reduced in the multiplicity measurement using "tracklets", reconstructed in the 2 innermost layers of the ITS. The charged particle multiplicity is measured in $|\eta| < 0.5$ to be $\mathrm{d}N_{ch}/\mathrm{d}\eta = 1601 \pm 60$ in 5% most central Pb--Pb collisions, indicating an energy density a factor $\sim 3$ higher than at RHIC. Its evolution with centrality shows a pattern strikingly similar to the one measured at RHIC. Intermediate ($5 \lesssim \pt \lesssim 15$ GeV/$c$) transverse momentum particles are found to be most strongly suppressed with respect to pp collisions, consistent with a large energy loss of hard-scattered partons in the hot and dense medium. The results are presented in terms of the nuclear modification factor $R_{\mathrm{AA}}$ and compared to theoretical expectations.