Measurement of the centrality dependence of the charged particle pseudorapidity distribution in lead-lead collisions at sqrt(s_NN) = 2.76 TeV with the ATLAS detector

TL;DR

This paper reports on the measurement of how the distribution of charged particles varies with collision centrality in lead-lead collisions at 2.76 TeV, using the ATLAS detector to analyze pseudorapidity distributions with low transverse momentum particles.

Contribution

It provides the first detailed measurement of charged particle pseudorapidity distributions over |eta|<2 at this energy, including low transverse momentum particles, and compares these to previous results.

Findings

Charged particle density distribution is consistent across centralities.

Shape of the pseudorapidity distribution is independent of centrality.

Results agree with lower energy measurements at RHIC and previous LHC data.

Abstract

The ATLAS experiment at the LHC has measured the centrality dependence of charged particle pseudorapidity distributions over |eta| < 2 in lead-lead collisions at a nucleon-nucleon centre-of-mass energy of sqrt(s_NN) = 2.76 TeV. In order to include particles with transverse momentum as low as 30 MeV, the data were recorded with the central solenoid magnet off. Charged particles were reconstructed with two algorithms (2-point "tracklets" and full tracks) using information from the pixel detector only. The lead-lead collision centrality was characterized by the total transverse energy in the forward calorimeter in the range 3.2 < |eta| < 4.9. Measurements are presented of the per-event charged particle density distribution, dN_ch/deta, and the average charged particle multiplicity in the pseudorapidity interval |eta|<0.5 in several intervals of collision centrality. The results are compared to previous mid-rapidity measurements at the LHC and RHIC. The variation of the mid-rapidity charged particle yield per colliding nucleon pair with the number of participants is consistent with the lower sqrt(s_NN) results. The shape of the dN_ch/deta distribution is found to be independent of centrality within the systematic uncertainties of the measurement.