Measurement of the centrality dependence of the charged particle pseudorapidity distribution in lead–lead collisions at √s_NN = 2.76 TeV with the ATLAS detector

Abstract

The ATLAS experiment at the LHC has measured the centrality dependence of charged particle pseudorapidity distributions over |η| < 2 in lead–lead collisions at a nucleon–nucleon centre-of-mass energy of √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 < |η| < 4.9. Measurements are presented of the per-event charged particle pseudorapidity distribution, dN_ch/dη, and the average charged particle multiplicity in the pseudorapidity interval |η| < 0.5 in several intervals of collision centrality. The results are compared to previous midrapidity 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 lower √s_NN results. The shape of the dN_ch/dη distribution is found to be independent of centrality within the systematic uncertainties of the measurement.