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 <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 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 lower sNN results. The shape of the dN ch/dη distribution is found to be independent of centrality within the systematic uncertainties of the measurement. © 2012 CERN.
Aad, G., Abbott, B., Abdallah, J., Abdelalim, A. A., Abdesselam, A., Abdinov, O., … Zwalinski, L. (2012). 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. Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics, 710(3), 363–382. https://doi.org/10.1016/j.physletb.2012.02.045