Scaling of charged particle production and centrality determination in p-Pb collisions at √s_NN = 5.02 TeV at ALICE

INFN, Via P. Giuria 1, 10125 Torino, ITALY

Published online: 29 May 2015

Abstract

Proton-nucleus collisions are studied in order to disentangle initial state effects, already present in cold nuclear matter, from final state effects, that are associated to the hot and strongly interacting medium created in A-A collisions. However, after the 2013 p-Pb data taking at LHC, the importance of p-A collisions on their own has been acknowledged. Several measurements clearly showed that p-A collisions cannot simply be explained by an incoherent superposition of proton-nucleon collisions, but indicate the presence of coherent and collective effects, with a strong dependence on the collision geometry. These measurements pointed out the need for a detailed characterization of the collision geometry.

Typically, parameters such as the number of binary collisions N_coll or the number of participating nucleons N_part are used to characterize the centrality. In nucleus-nucleus interactions the centrality is usually estimated by measuring the charged particle multiplicity. However, in contrast to A-A, in p-A collisions the large fluctuations in a much reduced multiplicity environment generate a dynamical bias in centrality classes based on particle multiplicity measurement.

The centrality determination is critically addressed and a different approach to extract the average N_coll values, needed to compare p-A to pp data, is presented. The centrality dependence of primary charged-particle production and of transverse momentum distributions in p-A collisions at √s_NN = 5.02 TeV measured by ALICE are presented.