Scaling of charged particle multiplicity in Pb-Pb collisions at SPS energies

The charged particle multiplicity distribution dN(ch)/deta has been measured by the NA50 experiment in Pb-Pb collisions at the CERN SPS. Measurements were done at incident energies of 40 and 158 GeV per nucleon over a broad impact parameter range. The multiplicity distributions are studied as a function of centrality using the number of participating nucleons (N-part), or the number of binary nucleon-nucleon collisions (N-coll). Their values at midrapidity exhibit a power law scaling behaviour given by N-part(1.00) and N-coll(0.75) at 158 GeV. Compatible results are found for the scaling behaviour at 40 GeV The width of the dN(ch)/deta distributions is larger at 158 than at 40 GeV/nucleon and decreases slightly with centrality at both energies. Our results are compared to similar studies performed by other experiments both at the CERN SPS and at RHIC. (C) 2002 Elsevier Science B.V. All rights reserved.