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

The charged particle multiplicity distribution dNch/dη 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 (Npart), or the number of binary nucleon–nucleon collisions (Ncoll). Their values at midrapidity exhibit a power law scaling behaviour given by Npart1.00 and Ncoll0.75 at 158 GeV. Compatible results are found for the scaling behaviour at 40 GeV. The width of the dNch/dη 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.