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The charged particle multiplicity distribution dN ch /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 (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 1.00 part and N 0.75 coll at 158 GeV. Compatible results are found for the scaling behaviour at 40 GeV. The width of the dN ch /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.

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