In the framework of the classical Glauber approach the exact analytical expression for the variance of the number of participants (wounded nucleons) for given centrality AA interactions is presented. Its shown, that in the case of nucleus-nucleus collisions along with the optical approximation term the additional contact term appears. The numerical calculations for PbPb collisions at SPS energies show that at intermediate values of the impact parameter the optical and contact terms contributions to the variance of the participants number are of the same order and their sum is in a good agreement with the results of independent MC simulations of this process. The correlation between the numbers of participants in colliding nuclei is taken into account. In particular its demonstrated that in PbPb collisions at SPS energies the variance of the total number of participants approximately three times exceeds the Poisson one in the impact parameter region 10-12 Fm. The fluctuations of the number of collisions are also discussed.
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