Multiplicity Fluctuations in Heavy Ion Collisions at CERN SPS

The system size and centrality dependence of multiplicity fluctuations in nuclear collisions at 158A GeV as well as the energy dependence for the most central Pb + Pb collisions were studied by the NA49 experiment at CERN SPS. A strong increase of fluctuations was observed with decreasing centrality in C + C, Si + Si and Pb + Pb collisions. The string hadronic models (UrQMD, Venus, HIJING, HSD) can not reproduce the observed increase. This may indicate a strong mixing of target and projectile contribution in a broad rapidity range. For the most central collisions at all SPS energies multiplicity distributions are significantly narrower than a corresponding Poisson one both for negatively and positively charged hadrons. The UrQMD model seems to reproduce the measured values on scaled variance. Statistical model calculations overpredict results when conservation laws are not taken into account.