Search for the QCD critical point in nuclear collisions at 158A GeV at the CERN Super Proton Synchrotron (SPS)

Pion production in nuclear collisions at the Super Proton Synchrotron (SPS) is investigated with the aim to search, in a restricted domain of the phase diagram, for power laws in the behavior of correlations that are compatible with critical QCD. We analyzed interactions of nuclei of different sizes (p+p, C+C, Si+Si, Pb+Pb) at 158A GeV adopting, as appropriate observables, scaled factorial moments in a search for intermittent fluctuations in transverse dimensions. The analysis is performed for π+π- pairs with an invariant mass very close to the two-pion threshold. In this sector one may capture critical fluctuations of the sigma component in a hadronic medium, even if the σ meson has no well-defined vacuum state. It turns out that for the Pb+Pb system the proposed analysis technique cannot be applied without entering the invariant mass region with strong Coulomb correlations. As a result the treatment becomes inconclusive in this case. Our results for the other systems indicate the presence of power-law fluctuations in the freeze-out state of Si+Si approaching in size the prediction of critical QCD