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 ($\mathrm{p}+\mathrm{p}$, $\mathrm{C}+\mathrm{C}$, $\mathrm{Si}+\mathrm{Si}$, $\mathrm{Pb}+\mathrm{Pb}$) at $158A$ GeV adopting, as appropriate observables, scaled factorial moments in a search for intermittent fluctuations in transverse dimensions. The analysis is performed for ${\pi }^{+}{\pi }^{-}$ 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 $\sigma$ meson has no well-defined vacuum state. It turns out that for the $\mathrm{Pb}+\mathrm{Pb}$ 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 $\mathrm{Si}+\mathrm{Si}$ approaching in size the prediction of critical QCD.

• Received 21 December 2009

DOI:https://doi.org/10.1103/PhysRevC.81.064907