Search for the QCD critical point in nuclear collisions at the CERN SPS

TL;DR

This study investigates pion production in nuclear collisions at CERN SPS to identify critical QCD phenomena through power-law correlations, finding indications of critical behavior in Si+Si collisions but inconclusive results for Pb+Pb.

Contribution

It introduces a novel analysis of scaled factorial moments near the two-pion threshold to search for critical QCD fluctuations in various nuclear collision systems.

Findings

Power-law fluctuations observed in Si+Si collisions.

Inconclusive results for Pb+Pb due to Coulomb correlations.

Indications of critical QCD behavior in the freeze-out state.

Abstract

Pion production in nuclear collisions at the SPS is investigated with the aim to search, in a restricted domain of the phase diagram, for power-laws in the behavior of correlations which are compatible with critical QCD. We have analyzed interactions of nuclei of different size (p+p, C+C, Si+Si, Pb+Pb) at 158$A$ 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 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 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.