Critical fluctuations of the proton density in A+A collisions at $158A$ GeV

TL;DR

This study investigates proton density fluctuations in heavy-ion collisions at 158A GeV to identify signs of the QCD critical point, finding evidence of critical-like power-law fluctuations in Si+Si collisions.

Contribution

It provides experimental evidence of critical fluctuations in proton densities at SPS energies, supporting the search for the QCD critical point.

Findings

Power-law fluctuations observed in Si+Si data.

Fitted exponent consistent with critical fluctuations.

Previous observations in pion pairs corroborate results.

Abstract

We look for fluctuations expected for the QCD critical point using an intermittency analysis in the transverse momentum phase space of protons produced around midrapidity in the 12.5\% most central C+C, Si+Si and Pb+Pb collisions at the maximum SPS energy of 158$A$~GeV. We find evidence of power-law fluctuations for the Si+Si data. The fitted power-law exponent $\phi_{2} = 0.96^{+0.38}_{-0.25}\text{ (stat.)}$ $\pm 0.16\text{ (syst.)}$ is consistent with the va\-lue expected for critical fluctuations. Power-law fluctuations had previously also been observed in low-mass $\pi^+ \pi^-$ pairs in the same Si+Si collisions.