Search for critical behavior of strongly interacting matter at the CERN Super Proton Synchrotron

TL;DR

This paper reviews the efforts and findings in searching for the critical point of strongly interacting matter in nucleus-nucleus collisions at CERN SPS, highlighting experimental signals and ongoing research to locate this critical point.

Contribution

It provides a comprehensive overview of search strategies, experimental results, and the potential evidence for critical behavior in strongly interacting matter at SPS energies.

Findings

Fluctuation signals suggest critical behavior.

Experimental results motivate further phase diagram scans.

Background effects are well-characterized.

Abstract

History, status and plans of the search for critical behavior of strongly interacting matter created in nucleus-nucleus collisions at the CERN Super Proton Synchrotron is reviewed. In particular, it is expected that the search should answer the question whether the critical point of strongly interacting matter exists and, if it does, where it is located. First, the search strategies are presented and a short introduction is given to expected fluctuation signals and to the quantities used by experiments to detect th The most important background effects are also discussed. Second, relevant experimental results are summarized and discussed. It is intriguing that both the fluctuations of quantities integrated over the full experimental acceptance (event multiplicity and transverse momentum) as well as the bin size dependence of the second factorial moment of pion and proton multiplicities in medium-sized Si+Si collisions at 158A GeV/c suggest critical behaviour of the created matter. These results provide strong motivation for the ongoing systematic scan of the phase diagram by the NA61/SHINE experiment at the SPS and the continuing search at the Brookhaven Relativistic Hadron Collider.