The parton to hadron phase transition observed in Pb+Pb collisions at 158 GeV per nucleon

TL;DR

This paper presents evidence that the observed hadronic yields in Pb+Pb collisions at 158 GeV per nucleon reflect a universal QCD phase transition from partons to hadrons, which occurs at a specific temperature and survives subsequent expansion.

Contribution

It demonstrates that the hadronization process in heavy-ion collisions results in a maximum entropy state consistent with a QCD phase transition, surviving hadronic rescattering.

Findings

Hadronic yields resemble a thermal equilibrium at T=180 MeV.

The maximum entropy state is due to the QCD parton to hadron phase transition.

This state persists through hadronic rescattering and expansion.

Abstract

Hadronic yields and yield ratios observed in Pb+Pb collisions at the SPS energy of 158 GeV per nucleon are known to resemble a thermal equilibrium population at T=180 +/- 10 MeV, also observed in elementary e+ + e- to hadron data at LEP. We argue that this is the universal consequence of the QCD parton to hadron phase transition populating the maximum entropy state. This state is shown to survive the hadronic rescattering and expansion phase, freezing in right after hadronization due to the very rapid longitudinal and transverse expansion that is inferred from Bose-Einstein pion correlation analysis of central Pb+Pb collisions.