Enhanced pion-to-proton ratio at the onset of the QCD phase transition

TL;DR

This paper investigates the pion-to-proton ratio as a signal for a first-order chiral phase transition in heavy-ion collisions, using non-equilibrium simulations to identify observable enhancements at specific energies.

Contribution

It introduces a non-equilibrium Bjorken expansion model to simulate entropy production and links it to hadron resonance gas predictions, highlighting the pion-to-proton ratio as a phase transition indicator.

Findings

Strong enhancement of pion-to-proton ratio at phase transition energies

Difference between first-order transition and crossover in particle ratios

Simulation results support experimental search for QCD phase transition

Abstract

The pion-to-proton ratio is identified as a potential signal for a non-equilibrium first-order chiral phase transition in heavy-ion collisions, as the pion multiplicity is directly related to entropy production. To showcase this effect, a non-equilibrium Bjorken expansion starting from realistic initial conditions along a Taub adiabat is used to simulate the entropy production. Different dynamical criteria to determine the final entropy-per-baryon number are investigated and matched to a hadron resonance gas model along the chemical freeze out curve to obtain the final pion and proton numbers. We detect a strong enhancement of their multiplicity ratio at the energies where the system experiences a strong phase transition as compared to a smooth crossover which shows almost no enhancement.