Effects of a phase transition on two-pion interferometry in heavy-ion collisions at $\sqrt{s_\mathrm{NN}}=2.4-7.7$ GeV

TL;DR

This study uses HBT correlations in heavy-ion collisions to investigate how phase transitions affect the equation of state at high baryon densities, providing insights into QCD matter properties.

Contribution

It demonstrates that pion HBT measurements can distinguish between different high-density equations of state and exclude certain phase transition scenarios.

Findings

HBT radii data favor a stiff EoS at low energies

Softening of EoS below 4 times nuclear saturation density is excluded

HBT parameters are sensitive to EoS stiffness and phase transition effects

Abstract

Hanbury-Brown-Twiss (HBT) correlations for charged pions in central Au+Au collisions at $\sqrt{s_\mathrm{NN}}=2.4 - 7.7~\text{GeV}$ (corresponding to beam kinetic energies in the fixed target frame from $E_{\rm{lab}}=1.23~\text{to}~30~\text{GeV/nucleon}$) are calculated using the UrQMD model with different equations of state. The effects of a phase transition at high baryon densities are clearly observed in the HBT parameters that are explored. It is found that the available data on the HBT radii, $R_{O}/R_{S}$ and $R^{2}_{O}-R^{2}_{S}$, in the investigated energy region favors a relatively stiff equation of state at low beam energies which then turns into a soft equation of state at high collision energies consistent with astrophysical constraints on the high density equation of state of QCD. The specific effects of two different phase transition scenarios on the $R_{O}/R_{S}$ and $R^{2}_{O}-R^{2}_{S}$ are investigated. It is found that a phase transition with a significant softening of the equation of state below 4 times nuclear saturation density can be excluded using HBT data. Our results highlight that the pion's $R_{O}/R_{S}$ and $R^{2}_{O}-R^{2}_{S}$ are sensitive to the stiffness of the equation of state, and can be used to constrain and understand the QCD equation of state in the high baryon density region.