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

Understand

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.