Global polarization of $\Lambda$ hyperons and its sensitivity to equations of state in low-energy heavy-ion collisions

TL;DR

This study investigates how the global polarization of $ ext{Lambda}$ hyperons in low-energy heavy-ion collisions is affected by the equation of state, using the SMASH transport model, and compares results with experimental data.

Contribution

It demonstrates that the hadron resonance gas EOS best matches experimental polarization data at low energies within the SMASH model.

Findings

Hadron resonance gas EOS describes experimental data well at low energies.

Polarization as a function of centrality, rapidity, and transverse momentum agrees with experiments at 3 GeV.

A possible peak in polarization around 2.4 GeV in Au+Au collisions is indicated.

Abstract

Significant global polarization of $\Lambda$ hyperons along the direction of the orbital angular momentum has been measured in non-central heavy-ion collisions where the equation of state (EOS) of the produced dense matter is expected to change from intermediate to low colliding energies. We study the sensitivity of the global $\Lambda$ polarization to EOS in heavy-ion collisions within the SMASH transport model. Among the three different EOS we considered, only the hadron resonance gas (HRG) describes the experimental data well at low colliding energies even when it is below the $\Lambda$ production threshold in nucleon-nucleon collisions. The polarization induced by thermal vorticity as a function of centrality, rapidity, and transverse momentum at $\sqrt{s_{NN}} = 3$ GeV in Au+Au collisions is shown to agree well with the experimental data. Our study also indicates a possible peak in the global $\Lambda$ polarization around $\sqrt{s_{NN}} = 2.4$ GeV in Au+Au collisions. Furthermore, we find that the rapidity and transverse momentum-dependent helicity polarization induced by thermal vorticity vanishes due to space-reversal symmetry.