Evolution of $\Lambda$ polarization in the hadronic phase of heavy-ion collisions

TL;DR

This study investigates how the spin polarization of $\\Lambda$ hyperons evolves during the hadronic phase of heavy-ion collisions, highlighting the roles of thermal vorticity and shear and their dependence on freeze-out temperature.

Contribution

It introduces a detailed analysis of spin polarization evolution in the hadronic phase using a hybrid model, emphasizing the impact of thermal vorticity and shear at different freeze-out temperatures.

Findings

Spin polarization decreases by a factor of two as temperature drops from 160 MeV to 110 MeV.

Thermal shear contributions become negligible below 140 MeV.

Results highlight the importance of spin dynamics in the hadronic stage of heavy-ion collisions.

Abstract

Using the AMPT + MUSIC+UrQMD hybrid model, we study the global and local spin polarizations of $\Lambda$ hyperons as functions of the freeze-out temperature of the spin degree of freedom in the hadronic phase of Au+Au collisions at $\sqrt{s_{NN}}=19.6$ GeV. Including contributions from both the thermal vorticity and thermal shear of the hadronic matter, we find that, with the spin freeze-out temperature dropping from the hadronization temperature of 160 MeV to 110 MeV at the kinetic freeze-out, both the global and local spin polarizations of $\Lambda$ hyperons due to the thermal vorticity decrease by a factor of two, while those due to the thermal shear decrease quickly and become negligibly small at 140 MeV. Our results suggest the importance of understanding the dynamical evolution of the spin degree of freedom in the hadronic stage in relativistic heavy-ion collisions.