Evolution of Λ polarization in the hadronic phase of heavy-ion collisions

Abstract

Using the ampt$+$music$+$urqmd hybrid model, we study the global and local spin polarizations of $\mathrm{\ensuremath{\Lambda}}$ 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 $\mathrm{\ensuremath{\Lambda}}$ hyperons due to the thermal vorticity decrease by a factor of 2, while those due to the thermal shear decrease quickly and become negligibly small at 140 MeV. Our results thus suggest that it is important to take into account the evolution of the spin degree of freedom in the hadronic stage of relativistic heavy-ion collisions when theoretically predicted global and local $\mathrm{\ensuremath{\Lambda}}$ spin polarizations are compared with the experimental measurements.