Relaxation time for the alignment between the spin of a finite-mass quark or antiquark and the thermal vorticity in relativistic heavy-ion collisions

Abstract

We study the relaxation time required for the alignment between the spin of a finite-mass quark or antiquark and the thermal vorticity, at finite temperature and baryon chemical potential, in the context of relativistic heavy-ion collisions. The relaxation time is computed as the inverse of the total reaction rate that in turn is obtained from the imaginary part of the quark or antiquark self-energy. We model the interaction between spin and thermal vorticity within the medium by means of a vertex coupling quarks and thermal gluons that, for a uniform temperature, is proportional to the global angular velocity and inversely proportional to the temperature. We use realistic estimates for the angular velocities for different collision energies and show that the effect of the quark mass is to reduce the relaxation times as compared to the massless quark case. Using these relaxation times we estimate the intrinsic quark and antiquark polarizations produced by the thermal vorticity. We conclude by pointing out that, although the intrinsic global polarization of the $s$-quark turns out to be larger than that of the $\overline{s}$-quark, there is room to explain recent STAR results which show that the global polarization of $\overline{\mathrm{\ensuremath{\Lambda}}}$ is larger than that of $\mathrm{\ensuremath{\Lambda}}$ when considering that these hyperons can be produced from different density regions in a core-corona model.