Impact of spin polarization on transport and thermodynamic coefficients

Abstract: In this work, we investigate the influence of parton spin polarization on effective transport and thermodynamic coefficients in noncentral light- and heavy-ion collisions. To model this influence, we consider two sources of spin polarization: thermal vorticity, induced by angular momentum, and thermal shear, arising from local velocity gradients. Using a novel kinetic theory framework, we find that transport and thermodynamic coefficients -- including the speed of sound squared $c_{s}^{2}$, specific shear viscosity $\eta/s$, specific bulk viscosity $\zeta/s$, and mean free path $\lambda$ -- are substantially modified by spin polarization effects. Among the two sources, thermal vorticity-induced spin polarization dominates the modifications to these coefficients. Moreover, both $c_{s}^{2}$ and $\zeta/s$ exhibit a nonmonotonic dependence on the collision energy, and the associated scaling behaviors potentially serve as indicators of the critical phenomena of QCD.