Impact of spin polarization on transport and thermodynamic coefficients
De-Xian Wei
TL;DR
This work addresses how spin polarization, induced by thermal vorticity $\varpi_{\rho\sigma}$ and thermal shear $\xi_{\rho\sigma}$, modifies transport and thermodynamic coefficients in noncentral heavy-ion collisions. It develops a kinetic theory with spin-dependent distribution, introduces VIP and VIP+SIP polarization, and derives spin-polarized TTCs such as $c_s^2$, $\eta/s$, $\zeta/s$, and $\lambda$. The results show that spin polarization can markedly alter these coefficients, with $c_s^2$ and $\zeta/s$ exhibiting nonmonotonic energy dependence and VIP usually dominating over SIP, while $\eta/s$ and $\lambda$ display monotonic behavior; the nonmonotonic features scale with system size and may signal proximity to the QCD critical point near $\sqrt{s_{NN}} \sim 19.6$–$27$ GeV. Using AMPT simulations across collision systems from O+O to Au+Au, the study suggests potential observable signatures of critical phenomena in spin-related transport, though it omits magnetic-field effects and a fully self-consistent dynamical evolution of spin polarization.
Abstract
This work investigates the influence of parton spin polarization on effective transport and thermodynamic coefficients in noncentral light- and heavy-ion collisions. To model this influence, I 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, one finds that transport and thermodynamic coefficients -- including the speed of sound squared $c_{s}^{2}$, specific shear viscosity $η/s$, specific bulk viscosity $ζ/s$, and mean free path $λ$ -- 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 $ζ/s$ exhibit a nonmonotonic dependence on the collision energy, and the associated scaling behaviors potentially serve as indicators of the critical phenomena of quantum chromodynamics.