Collective dynamics of polarized spin-half fermions in relativistic heavy-ion collisions

Abstract: Relativistic hydrodynamics has been quite successful in describing the properties of strongly-interacting matter produced in heavy-ion collision experiments. Recently, there has been a significant advancement in this field to explain the spin polarization of hadrons emitted in these processes. Although current models have successfully explained some of the experimental data based on spin-vorticity coupling, they still lack a clear understanding of differential measurements. This is an indication that the spin needs to be treated as an independent degree of freedom whose dynamics is not entirely bound to flow circulation. In particular, if the spin is a macroscopic property of the system, in equilibrium its dynamics should follow hydrodynamic laws. In this thesis, we develop a framework of relativistic perfect-fluid hydrodynamics which includes spin degrees of freedom from kinetic theory, and use it for modeling the dynamics of matter produced in relativistic heavy-ion collisions. Following experimental observations, we assume that the polarization effects are small and derive conservation laws for net-baryon current, energy-momentum tensor, and spin tensor based on the de Groot-van Leeuwen-van Weert pseudogauge. Subsequently, we present various properties of the spin polarization tensor and its components, analyze the propagation properties of the spin polarization components, and derive the spin-wave velocity for arbitrary statistics. We find that only the transverse spin components propagate, analogously to the EM waves. Finally, we study the spacetime evolution of spin polarization for the systems respecting certain spacetime symmetries and calculate the mean spin polarization per particle, which can be compared to the experimental data. We find that, for some observables, our spin polarization results agree qualitatively with the experimental findings and other model calculations.