Spin polarization in non equilibrated relativistic plasmas

This project addresses non-equilibrium effects and equilibration in relativistic plasmas focusing on spin physics. I will use techniques from quantum-relativistic statistical mechanics, quantum field theory, and many-body theory to answer some of the most pressing questions concerning spin polarization in the quark-gluon plasma (QGP). The QGP is created in laboratories such as the “Large Hadron Collider” and the “Relativistic Heavy Ion Collider” by smashing heavy ions at relativistic speed. It is a state of matter well described by relativistic hydrodynamics where quarks and gluons are no longer confined inside nuclei, and it is the hottest and least viscous fluid in nature. Due to the initial orbital angular momentum, non-central collisions create a rotating QGP, which is the most vortical fluid ever observed. Vorticity couples to the spin of particles, aligning it with the total angular momentum and producing a spin polarization that is measured experimentally. This effect connects the macroscopic motion of the plasma to a microscopic and entirely quantum property such as spin. Recently, another source of polarization has been found in the “thermal shear flow”, which is only present in out-of-equilibrium fluids. Indeed, only the thermal vorticity can be present in equilibrium, and the thermal shear is zero.Studying spin in out-of-equilibrium systems requires tackling some important theoretical questions. The coupling of spin to shear is surprising compared to its coupling to vorticity: how does it happen? The results of any spin calculation depend on the phenomenological definition of a spin density of the fluid: can such a density be fixed microscopically? Finally, any out-of-equilibrium state should relax to equilibrium: how does the relaxation to a vortical equilibrium state happen for a relativistic plasma? These questions will be addressed in my project, SPINNERET.