Spin polarization in a relativistic fluid: dissipative contributions

The Quark-Gluon Plasma (QGP), a state of nuclear matter composed of deconfined quarks and gluons produced in relativistic heavy-ion collisions (HIC), is a strongly coupled fluid. Spin polarization measurements in HIC have opened new ways to probe the QGP, and lead to the discovery that QGP possesses large vorticity.

The observation of global and local spin polarization is explained as being induced by gradients of hydrodynamic quantities of the QGP at local thermal equilibrium, such as the thermal vorticity and the thermal shear. However, since not all the features of the measurements can be well reproduced by the models, the assumptions that dissipative effects are negligible and that the spin degrees of freedom of the QGP are equilibrated can be questioned. In this seminar, I will derive and classify the dissipative, out-of-equilibrium contributions to spin polarization at first order in gradients using statistical field theory methods. I will show that thermal shear can not give rise to dissipative corrections to spin polarization and I will discuss consequences on the interpretation of the measurements.