  The vorticity is a quantity defined in a relativistic fluid that describes
how much a fluid element is rotating and accelerating. By measuring the spin
polarization of hadrons, it was found that the quark gluon plasma produced in
heavy-ion collisions is the most "vorticous" fluid ever observed. More
generally, this opens the possibility to study the physics of QCD matter using
spin. Here I use statistical quantum field theory applied to a fluid in local
thermodynamic equilibrium to show how to connect the average spin of a fermion
with hydrodynamic quantities, and in particular with the thermal vorticity and
the thermal shear. I show that the spin polarization of a Dirac particle
induced by thermal vorticity is related to the gravitational (in medium) form
factor related to spin-rotation coupling. For these reasons, as we are
understanding the role of spin in hydrodynamics and in heavy-ion collisions,
spin is becoming a promising tool to investigate the properties of QCD and
whose applications are just begun to be explored.
