Exact spin polarization of massive and massless particles in relativistic fluids at global equilibrium

Abstract

We present the exact form of the spin polarization vector and the spin density matrix of massive and massless free particles of any spin and helicity at general global equilibrium in a relativistic fluid with non-vanishing thermal vorticity, thus extending the known expression at the linear order. The exact form is obtained by means of the analytic continuation of the relativistic density operator to imaginary thermal vorticity and the resummation of the obtained series. The phenomenological implications for the polarization of the $\Lambda$ hyperon in relativistic heavy-ion collisions are addressed.