Spin and polarization: a new direction in relativistic heavy ion physics

Abstract

Since the first evidence of a global polarization of Λ hyperons in relativistic nuclear collisions in 2017, spin has opened a new window in the field, both at experimental and theoretical level, and an exciting perspective. The current state of the field is reviewed with regard to the theoretical understanding of the data, reporting on the most recent achievements and envisioning possible developments. The intriguing connections of spin physics in relativistic matter with fundamental questions in quantum field theory and applications in the non-relativistic domain are discussed.

Spin alignment of vector mesons by glasma fields

“…Surprisingly, the predictions for the local spin polarization, i. e. the momentum dependence of the longitudinal spin polarization [19,26], fails to agree with the measured values [11]. This result has motivated further theoretical developments, trying to clarify the origins of spin polarization and spin transport phenomena in relativistic heavy ion collisions (see references [54,55] for recent reviews). Such developments include investigating the roles of the symmetric gradients of hydrodynamic variables (known as thermal shear) [27,29,30], gradients of chemical potentials [27,28] and spin potentials [35].…”

The exploration of hot and dense nuclear matter: introduction to relativistic heavy-ion physics

“…The orientation of the polarization vector depends on the direction of the hyperon transverse momentum and shows a quadrupole pattern with respect to the beam axis [391]. This effect is now understood as a result of the shear caused by the anisotropy of the transverse flow (see [392] for a review).…”