Tensor Polarization and Spectral Properties of Vector Meson in QCD Medium
Published 2022-06-23Version 1
Under the framework of thermal field theory, we calculate the tensor polarization of a generic vector meson in local equilibrium, which is closely related to the spin alignment measured in the heavy-ion collision experiments, up to the first order in the hydrodynamics gradient. At the zeroth order of the hydrodynamics gradient, the spin alignment of the vector meson emerges due to the divergence between the transverse and longitudinal spectral functions. At the first order, the tensor polarization can be induced by all the hydrodynamic gradients allowed by symmetry, including the shear strength tensor. The coefficient of the shear-induced tensor polarization (SITP) is calculated using linear response theory up to the order of one loop, and turns out related to the in-medium spectral properties of vector mesons, e.g. width and the in-medium mass-shift. Our results indicate that spin alignment can be utilized as a new probe revealing the in-medium spectral properties of vector mesons.