Speaker
Description
Spin polarization has become a sensitive probe of vorticity and shear structures in the quark-gluon plasma. While global hyperon polarization can be largely understood from spin-vorticity coupling, the tensor polarization, or spin alignment, of vector mesons remains a challenging observable whose magnitude and sign depend strongly on collision energy, momentum, and meson species.
In this talk, I will present a quantum-statistical formulation of vector-meson spin alignment at local thermodynamic equilibrium based on 2412.19416. Starting from the Proca field with canonical stress-energy and spin tensors, we derive Cooper-Frye-type expressions for the vector and tensor spin polarization using the local equilibrium density operator and Wigner-function formalism. The calculation shows that, unlike the vector polarization of spin-1/2 particles, the leading purely hydrodynamic contribution arises at second order of derivative expansion and provides a controlled baseline involving thermal vorticity, thermal shear, spin potential, and their gradients.
I will then discuss how the geometry of the freeze-out hypersurface modifies this baseline based on 2509.20200. A curved space-like freeze-out surface generates an additional tensor polarization at the first order of derivative expansion induced by its curvature tensor. Analytic estimates in Bjorken and Gubser flows indicate a negative contribution to the spin alignment in gold-gold collisions, typically at the $10^{-4} -10^{-3}$ level. Further study of the Gubser flow shows that the magnitude depends largely on the transverse size of the collision system. A rough estimation shows that in O-O collisions, the magnitude of the geometry effect would reach $10^{-2}$, making spin alignment in small systems a cleaner probe of freeze-out geometry effects.