Meson spin alignment and baryon polarization from coalescence with spin-vorticity non-equilibrium

TL;DR

This paper presents a thermal coalescence model incorporating vorticity and polarization to estimate baryon polarization and meson spin alignment, aiming to explain experimental spin phenomena in heavy-ion collisions.

Contribution

It introduces a non-equilibrium coalescence framework for spin and vorticity, providing a potential explanation for puzzling experimental spin measurements.

Findings

Model can explain the absence of baryon polarization with significant meson spin alignment.

Suggests measuring meson spin off-diagonal elements to test the model.

Proposes analyzing azimuthal angle dependence of polarization observables.

Abstract

We estimate both the polarization of spin $1/2$ baryons and the spin density matrix coefficients of spin $1$ mesons using a thermal model that incorporates vorticity and polarization to describe quarks, along with a coalescence formalism where spin and vorticity are not in equilibrium. We find that while our model is not predictive due to its considerable number of free parameters, it has the potential to explain several seemingly puzzling features of experimental spin measurements, such as the absence of baryon polarization alongside significant vector meson spin alignment. We suggest the measuring meson spin off-diagonal matrix elements, and examining the dependence of polarization observables on azimuthal angles, as methods to falsify this model and gain insights into the freezeout details of baryon and meson spin structure.