Effect of thermal shear on longitudinal spin polarization in a thermal model

TL;DR

This paper investigates how thermal shear influences the longitudinal spin polarization of bc hyperons in a hot, rotating medium, revealing that thermal shear alone can produce the correct polarization sign but cancels with vorticity effects, leading to discrepancies with experimental data.

Contribution

It introduces the thermal shear term into the polarization model and systematically analyzes its impact, highlighting the cancellation effects with vorticity in a thermal model at RHIC energies.

Findings

Thermal shear alone yields the correct polarization sign.

Thermal shear and vorticity effects largely cancel each other.

Model predictions do not fully agree with experimental data.

Abstract

By including the recently introduced thermal shear term that contributes to the spin polarization vector at local equilibrium, we determine longitudinal polarization of $\Lambda$ hyperons emitted from a hot and rotating hadronic medium using the thermal model with single freeze-out. In our analysis, we consider the RHIC top energies and use the model parameters which were determined in the earlier analyses of particle spectra and elliptic flow. We confirm that, unlike the previous calculations done by using only the thermal vorticity, the thermal shear term alone leads to the correct sign of the quadrupole structure of the longitudinal component of the polarization three-vector measured in experiments. However, we find almost complete cancellation between thermal shear and vorticity terms, which eventually leads to disagreement with the data. To clarify the role played by velocity and temperature gradient terms, we present a systematic analysis of different contributions to the longitudinal polarization.