Hadronic scattering effects on $\Lambda$ polarization in relativistic heavy ion collisions

TL;DR

This paper investigates how hadronic scatterings affect $ ext{Lambda}$ hyperon polarization in heavy ion collisions, finding only a modest decrease during the hadronic phase, supporting previous assumptions in polarization studies.

Contribution

It introduces a simple hadronic model including specific resonance and meson exchange processes to quantify hadronic effects on $ ext{Lambda}$ polarization.

Findings

$ ext{Lambda}$ polarization decreases by only 7-12% during hadronic stage.

The $s$-channel process dominates the spin flip cross section.

The results support the assumption that polarization at chemical freezeout approximates that at kinetic freezeout.

Abstract

The $\Lambda$ hyperon spin flip and non-flip cross sections are calculated in a simple hadronic model by including both the $s$-channel process involving the spin 3/2, positive parity $\Sigma^*(1358)$ resonance and the $t$-channel process via the exchange of a scalar $\sigma$ meson. Because of its large mass, the $\Lambda$ spin flip to non-flip cross sections is negligibly small in the $t$-channel process compared to the constant value of 1/3.5 in the $s$-channel process. With the $s-$channel $\Lambda-\pi$ spin-dependent cross sections included in a schematic kinetic model, the effects of hadronic scatterings on the $\Lambda$ spin polarization in Au-Au collisions at $\sqrt{s_{NN}}=7.7$ GeV are studied. It is found that the $\Lambda$ spin polarization only decreases by 7-12\% during the hadronic stage of these collisions, which justifies the assumption in theoretical studies that compare the $\Lambda$ polarization calculated at the chemical freezeout to the measured one at the kinetic freezeout.