Hadronic effects on Λ polarization in relativistic heavy ion collisions

The Λ 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 Σ^*(1358) resonance and the t-channel process via the exchange of a scalar σ meson. Although the t-channel process gives a thermally averaged cross section that is about a factor of 1.3 larger than that from the s-channel process, the Λ spin flip to non-flip cross sections is negligibly small in the t-channel compared to the constant value of 1/3.5 in the s-channel process. With these cross sections included in a schematic kinetic model, the effects of hadronic scatterings on the Λ spin polarization in Au-Au collisions at √sNN=7.7 GeV are studied. It is found that the Λ spin polarization only decreases by about 7% during the hadronic stage of these collisions, which thus justifies the assumption in theoretical studies that compare the Λ polarization calculated at the chemical freeze out to the measured one at the kinetic freeze out.