Hadronic scattering 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. Because of its large mass, the Λ 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 Λ − π spin-dependent cross sections included in a schematic kinetic model, the effects of hadronic scatterings on the Λ spin polarization in Au-Au collisions at s N N = 7.7 GeV are studied. It is found that the Λ spin polarization only decreases by 7-12% during the hadronic stage of these collisions, which justifies the assumption in theoretical studies that compare the Λ polarization calculated at the chemical freezeout to the measured one at the kinetic freezeout.