Collision cross sections for the thermalization of cold gases

Abstract

The collision cross section that controls thermalization of gas mixtures is the transport cross section ση(1) and not the elastic cross section σel. The two are the same for pure s-wave scattering but not when higher partial waves contribute. We investigate the differences between them for prototype atomic mixtures and show that the distinction is important at energies above 100μK for LiYb and 3μK for RbYb and RbCs. For simple systems, both ση(1) and σel follow universal energy dependence that depends only on the s-wave scattering length when expressed in reduced length and energy units.