Field Dependence of Spin Relaxation in a Dense Rb Vapor

We observe that the Rb-Rb relaxation rate for spin polarized Rb is reduced by a factor of 3 in magnetic fields of a few kG, even at multiatmosphere buffer gas pressures. This reduction is proportional to the Rb density and is independent of buffer gas pressure between 100 and 3000 Torr. We also report anomalously large relaxation rates below 100 Torr. Both of these observations are inconsistent with the previously held assumption that the Rb-Rb relaxation arises from sudden binary collisions.