Measurement of elastic cross section for cold cesium collisions

We have measured the time taken for a magnetically trapped cloud of cold cesium atoms in the ${(F=3,m}_F=-3)$ ground state to rethermalize from a nonequilibrium spatial and velocity distribution. From these measurements we infer the dependences of the elastic scattering cross section on temperature and magnetic field in the ranges $1-30\mu \mathrm{K}$ and 0.05–2.0 mT, respectively. We determine a lower bound on the magnitude of the $(3,-3)+(3,-3)$ s-wave scattering length of ${940a}_0.$