Observation of a zero-energy resonance in Cs-Cs collisions

We measure the cross section for s-wave collisions of Cs atoms in a magnetic trap. From the relaxation of the atomic cloud towards thermal equilibrium, we infer that the cross section varies as 1/T over the temperature range T = 5-60 mu K. It takes at each measured temperature the maximum allowed value for polarized bosons. This indicates a zero-energy resonance generated by a bound or virtual state less than 5 mu K from the dissociation threshold.