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 T0ex{0ex}=0ex{0ex}5--60. 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 from the dissociation threshold.