OBSERVATION OF A ZERO-ENERGY RESONANCE IN CS-CS COLLISIONS

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