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 $T=5\mathrm{}–60\mu \mathrm{K}$. 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 $5\mu \mathrm{K}$ from the dissociation threshold.

• Received 28 January 1997

DOI:https://doi.org/10.1103/PhysRevLett.79.625