Observation of a Zero-Energy Resonance in Cs-Cs Collisions

Abstract

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 $T\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}5--60\ensuremath{\mu}\mathrm{K}$. 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 $5\ensuremath{\mu}\mathrm{K}$ from the dissociation threshold.