Direct Observation of Resonant Scattering Phase Shifts and their Energy   Dependence
  We scan the collision energy of two clouds of cesium atoms between 12 and 50 $\mu$K in atomic fountain clock. By directly detecting the difference of s-wave scattering phase shifts, we observe a rapid variation of a scattering phase shift through a series of Feshbach resonances. At the energies we use, resonances that overlap at threshold become resolved. Our statistical phase uncertainty of 8 mrad can be improved in future precision measurements of Feshbach resonances to accurately determine the Cs-Cs interactions, which may provide stringent limits on the time variation of fundamental constants. 
Stephen D. Gensemer, Ross B. Martin-Wells, Aaron W. Bennett, Kurt   Gibble
physics.atom-ph cond-mat.quant-gas quant-ph