Resonant scattering phase shifts of ultracold cesium and their energy dependence

This dissertation describes the first measurement of an atomic Feshbach resonance as a function of the collision energy between two clouds of ultracold cesium atoms. The resonance is observed as a large phase excursion in the s-wave collisional phase shift. The collision energy between two ultracold clouds of cesium in a juggling atomic fountain is adjusted from 12-50 muK in five energy steps. The magnetic field is tuned each collision energy from 0-1.8G to see several discernible resonances. Accurately measuring the position and width of Feshbach resonances can place stringent limits on the time variation of fundamental constants.;Atoms are prepared in a superposition of the clock states of cesium and scatter off target atoms in different states. We observe the scattering phase shift due to the interaction of the superposition with the target atoms near resonances.;This dissertation also includes: a detailed procedure that traces the course from atom cooling and launching to analysis of the collision data, descriptions of changes made to the apparatus, brief explanations of the laser cooling techniques employed, and the basic quantum scattering theory necessary to understand the experiment.