Quantum Scattering in a Juggling Atomic Fountain

We demonstrate a juggling atomic fountain by launching two laser-cooled balls of ${}^{133}\mathrm{Cs}$ atoms in rapid succession. The atoms collide near the top of the fountain and, by varying the delay between launches, we scan the collision energy from 19 to $150\mu \mathrm{K}$. We measure the differential scattering cross section and isolate the contributions from different partial waves including their interferences. We observe the $s$-wave energy dependence and the $p$-wave scattering threshold. We infer a triplet $s$-wave scattering length of ${-400a}_0$ and a $p$ wave of $({-107a}_0{)}^3$ for $|F=4,m=4\mathrm{}〉+|3,3〉$.