We have measured independently both $s$-wave and $p$-wave cross-dimensional thermalization rates for ultracold ${}^{40}\mathrm{K}$ atoms held in a magnetic trap. These measurements reveal that this fermionic isotope has a large positive $s$-wave triplet scattering length in addition to a low temperature $p$-wave shape resonance. We have measured directly the $p$-wave threshold law which, combined with the Fermi statistics, dramatically suppresses elastic collision rates at low temperatures. In addition, we present initial evaporative cooling results that make possible these collision measurements and are a precursor to achieving quantum degeneracy in this neutral, low-density Fermi system.

• Received 21 December 1998

DOI:https://doi.org/10.1103/PhysRevLett.82.4208