  We study ultracold collisions in fermionic ytterbium by precisely measuring
the energy shifts they impart on the atom's internal clock states. Exploiting
Fermi statistics, we uncover p-wave collisions, in both weakly and strongly
interacting regimes. With the higher density afforded by two-dimensional
lattice confinement, we demonstrate that strong interactions can lead to a
novel suppression of this collision shift. In addition to reducing the
systematic errors of lattice clocks, this work has application to quantum
information and quantum simulation with alkaline-earth atoms.
