  The wavefunction for indistinguishable fermions is anti-symmetric under
particle exchange, which directly leads to the Pauli exclusion principle, and
hence underlies the structure of atoms and the properties of almost all
materials. In the dynamics of collisions between two indistinguishable fermions
this requirement strictly prohibits scattering into 90 degree angles. Here we
experimentally investigate the collisions of ultracold clouds fermionic
$\rm^{40}K$ atoms by directly measuring scattering distributions. With
increasing collision energy we identify the Wigner threshold for p-wave
scattering with its tell-tale dumb-bell shape and no $90^\circ$ yield. Above
this threshold effects of multiple scattering become manifest as deviations
from the underlying binary p-wave shape, adding particles either isotropically
or axially. A shape resonance for $\rm^{40}K$ facilitates the separate
observation of these two processes. The isotropically enhanced multiple
scattering mode is a generic p-wave threshold phenomenon, while the axially
enhanced mode should occur in any colliding particle system with an elastic
scattering resonance.
