  We investigate the polarization dependence of the carrier excitation and
relaxation in epitaxial multilayer graphene. Degenerate pump-probe experiments
with a temporal resolution of 30 fs are performed for different rotation angles
of the pump-pulse polarization with respect to the polarization of the probe
pulse. A pronounced dependence of the pump-induced transmission on this angle
is found. It reflects a strong anisotropy of the pump-induced occupation of
photogenerated carriers in momentum space even though the band structure is
isotropic. Within 150 fs after excitation an isotropic carrier distribution is
established. Our observations imply the predominant role of collinear
scattering preserving the initially optically generated anisotropy in the
carrier distribution. The experiments are well described by microscopic time-,
momentum, and angle-resolved modelling, which allows us to unambiguously
identify non-collinear carrier-phonon scattering to be the main relaxation
mechanism giving rise to an isotropic distribution in the first hundred fs
after optical excitation.
