  We present a microscopic explanation of the controversially discussed
transient negative differential transmission observed in degenerate optical
pump-probe measurements in graphene. Our approach is based on the density
matrix formalism allowing a time- and momentum-resolved study of carrier-light,
carrier-carrier, and carrier-phonon interaction on microscopic footing. We show
that phonon-assisted optical intraband transitions give rise to transient
absorption in the optically excited hot carrier system counteracting pure
absorption bleaching of interband transitions. While interband transition
bleaching is relevant in the first hundreds of fs after the excitation,
intraband absorption sets in at later times. In particular, in the low
excitation regime, these intraband absorption processes prevail over the
absorption bleaching resulting in a zero-crossing of the differential
transmission. Our findings are in good agreement with recent experimental
pump-probe studies.
