  A novel approach based on the merging of the out--of--equilibrium Green's
function method with the ab-initio, Density--Functional--Theory is used to
describe the ultra--fast carriers relaxation in Silicon. The results are
compared with recent two photon photo--emission measurements. We show that the
interpretation of the carrier relaxation in terms of L -> X inter--valley
scattering is not correct. The ultra--fast dynamics measured experimentally is,
instead, due to the scattering between degenerate $L$ states that is activated
by the non symmetric population of the conduction bands induced by the laser
field. This ultra--fast relaxation is, then, entirely due to the specific
experimental setup and it can be interpreted by introducing a novel definition
of the quasi--particle lifetimes in an out--of--equilibrium context.
