  The non-equilibrium semiconductors physics is based on the paradigm that
different degrees of freedom interact on different timescales. In this context
the photo-excitation is often treated as an impulsive injection of electronic
energy that is transferred to other degrees of freedom only at later times.
Here, by studying the ultrafast particles dynamics in a archetypal strongly
correlated charge-transfer insulator (La2CuO4), we show that the interaction
between electrons and bosons manifest itself directly in the photo-excitation
processes of a correlated material. With the aid of a general theoretical
framework (Hubbard Holstein Hamiltonian), we reveal that sub-gap excitation
pilots the formation of itinerant quasi-particles which are suddently dressed
(<100 fs) by an ultrafast reaction of the bosonic field.
