Photon-gluon fusion at HERA: measurement of the fractional momentum of the gluon in lowQ 2 events

We have performed a Monte Carlo study of photon-gluon interactions at theep collider HERA. We show that it is possible to determine the fractional momentum carried by the gluon (x _g) from hadronic informations alone. In particular, in the photoproduction region, where thex andQ ² are badly measured, this could be the only global physical variable of interest, together with the hadronic invariant mass. The study of these quantities would allow a direct measurement of the gluon density of the proton in the rangex _g=2.5×10^?3∶5×10^?1.