Q2 dependence of the hard diffractive photoproduction of a vector meson or photon and the range of PQCD validity

We consider two coupled problems. We study the dependence on the photon virtuality Q2 for the semihard quasi-elastic photoproduction of neutral vector mesons on a quark, gluon, or real photon [at s>pperpendicular2,Q2;pperpendicular2>μ2≅(0.3 GeV)2]. To this end we calculate the corresponding amplitudes (in an analytical form) in the lowest nontrivial approximation of perturbative QCD. It is shown that the amplitude for the production of a light meson varies very rapidly with the photon virtuality near Q2=0. We estimate the bound of the PQCD validity region for such processes. For a real incident photon the obtained bound for ρ meson production is very high. This bound decreases fast with the increase of Q2, and we expect that the virtual photoproduction at DESY HERA will give us the opportunity to test the PQCD results. The signature of this region is discussed. For the hard Compton effect PQCD should work well at not too high pperpendicular, and this effect seems measurable at HERA. copyright 1996 The American Physical Society