Semihard diffractive production of neutral mesons by off shell photons and the range of pQCD validity

We study the dependence on photon virtuality $Q^2$ for the semihard quasi--elastic photoproduction of neutral vector mesons on a quark, gluon or real photon (at $s\gg p_{\bot}^2, Q^2; p_{\bot}^2 \gg \mu^2 \approx (0.3$ GeV)$^2$). To this end we calculate the corresponding amplitudes (in an analytical form) in the lowest nontrivial approximation of the perturbative QCD. The amplitudes for the production of mesons consisting of light quarks vary very rapidly with the photon virtuality near $Q^2=0$. We estimate the bound of the pQCD validity region for such processes. For the process with mass shell photon the obtained bound is very high, and this region seems beyond opportunities of real experiment. This bound decreases fast with the increase of $Q^2$, and we expect that the virtual photoproduction at HERA provides the opportunity to test the pQCD results. The signature of this region is discussed.