Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The $Q^2$ dependence of the hard diffractive photoproduction of vector
  meson or photon and the range of pQCD validity   We consider two coupled problems.
  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 perturbative QCD. It
is shown that the amplitude for the production of light meson varies very
rapidly with the photon virtuality near $Q^2=0$.
  We estimate the bound of the pQCD validity region for such processes. For the
real incident photon the obtained bound for the $\rho$ meson production is very
high. This bound decreases fast with the increase of $Q^2$, and we expect that
the virtual photoproduction at HERA gives opportunity to test the pQCD results.
The signature of this region is discussed. For the hard Compton effect the pQCD
should work good at not too high $p_{\bot}$, and this effect seems measurable
at HERA.

12
The categories of this paper are: nucl-th
28
The categories of this paper are: alg-geom
31
The categories of this paper are: ao-sci
4
The categories of this paper are: cond-mat
17
The categories of this paper are: q-fin
22
The categories of this paper are: chao-dyn
29
The categories of this paper are: funct-an
10
The categories of this paper are: q-bio
2
The categories of this paper are: math
14
The categories of this paper are: math-ph
36
The categories of this paper are: cmp-lg
32
The categories of this paper are: atom-ph
11
The categories of this paper are: hep-lat
25
The categories of this paper are: patt-sol
30
The categories of this paper are: q-alg
7
The categories of this paper are: hep-th