QCD description of diffractive ρ meson electroproduction

We critically review the QCD predictions for the cross sections σL and σT for diffractive ρ meson electroproduction in longitudinally and transversely polarized states in the DESY HERA energy region. We show that both perturbative and nonperturbative approaches, which involve convolution with the ρ meson wave function, predict values of σT which fall off too quickly with increasing Q², in comparison with the data. We present a perturbative QCD model based on the open production of light qq pairs and parton-hadron duality, which describes all features of the data for ρ electroproduction at high Q² and, in particular, predicts a satisfactory Q² behavior of σL/σT. We find that precise measurements of the latter can give valuable information on the Q² behavior of the gluon distribution at small x.