The QCD description of diffractive ρ meson electroproduction

We critically review the QCD predictions for the cross sections σL and σT for diffractive ρmeson electroproduction in longitudinally and transversely polarised states in the HERA energy region. We show that both perturbative and non-perturbative 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 behaviour of σL/σT . We find that precise measurements of the latter can give valuable information on the Q2 behaviour of the gluon distribution at small x. Permanent address: Laboratory of Theoretical Nuclear Physics, St. Petersburg Nuclear Physics Institute, Gatchina, St. Petersburg 188350, Russia.