QCD description of diffractiveρmeson electroproduction

Abstract

We critically review the QCD predictions for the cross sections ${\sigma }_L$ and ${\sigma }_T$ for diffractive $\rho$ 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 $\rho$ meson wave function, predict values of ${\sigma }_T$ which fall off too quickly with increasing $Q^2$, in comparison with the data. We present a perturbative QCD model based on the open production of light $q\mathrm{q¯}$ pairs and parton-hadron duality, which describes all features of the data for $\rho$ electroproduction at high $Q^2$ and, in particular, predicts a satisfactory $Q^2$ behavior of ${\sigma }_L/{\sigma }_T.$ We find that precise measurements of the latter can give valuable information on the $Q^2$ behavior of the gluon distribution at small $x$.