DIFFRACTIVE VECTOR-MESON PRODUCTION AT LARGE MOMENTUM-TRANSFER

The diffractive process gamma p --> V + X (where V is a vector meson a nd X results from the dissociation of the proton) is studied. In parti cular, we consider the region of large momentum transfer (i.e. \t\ muc h greater than Lambda(QCD)(2)) and large centre-of-mass (CM) energy, s . The asymptotic (s --> infinity, s/\t\ much greater than 1) behaviour is derived from the BFKL equation and compared to that which is obtai ned in the Born approximation (two-gluon exchange). We also calculate the corrections to the Born graphs by iterating the BFKL kernel numeri cally. Improved convergence of the BFKL series is found by summing the logarithms which occur when an exchanged gluon goes nearly on shell. Importantly, we find evidence that the asymptotic solution to the BFKL equation is inappropriate over most of the HERA range and we provide more realistic predictions for the cross section. The predicted cross section is not too small and can be measured at HERA, up to momentum t ransfers \t\ similar to 10 GeV2