Shrinkage vs. anti-shrinkage of the diffraction cone in the exclusive vector mesons production

We investigate the energy behavior of the diffraction cone in the exclusive vector meson production in diffractive DIS within the k_t-factorization approach. In our calculations, we make full use of fits to the unintegrated gluon structure functions extracted recently from experimental data on F_2p. Confirming early predictions, we observe that shrinkage of the diffraction cone due to the slope of the Pomeron trajectory is significantly compensated by the anti-shrinkage behavior of the gamma -> V transition. In order to match recent ZEUS data on the energy behavior of the diffraction slope, alpha^prime_eff(J/psi, exp.) = 0.115 +/- 0.018 (stat.) +0.008-0.015(syst.) GeV^-2, we had to use an input value alpha^\prime_Pomeron = 0.25 GeV^-2. We investigate the compensation effect in detail and give predictions for Q^2-dependence of the rate of cone shrinkage for different vector mesons.