DGLAP versus perturbative Pomeron in large momentum transfer hard diffractive processes at HERA and LHC

Abstract

We evaluate within the LO DGLAP approximation the dependence on energy of the cross section of the photo(electro)production of vector meson V (V=J/ψ,…) in the hard elastic processes off a parton γ∗+g→V+g as the function of momentum transfer t=(qγ−pV)2. We demonstrate that in the limit −t⩾mV2+Q2 the cross section does not contain double logarithmic terms in any order of the DGLAP approximation leading to the energy independent cross section. Thus the energy dependence of cross section γ∗+p→J/ψ+rapidity gap+X is governed at large t by the gluon distribution within a proton, i.e. it is unambiguously predicted within the DGLAP approximation including the stronger WγN dependence at larger −t. This prediction explains recent HERA data. The calculations which follow perturbative Pomeron logic predict opposite trend of a weaker WγN dependence at larger t. We explain that at the HERA energies double logarithmic terms characteristic for DGLAP approximation dominate in the hard processes as the consequence of the constraints due to the energy–momentum conservation. We give predictions for the ultraperipheral hard diffractive processes at the LHC and show that these processes are well suited for looking for the contribution of the single logarithmic terms due to the gluon emission in the multi-Regge kinematics. We also comment on the interrelation between energy and t dependence of the cross sections of the hard exclusive processes.