Using impact-factor representation, we consider the lepton pair production by an incident high-energy photon in a strong electromagnetic field of a nucleus. By summing leading terms of perturbation series, we obtain a simple formula for the amplitude, valid to all orders in O(αZ) and arbitrary field of the nucleus. Using these results, we derive, in a simple manner, the results for the lepton pair production by a virtual incident photon in a Coulomb field. For real incident photon our results coincide with the known ones. Also, a particular example of a non-Coulomb potential is discussed in some detail.

The Q 2 dependence of the hard diffractive photoproduction of vector meson or photon and the range of pQCD validity AbstractWe consider two coupled problems.We study the dependence on photon virtuality Q 2 for the semihard quasielastic photoproduction of neutral vector mesons on a quark, gluon or realTo this end we calculate the corresponding amplitudes (in an analytical form) in the lowest nontrivial approximation of perturbative QCD. It is shown that the amplitude for the production of light meson varies very rapidly with the photon virtuality nearWe estimate the bound of the pQCD validity region for such processes.For the real incident photon the obtained bound for the ρ meson production is very high. This bound decreases fast with the increase of Q 2 , and we expect that the virtual photoproduction at HERA gives opportunity to test the pQCD results. The signature of this region is discussed. For the hardCompton effect the pQCD should work good at not too high p ⊥ , and this *

We consider the Coulomb correction (CC) to the e + e − pair production related to multiphoton exchange of the produced e ± with nuclei. The contribution of CC to the energy distribution of e + and e − as well as to the total pair production cross section are calculated with an accuracy of the order of 1 %. The found correction to the total Born cross section is negative and equal to −25 % at the RHIC for Au-Au and −14 % at the LHC for Pb-Pb collisions.Introduction. Two new large colliders with relativistic heavy nuclei, the RHIC and the LHC, are scheduled to be in operation in the nearest future. The charge numbers Z 1 = Z 2 = Z of the nuclei with masses M 1 = M 2 = M and their Lorentz factors γ 1 = γ 2 = γ = E/M are the following

We study in perturbative QCD the helicity ampltiudes of the process γ * p → ρp at large virtualities Q of the photon γ * . We estimate all spin flip amplitudes taking into account an important effect of the scale behaviour of the gluon density. The transition of a transverse virtual photon to a longitudinal vector meson is not small at typical HERA conditions. This helicity non-conserving amplitude leads by interference to a measurable effect in the distribution of the angle between the electron scattering and the meson production planes.