Sudakov Form Factors in Leptoproduction of Vector Mesons

It is shown that Sudakov form factors for a colour dipole in a QCDinspired model of leptoproduction of vector mesons reduce the value of the cross section of the process by an order of magnitude. They suppress the large size quark-antiquark pairs and unequal sharing of energy among the components of the dipole. Some freedom in the choice of the model parameters is also discussed. The idea of inelastic diffraction goes back to the papers of I.Ya. Pomeranchuk and E.L. Feinberg [1]. The particular example is provided by the diffraction dissociation of a virtual photon to any vector meson on a nuclear target. Recently, the processes of diffractive leptoproduction of vector mesons in interactions of high energy electrons and muons with nucleons and nuclei were intensively investigated both experimentally and theoretically. The most popular approach to the problem of their description is provided by a model inspired by ideas of quantum chromodynamics (QCD) where the exchanged photon with the four-momentum squared (the virtuality) Q2 is treated as a quark-antiquark pair which interacts with a target nucleon by the two-gluon exchange [2, 3, 4, 5]. Correspondingly, in the simplest approximation the diffractive cross section of ”elastic” leptoproduction of vector mesons is defined by a product of wave functions of the virtual photon and of the vector meson with the amplitude of elastic scattering of the quark-antiquark dipole off the nucleon. This factorization model becomes valid at high energies because, due to the Lorentz transformation, those longitudinal distances at which the photon separates as the colour dipole (and, later, this dipole transforms into the vector meson) exceed strongly the range of distances of the intermediate interaction of the dipole with the target nucleon. That is why the whole process can be treated as if separated into three independent stages. First, long before its contact with the target nucleon, the virtual photon forms the quark-antiquark component of its wave function. At the second stage, the scattering of this colour dipole on the target takes place during the rather short time when the transverse size of the dipole does not change