Vector meson electroproduction: recent results and universality

Abstract

In a nonperturbative approach we have made extensive and comprehensive calculations of the processes of vector meson electroproduction. We here confront our results with recent data on the elastic processes of p and <j) electroproduction. The description is unified for all vector mesons, without free parameters, covering 5 scales of magnitude in cross sections, exhibiting a universality that is determined by properties of the overlap of photon and vector meson wave functions. Some recent results on p and 0 electroproductionIn a nonperturbative QCD framework [1] based on the Stochastic Vacuum Model, we have calculated the observables of vector meson electroproduction, with very good description of the data in all cases, without any free parameter. New measurements in the p elastic case [2], and preliminary information from HI collaboration [3] on -fp -^ pp and "fp ^ p(j> aie now available, including t distribution, polarization matrix elements and Q^ and W dependencies. It is important to study these data, since the broader wave functions of the light vector mesons impose more rigorous tests on the dynamical mechanisms [4] .Our calculation gives definite predictions for the t dependence of the differential cross sections, with a curvature in the log plot, described by the form da d\r\ da dW e-b\t\ ,." (l + «kl)^ (1) The parameters a and b are connected to ranges of the photon and vector meson wave functions, to the proton size and to interaction range. Numerical values, showing regular and universal behaviour of the form factors, have been given before [1].The curvature is obvious in some experimental results, such as in p production for g^ = 0 shown in the LHS of fig. 1, and sometimes is not so obvious as in recent data at Q^ = 40 GeV^ shown in the RHS, since the straight lines may look satisfactory within the errors. A more dramatic demonstration of curvature can be given by plotting the CPl 105, Diffraction 2008, edited by R. Fiore, I. Ivanov, A. Papa, and J. Soffer