Elastic production of Vector Mesons at HERA: study of the scale of the interaction and measurement of the helicity amplitudes

Abstract

A compilation of H1 and ZEUS cross section measurements for elastic vector meson production is presented as a function of the scale $K^2 = (Q^2+M_V^2)/4$, where Q^2 is the exchanged photon virtuality and M_V is the mass of the vector meson. The ratio of longitudinal to transverse cross sections R = $σ_L/σ_T$ is presented as a function of Q^2/M^2_V. The cross sections are separated in a transverse and a longitudinal component, and are presented as a function of K^2. The intercept $α(0)$ -- 1 governing the energy dependence of the vector meson cross sections is compared with the $λ$ parameter measured in inclusive F_2 analysis. For $ρ$ meson production, the helicity amplitude ratios $|T_{ij}|/|T_{00}|$, extracted from the H1 and ZEUS measurements of the spin density matrix elements are presented as a function of Q^2 and W and are compared to recent predictions.