Saturation effects in exclusive ρ T,L meson electroproduction

Abstract

Abstract We use recent results for the $ \gamma_L^{*}\to {\rho_L} $ and $ \gamma_T^{*}\to {\rho_T} $ impact factors, computed in the impact parameter representation within the collinear factorization scheme, to get predictions for the polarized cross-sections σ T and σ L of the diffractive leptoproduction of the ρ meson at high energy. In this approach the helicity amplitude is a convolution of the scattering amplitude of a color dipole with a target, together with the virtual gamma wave function and with the first moments of the ρ meson wave function (in the transverse momentum space), given by the distribution amplitudes up to twist 3 for the $ \gamma_T^{*}\to {\rho_T} $ impact factor and up to twist 2 for the $ \gamma_L^{*}\to {\rho_L} $ impact factor. Combining these results with recent dipole models fitted to DIS data, which include saturation effects, we show that the predictions are in good agreement with HERA data for photon virtuality (Q 2) larger than typically 5 GeV2, without free parameters and with a weak dependence on the choice of the factorization scale, i.e. the shape of the DAs, for both longitudinally and transversely polarized ρ meson. For lower values of Q 2, the inclusion of saturation effects is not enough to provide a good description of HERA data. We believe that it is a signal of a need for higher twist contributions in the ρ meson DAs. We also analyze the radial distributions of dipoles between the initial γ * and the final ρ meson states.