Saturation effects in exclusive ρ T,L meson electroproduction

Abstract

We use recent results for the γ * L → ρ L and γ * T → ρ 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 γ * T → ρ T impact factor and up to twist 2 for the γ * L → ρ 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 GeV 2 , 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.

Unintegrated gluon distribution from forward polarized $$\rho $$ ρ -electroproduction

“…Moreover, as for the HSS UGD parametrization, we checked that the discrepancy between the so-called improved setup given in Eq. (19) and the standard one (see, e.g. Ref.…”

“…The dipole approach is based on similar physical ideas, but formulated not in κbut in the transverse coordinate space; this scheme is especially suitable to account for nonlinear evolution and gluon saturation effects. Interesting developments are the results of the papers [18,19], where the helicity amplitudes of VM production were considered in the dipole approach.…”

Phenomenology of the polarized cross-sections of the rho meson leptoproduction at high energy

“…2 are shown the predictions we obtained in ref. [1] compared to H1 data [9]. The asymptotic cross-section (AS) corresponds to the case where the renormalization scale in the model of distribution amplitudes is chosen asymptotically large.…”

Exclusive Processes: Theory Introduction

“…This dipole representation is consistent with the twist 2 collinear factorization, and it has been recently proven that it remains valid beyond leading twist. This leads to a very good description of HERA data, except at low Q 2 where higher twist corrections seem to be rather important [35]. An impact parameter analysis in this spirit would be very interesting, since it provides a probe of the proton shape, in particular through local geometrical scaling [36].…”