Quarks vs. Gluons in Exclusive Ρ Electroproduction

We compare the contributions from quark and from gluon exchange to the exclusive process γ * p → ρ 0 p. We present evidence that the gluon contribution is substantial for values of the Bjorken variable x B around 0.1. 1. There is an ongoing experimental and theoretical effort to determine generalized parton distributions [1, 2] from hard exclusive processes like deeply virtual Compton scattering and electroproduction of mesons. These distributions encode fundamental information about nu-cleon structure, in particular about the angular momentum carried by partons [2] and about their spatial distribution [3, 4]. An important process is the production of ρ 0 mesons, well suited for experimental study because of its relatively high cross section and its clean final state signature from the decay ρ 0 → π + π −. As pointed out in [5], the transverse target polarization asymmetry of this channel is sensitive to the nucleon spin-flip distribution E appearing in the angular momentum sum rule [2]. Quark and gluon distributions contribute to ρ production at the same order in α s , as seen in Fig. 1. For the separation of quark and gluon degrees of freedom this channel is thus a valuable complement to deeply virtual Compton scattering, which offers the cleanest and most detailed access to generalized parton distributions [6, 7], but is sensitive to gluons only at the level of α s