Multiquark matrix elements in the proton and three gluon exchange for exclusive ηc production in photon-proton diffractive scattering

Exclusive production of a eta(c) pseudoscalar meson in gamma((*)) + p -> eta(c) + p scattering at high energies involves a C-odd exchange in the t-channel. We formulate the description of this process within the highenergy framework of eikonal dipole scattering. We obtain expressions for the light-cone wave function of the eta(c) required in this framework as well as for the C-odd amplitudes due to exchange of a single photon, of a photon plus two gluons, and of three gluons. We relate these amplitudes to correlators of the plus component of the quark current in the light-cone wave function of the proton. For high transverse momenta, these correlators correspond to (leading twist) generalized parton distributions given by diagrams where all exchanged gauge bosons attach to a single quark in the proton. Diagrams involving multiquark matrix elements potentially sensitive to correlations screen infrared singularities. Moreover, they are numerically important for configurations where the exchanged bosons nearly share the total momentum transfer. Using two simple models for the three-quark Fock state of the proton at x similar or equal to 0.1, we find that single photon exchange dominates fort vertical bar t vertical bar 1.5 GeV2. Here, the quark generalized parton distribution could be measured cleanly in gamma((*)) + p -> eta(c) + p via single photon exchange. For higher momentum transfer, three-gluon (Odderon) exchange is dominant.