New Results on Deeply Virtual Compton Scattering at H1, Zeus and Hermes

The process of deeply virtual Compton scattering will be shortly introduced, and the latest results from measurements at the HERA ep–collider at DESY will be given. In particular, the cross section has been measured with increased statistics at the collider experiments H1 and ZEUS, while HERMES for the first time reports measurements of the t–dependence of the beam–charge asymmetry on hydrogen and of a beam–charge asymmetry on deuterium. Hard exclusive processes such as the Deeply Virtual Compton Scattering (DVCS) process γ * p → γp can be expressed in terms of Generalized Parton Distributions (GPDs) 1,2. GPDs give a generalized description of the partonic structure of the nu-cleon, where the ordinary Parton Distribution functions (PDFs) and the nucleon form factors turn out to be the kinematic limits and the moments of GPDs, respectively 3. Besides the fact that the DVCS process appears to provide the theoretically most direct access to GPDs, it is also investigated in the framework of diffractive processes. While the DVCS process is similar to the diffrac-tive electroproduction of vector mesons, it avoids the theoretical complications of needing further non–perturbative information due to the vector meson distribution amplitude in the final state. Furthermore, it is also unique among the hard scattering processes in that DVCS amplitudes (magnitude and phase) can be determined. This is possible through a measurement of the interference between the DVCS and Bethe–Heitler (BH) processes, in which the photon is radiated from a parton in the former and from the lepton in the latter process. These processes have an identical final state, i.e., they are indistinguishable and thus the photon production amplitude τ is given as the coherent sum of the amplitudes of the DVCS (τ DV CS) and BH (τ BH) processes. The cross section for the exclusive leptoproduction of photons is then given as (see Ref.