HERA DVCS Results

Abstract

Deeply Virtual Compton Scattering is the simplest interaction that allows access to Generalised Parton Distributions (GPDs), a theoretical framework that can be used to describe nucleon structure. The strong interest in GPDs results from the fact that they offer access to the total angular momentum of quarks inside the nucleon and to a 3-dimensional picture of nucleon structure. The measurement of the DVCS process is facilitated by a competing interaction known as the Bethe-Heitler process which has the same final state. GPDs sensitive to asymmetries are accessed from the interference of these two processes. Measurements of DVCS data recorded at H1, ZEUS and HERMES are presented. The cross section results from H1 and ZEUS are measured as a function of the virtuality of the exchanged photon and the centre-of-mass energy W of the γ*p system. A beam charge asymmetry is extracted for the first time at a collider using H1 data in the low x≈Q2/ W2 kinematic domain. A subsample of events from ZEUS is used to produce the first direct measurement of the differential cross section as a function of t. Beam charge, beam spin and target asymmetries on both unpolarised and polarised hydrogen and deuterium targets have been extracted at HERMES. The extracted asymmetries are presented over the range of HERMES kinematic acceptance, with their dependence on kinematic variables t , xB and Q2 also shown and compared to a phenomenological model.