Project for a DVCS measurement at COMPASS

The high energy polarised muon beam available at CERN with the option of using positive or negative muons with opposite polarisation gives COMPASS an excellent possibility to study generalised parton distributions via deeply virtual Compton scattering. In a first step we propose to use an unpolarised proton target to study the slope of the $t$ dependence of the differential cross section as a function of $x_{Bj}$ to observe a possible shrinkage of the nucleon with increasing $x_{Bj}$. Furthermore, the beam charge and spin difference will be measured over a wide kinematical range to determine the Compton form factor related to the GPD $H$. As a second step we consider to use a transversely polarised proton target to collect data to constrain the GPD $E$. In preparation of the future measurements two DVCS test runs were performed in 2008 and 2009