The $t$-dependence of the pure DVCS cross section at COMPASS

The key reactions to study the Generalised Parton Distributions are Deeply Virtual Compton Scattering (DVCS) and Deeply Virtual Meson Production (DVMP). At COMPASS, these processes are investigated using a high intensity muon beam with a momentum of 160\,GeV/c and a 2.5\,m-long liquid hydrogen target. In order to optimize the selection of exclusive reactions at these energies, the target is surrounded by a new barrel-shaped time-of-flight system to detect the recoiling particles. COMPASS-II covers the up to now unexplored $x_{B}$ domain ranging from 0.01 to 0.15. From the sum of cross sections measured with positive and negative beam polarities, the pure DVCS cross-section and its $t$-dependence have been extracted resulting in a first model-independent determination of the transverse size of the partonic distribution of the nucleon $\sqrt{<r_{\perp}^2>}= (0.578 \ \pm \ 0.042\ _{- \ 0.018}^{+ \ 0.006})\,\textsf{fm} $ at a mean $x_{B}$ value of 0.056.