Studies of DVCS and photoproduction of photons at high t with the H1 Detector

The diffractive scattering of photon off proton $\gamma^{(*)} p \to \gamma Y$ (where $Y$ denotes the proton dissociative system or the elastically scattered proton) is measured in two different kinematic regimes at high energy with the H1 detector at HERA. i) the process is studied for the first time at large transverse photon momentum $p_t^{\gamma} > 2$ GeV in photoproduction regime ($Q^2 < 0.01$ GeV$^2$)and photon-proton c.m.s. energy $175 < W < 247 {\rm GeV}$. The cross section is measured differentially in $t$ and in $\xpom$. The measurement is compared to a LL BFKL prediction of QCD. ii) in electroproduction, the Deeply Virtual Compton Scattering (DVCS) $\gamma^* p \to \gamma p$ cross section has been measured more precisely and in an extended kinematic domain: at photon virtualities $4<Q^2<80$ GeV$^2$ and $30<W<140$ GeV. The measurement is compared to NLO QCD calculations using the DGLAP approximation and to Colour Dipole model predictions.