The Photon in Diffraction at HERA

The universality of the diffractive proton structure is investigated at HERA using structure function measurements in diffractive deep-inelastic scattering (DIS) and diffractive jet and charm production in DIS and photoproduction. While all point-like photon scattering processes can be described using the same parton densities (factorisation), the situation is different in real photon scattering where the diffractive dijet cross section measured by H1 for is a factor of approximately 0.5 lower than expectations based on the DIS parton densities (factorisation breaking). Strikingly, the suppression factor (or ‘gap survival probability’) is independent of the photon energy that manifests itself outside the jet system—direct and resolved photon processes are equally suppressed. This can be understood qualitatively in the dipole picture. A reduced suppression with respect to theoretical predictions is found in a ZEUS analysis of diffractive photoproduction of dijets with higher transverse momenta .