Studies of the diffractive photoproduction of isolated photons at HERA

The photoproduction of isolated photons has been measured in diffractive events recorded by the ZEUS detector at HERA. Cross sections are evaluated in the photon transverse-energy and pseudorapidity ranges $5 < E_T < 15$ GeV and $-0.7 < \eta < 0.9,$ inclusively and also with a jet with transverse energy and pseudorapidity in the ranges $4 < E_{jet} < 35$ GeV and $-1.5 < \eta_{jet} < 1.8,$ using a total integrated electron-proton luminosity of 456 pb$^{-1}$. A number of kinematic variables were studied and compared to predictions from the Rapgap Monte Carlo model. An excess of data is observed above the Rapgap predictions for $z^{meas}_{Pomeron} > 0.9,$ where $z^{meas}_{Pomeron}$ is the fraction of the longitudinal momentum of the colourless 'Pomeron' exchange that is transferred to the photon-jet final state, giving evidence for direct Pomeron interactions.