Speaker
Description
The photoproduction of isolated photons is measured using diffractive events
recorded by the ZEUS detector at HERA. Cross sections are evaluated in the
photon transverse-energy and pseudorapidity ranges $5 < E_T^{\gamma} < 15$ GeV
and $-0.7 < \eta^{\gamma} < 0.9$, inclusively and with a jet with
transverse-energy and pseudorapidity in the ranges $4 < E_T^{\rm jet} < 35$ GeV
and $-1.5 < \eta^{\rm jet} < 1.8$, for an integrated electron-proton luminosity
of 374 ${\rm pb}^{-1}$. A number of kinematic variables are studied and compared
to predictions from the RAPGAP Monte Carlo model. In considering the fraction of
the energy of the colourless (``Pomeron'') exchange that is transferred to the
photon--jet final state, $z_P$, it is found that the data lie above the RAPGAP
predictions for $z_P > 0.9$, giving evidence for direct-Pomeron interactions. The
shapes of the kinematic distributions of events below and above this value of
$z_P$ are separately well described by RAPGAP. This and other features provide
evidence for a universal set of parton distribution functions in the Pomeron.
