Measurement of dijet cross sections for events with a leading 
   	         neutron in photoproduction at HERA

                          ZEUS Collaboration

                              Abstract

Differential cross sections for dijet photoproduction and in
association with a leading neutron have been measured in the reaction
e+ p -> e+ jet jet X n
with the ZEUS detector at HERA using an integrated luminosity of 40pb-1.
The data are consistent with a simple pion exchange model. The ratio of
the neutron-tagged and dijet cross sections show violations of
factorization of the lepton and photon vertices which can be explained by
kinematic effects constraining the phase space for neutron production.
Normalised double-differential leading-neutron cross sections have been
measured in dijet photoproduction for the first time. The distributions
can be fully characterised by only two energy dependent parameters
extracted from fits to the data. Absorption effects were studied by
comparing the dijet photoproduction measurements and similar results in
deep inelastic scattering. No clear effect, not related to kinematics, was
observed. In a resolved-enriched dijet photoproduction sample,
significantly fewer neutrons were seen than for direct. This depletion can
also be accounted for by kinematic constraints.