Abstract submitted to the
         XXXth International Conference on High Energy Physics
                July 27 - August 2, 2000, Osaka, Japan
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                Substructure dependence of jet cross sections
                        in photoproduction at HERA


                    
                               ZEUS Collaboration





                                    Abstract


    Differential cross sections for jets produced in photoproduction
at centre-of-mass energies in the range 134-277 GeV have been measured
with the ZEUS detector at HERA using an integrated luminosity of 42.5
pb^-1. Jets are identified using the longitudinally invariant
kt-cluster algorithm. Measurements of the differential cross
sections as a function of the jet pseudorapidity and transverse energy
are presented for jets characterised either by their size or by their
subjet multiplicity.  Leading-logarithm parton-shower Monte Carlo
calculations of resolved- and direct-photon processes are compared to
the results and used to disentangle the contributing two-body
hard-scattering subprocesses.