Abstract submitted to the
         International Europhysics Conference on High Energy Physics
                  July 12 - 18, 2001, Budapest, Hungary
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         Measurements of jet substructure in inclusive and charm-induced 
                         dijet photoproduction at HERA

                    
                               ZEUS Collaboration



                                    Abstract


         The substructure of jets produced in quasi-real photon-proton 
         collisions at centre-of-mass energies in the range 134-277 GeV 
         has been investigated with the ZEUS detector at HERA using an
         integrated luminosity of ~100 pb^{-1}. Jets were identified 
         using the longitudinally invariant k_T-cluster algorithm. The 
         substructure of jets has been studied in terms of the jet shape 
         and subjet multiplicity defined as the number of clusters 
         resolved in a jet by reapplying the jet algorithm at a smaller 
         resolution scale. Measurements of the jet shape and mean subjet 
         multiplicity are presented for an inclusive and a charm-induced 
         dijet samples. The charm-induced sample has been obtained by 
         selecting dijet events in which at least one of the two jets is 
         accompanied by a D^* meson; the substructure of the untagged 
         jet has been studied and compared to that of the jets in the 
         inclusive dijet sample. This comparison allows an extraction of 
         the jet substructure for quarks and gluons. Leading-logarithm 
         parton-shower Monte Carlo calculations of resolved- and direct-
         photon processes give a good description of the differences 
         between quark- and gluon-initiated jets.