Abstract submitted to the
         International Europhysics Conference on High Energy Physics
                  July 12 - 18, 2001, Budapest, Hungary
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        Dijet angular distributions in D* photoproduction at HERA

                    
                               ZEUS Collaboration



                                    Abstract


      Angular distributions of dijets in the photoproduction regime, 
      with at least one of the jets associated with a D^{*\pm} meson,
      have been studied with the ZEUS detector at HERA using an 
      integrated luminosity of 105 pb^{-1}. Direct and resolved 
      photoproduction samples have been separated using the fraction of 
      the photon momentum which participates in the dijet photoproduction, 
      x_{gamma}^{obs}. The angular distribution dN/d|cos(theta^*)|, 
      where theta^* is the angle between the jet-jet axis and the 
      beam direction in the dijet rest frame, has been measured for 
      the two samples. This distribution is sensitive to the parton 
      dynamics of the underlying processes. The resolved processes 
      show an enhancement at high |cos(theta^*)|, in agreement with 
      leading-order QCD Monte Carlo predictions, as expected for 
      dominant gluon exchange diagrams. The direct processes are much 
      flatter in |cos(theta^*)|, consistent with quark exchange.        
      The results are compared with previous dijet measurements which 
      did not require the presence of charm and are also presented 
      separately for backward- and forward-going D^* in the laboratory 
      frame.