Abstract submitted to the
         International Europhysics Conference on High Energy Physics
                  July 12 - 18, 2001, Budapest, Hungary
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      Three-jet production in diffractive deep inelastic scattering at HERA

                    
                               ZEUS Collaboration



                                    Abstract


     First measurements are reported of three-jet production in diffractive
     deep inelastic scattering with the ZEUS detector at HERA using an
     integrated luminosity of 42.7 pb^{-1}.  The data were measured in the
     kinematic region 5 < Q^2 < 100 GeV^2, 200 < W < 250 GeV, 23 < M_X < 40 
     GeV and x_{Pom} < 0.025. The diffractive signal was selected by 
     requiring a large rapidity gap in the outgoing proton direction. Jets 
     were reconstructed in the centre-of-mass system of the observed 
     hadronic final state with the exclusive-k_T jet-algorithm. The three-jet 
     final-state topology is studied. Differential cross sections are 
     measured as a function of the jet pseudorapidity and jet transverse 
     momentum with respect to the gamma^*-{Pom} axis. Finally, the shapes of 
     the jets are studied with the aim of distinguishing between quark- 
     and gluon-initiated jets. The measurements are compared to models for 
     diffraction in deep inelastic scattering.