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

                    
                               ZEUS Collaboration





                                    Abstract


Dijet differential cross sections for the reaction e+p -> e+ jet jet X
with quasi-real photons have been measured with the ZEUS detector at
HERA using an integrated luminosity of 42.5 pb^-1. The cross sections
are given for photon-proton centre-of-mass energies in the range 134
<W< 277 GeV and refer to jets at the hadron level using the kt-cluster
algorithm. The differential cross sections as a function of the dijet
mass (M^{JJ}) and as a function of the angle between the jet-jet axis
and the beam direction in the dijet centre-of-mass system
(cos(theta^*)) have been measured for M^{JJ} above 47 GeV and
|cos(theta^*)|<0.8. The dijet cross section as a function of
chi=(1+|cos(theta^*))/(1-|cos(theta^*)|) has been measured in
different M^{JJ} regions. Next-to-leading-order (NLO) QCD calculations
describe the shape of the measured cross sections well. No significant
discrepancy between data and NLO QCD calculations has been observed up
to M^{JJ} ~ 140 GeV.