Abstract submitted to the
         XXXth International Conference on High Energy Physics
                July 27 - August 2, 2000, Osaka, Japan
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     Measurement of the spin-density matrix elements in exclusive 
             electroproduction of rho^0 Mesons at HERA

                    
                               ZEUS Collaboration





                                    Abstract


Exclusive electroproduction of rho^0 mesons has been measured using
the ZEUS detector at HERA in two Q^2 ranges, 0.25<Q^2<0.85 GeV^2 and
3<Q^2<30 GeV^2. The low-Q^2 data span the range 20<W<90 GeV; the
high-Q^2 data cover the 40<W<120 GeV interval. Both samples extend up
to four-momentum transfers of |t|=0.6 GeV^2. The distribution in the
azimuthal angle between the positron scattering plane and the rho^0
production plane shows a small but significant violation of s-channel
helicity conservation, corresponding to the production of
longitudinally polarised (i.e. helicity zero) rho^0 mesons from
transverse photons.  Measurements of the 15 combinations of
spin-density matrix elements which completely define the angular
distributions are presented and discussed.