Exclusive electroproduction of ρ mesons has been measured using the ZEUS detector at HERA in two Q ranges, 0.25 < Q < 0.85 GeV and 3 < Q < 30 GeV. The low-Q data span the range 20 < W < 90 GeV; the high-Q data cover the 40 < W < 120 GeV interval. Both samples extend up to four-momentum transfers of |t| = 0.6 GeV. The distribution in the azimuthal angle between the positron scattering plane and the ρ production plane shows a small but significant violation of s-channel helicity conservation, corresponding to the production of longitudinally polarised (i.e. helicity zero) ρ mesons from transverse photons. Measurements of the 15 combinations of spin-density matrix elements which completely define the angular distributions are presented and discussed. DESY 99-102 July 1999
