Measurement of the spin-density matrix elements in exclusive electroproduction of ρ0 mesons at HERA

Exclusive electroproduction of rho(o) mesons has been measured using the ZEUS detector at HERA in two Q(2) ranges, 0.25 < Q(2) < 0.85 GeV2 and 3 < Q(2) < 30 GeV2. 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 GeV2. The distribution in the azimuthal angle between the positron scattering plane and the rho(o) 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(o) mesons from transverse photons. Measurements of the 15 combinations of spin-density matrix elements which completely define the angular distributions are presented and discussed.