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

Exclusive electroproduction of ρ0 mesons has been measured using the ZEUS detector at HERA in two Q(2) ranges, 0.25 lt Q(2) lt 0.85 GeV2 and 3 lt Q(2) lt 30 GeV2. The low-Q(2) data span the range 20 lt W lt 90 GeV; the high-Q(2) data cover the 40 lt W lt 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 ρ0 production plane shows a small but significant violation of s-channel helicity conservation, corresponding to the production of longitudinally polarised (i.e, helicity zero) ρ0 mesons from transverse photons. Measurements of the 15 combinations of spin-density matrix elements which completely define the angular distributions are presented and discussed