Spin Density Matrix Elements in Exclusive ρ 0 Electro - production on 1 H and 2 H Targets at 27 . 5 GeV Beam Energy

Spin Density Matrix Elements (SDMEs) describing the angular distribution of exclusive ρ electroproduction and decay are determined in the HERMES experiment with 27.6 GeV beam energy and unpolarized hydrogen and deuterium targets. Eight (fifteen) SDMEs that are related (unrelated) to the longitudinal polarization of the beam are extracted in the kinematic region 1 GeV < Q < 7 GeV, 3.0 GeV < W < 6.3 GeV, and −t < 0.4 GeV. Within the given experimental uncertainties, a hierarchy of relative sizes of helicity amplitudes is observed. Kinematic dependences of all SDMEs on Q and t are presented, as well as the longitudinal-to-transverse ρ electroproduction cross section ratio as a function of Q. A small but statistically significant deviation from the hypothesis of s-channel helicity conservation is observed. An indication is seen of a contribution of unnatural-parity-exchange amplitudes; these amplitudes are naturally generated with a quark-exchange mechanism. PACS. 13.60.-r,13.60.Le,13.88.+e