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Spin Density Matrix Elements (SDMEs) describing the angular distribution ofexclusive rho^0 electroproduction and decay are determined in the HERMESexperiment with 27.6 GeV beam energy and unpolarized hydrogen and deuteriumtargets. Eight (fifteen) SDMEs that are related (unrelated) to the longitudinalpolarization of the beam are extracted in the kinematic region 1 GeV^2 < Q^2 <7 GeV^2, 3.0 GeV < W < 6.3 GeV, and -t < 0.4 GeV^2. Within the givenexperimental uncertainties, a hierarchy of relative sizes of helicityamplitudes is observed. Kinematic dependences of all SDMEs on Q^2 and t arepresented, as well as the longitudinal-to-transverse rho^0 electroproductioncross section ratio as a function of Q^2. A small but statistically significantdeviation from the hypothesis of s-channel helicity conservation is observed.An indication is seen of a contribution of unnatural-parity-exchangeamplitudes; these amplitudes are naturally generated with a quark-exchangemechanism.

Spin Density Matrix Elements in Exclusive rho^0 Electroproduction on 1H and 2H Targets at 27.5 GeV Beam Energy