  The exclusive electroproduction of rho-0 mesons was studied with the HERMES
spectrometer at the DESY laboratory by scattering 27.6 GeV positron and
electron beams off a transversely polarized hydrogen target. Spin density
matrix elements for this process were determined from the measured production-
and decay-angle distributions of the produced rho-0 mesons. These matrix
elements embody information on helicity transfer and the validity of s-channel
helicity conservation in the case of a transversely polarized target. From the
spin density matrix elements, the leading-twist term in the single-spin
asymmetry was calculated separately for longitudinally and transversely
polarized rho-0 mesons. Neglecting s-channel helicity changing matrix elements,
results for the former can be compared to calculations based on generalized
parton distributions, which are sensitive to the contribution of the total
angular momentum of the quarks to the proton spin.
