1 research outputs found
Spin density matrix elements in exclusive electroproduction on and targets at 27.5 GeV beam energy
Spin Density Matrix Elements (SDMEs) describing the angular distribution of
exclusive rho^0 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^2 < Q^2 <
7 GeV^2, 3.0 GeV < W < 6.3 GeV, and -t < 0.4 GeV^2. Within the given
experimental uncertainties, a hierarchy of relative sizes of helicity
amplitudes is observed. Kinematic dependences of all SDMEs on Q^2 and t are
presented, as well as the longitudinal-to-transverse rho^0 electroproduction
cross section ratio as a function of Q^2. 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.Comment: 41 pages, 20 figures, 16 table; revised version corrects small typos,
text clarified on 6 pages, two references added and new data added to figure
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