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Exclusive electroproduction of ω mesons on unpolarized hydrogen and deuterium targets is studied at HERMES in the kinematic region of Q2 > 1.0GeV2, 3.0GeV < W < 6.3GeV, and − t′ < 0.2GeV2. The data were accumulated during the 1996-2007 running period using the 27.6GeV longitudinally polarized electron or positron beams at HERA. The determination of the virtual-photon longitudinal-to-transverse cross-section ratio shows that a considerable part of the cross section arises from transversely polarized photons. Spin density matrix elements are derived and presented in projections of Q2 or − t′. Violation of s-channel helicity conservation is observed for some of these elements. A sizable contribution from unnatural-parity-exchange amplitudes is found and the phase shift between those amplitudes that describe transverse ω production by longitudinal and transverse virtual photons is determined for the first time. Good agreement is found between the HERMES proton data and results of a pQCD-inspired phenomenological model that includes pion-pole contributions.

Spin Density Matrix Elements in Exclusive Production of Omega Mesons at HERMES