Spin density matrix elements in exclusive \(\omega \) electroproduction on
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Exclusive electroproduction of \(\omega \) mesons on unpolarized hydrogen and deuterium targets is studied in the kinematic region of \(Q^2> 1.0\) ?GeV \(^2\) , 3.0?GeV? \( ?6.3 GeV, and \(-t' ?GeV \(^2\) . Results on the angular distribution of the \(\omega \) meson, including its decay products, are presented. The data were accumulated with the HERMES forward spectrometer during the 1996-007 running period using the 27.6?GeV longitudinally polarized electron or positron beam of HERA. The determination of the virtual-photon longitudinal-to-transverse cross-section ratio reveals that a considerable part of the cross section arises from transversely polarized photons. Spin density matrix elements are presented in projections of \(Q^2\) 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 \(\omega \) production by longitudinal and transverse virtual photons, \(\gamma ^{*}_{L} \rightarrow \omega _{T}\) and \(\gamma ^{*}_{T} \rightarrow \omega _{T}\) , is determined for the first time. A hierarchy of helicity amplitudes is established, which mainly means that the unnatural-parity-exchange amplitude describing the \(\gamma ^*_T \rightarrow \omega _T\) transition dominates over the two natural-parity-exchange amplitudes describing the \(\gamma ^*_L \rightarrow \omega _L\) and \(\gamma ^*_T \rightarrow \omega _T\) transitions, with the latter two being of similar magnitude. Good agreement is found between the HERMES proton data and results of a pQCD-inspired phenomenological model that includes pion-pole contributions, which are of unnatural parity.

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