Spin density matrix elements in exclusive ω electroproduction on 1H and 2H targets at 27.5 GeV beam energy

Exclusive electroproduction of. mesons on unpolarized hydrogen and deuterium targets is studied in the kinematic region of Q(2) > 1.0 GeV2, 3.0 GeV < W < 6.3 GeV, and -t ' < 0.2 GeV2. Results on the angular distribution of the omega meson, including its decay products, are presented. The data were accumulated with the HER-MES forward spectrometer during the 1996-2007 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. Asizable 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)* -> omega(T) and gamma(T)* -> 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)* -> omega(T) transition dominates over the two natural-parity-exchange amplitudes describing the gamma(L)* -> omega(L) and gamma(T)* -> 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.