Spin density matrix elements in exclusive $\omega$ electroproduction on ${}^1$H and ${}^2$H targets at 27.5 GeV beam energy

Abstract

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 < W < 6.3 GeV, and -t'< 0.2 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 1996-2007 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. 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, ${\gamma }_L^{\ast }\to {\omega }_T$ and ${\gamma }_T^{\ast }\to {\omega }_T$, is determined for the first time. A hierarchy of helicity amplitudes is established, which mainly means that the unnatural-parity-exchange amplitude describing the ${\gamma }_T^{\ast }\to {\omega }_T$ transition dominates over the two natural-parity-exchange amplitudes describing the ${\gamma }_L^{\ast }\to {\omega }_L$ and ${\gamma }_T^{\ast }\to {\omega }_T$ 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.

Comments: "Erratum version", e.g., includes the corrected version of Figure 6, to be published in "Erratum to: Spin density matrix elements in exclusive omega electroproduction on 1H and 2H targets at 27.5 GeV beam energy". Three SDMEs were mislabeled in the figure of the previously published version [EPJC74(2014)3110]. The labels have been fixed, while none of the results or conclusions changed