Exclusive meson production at HERMES
  The data were accumulated with the HERMES forward spectrometer using the 27.6 GeV longitudinally polarized electron or positron beam of HERA. 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$^{2}$, while for $\rho^0$-meson production on a transversely polarized hydrogen target $-t'< 0.4$ GeV$^{2}$ is used.   Spin-density matrix elements for $\omega$ production are presented in projections of $Q^2$ or $-t'$, while the ratios of the helicity amplitudes for the reaction $\gamma^*+p \to \rho^0+p$ are obtained in the entire kinematic region. The usage of the transversely polarized target allows for the first time the extraction of the ratios of certain nucleon-helicity-flip amplitudes to the natural-parity exchange amplitude $T_{0\frac{1}{2}0\frac{1}{2}}$ without the nucleon-helicity flip describing the longitudinal $\rho^0$-meson production by the longitudinal virtual photon. Violation of $s$-channel helicity conservation is observed for both mesons.   A dominant contribution from unnatural-parity exchange (UPE) is established for special combinations of the $\omega$ spin-density matrix elements. For $\rho^0$-meson production, the UPE amplitude without the nucleon-helicity flip, $U_{1\frac{1}{2}1\frac{1}{2}}$, is shown to be the largest UPE amplitude. Good agreement is found between the HERMES data on $\omega$ production on the proton and results of the perturbative QCD-inspired Goloskokov-Kroll model that includes pion-pole contribution. 
Sergey Manaenkov (on behalf of the HERMES collaboration)
hep-ph