Spin Density Matrix Elements from diffractive $\phi$ meson production at   HERMES
  Exclusive production of $\phi$ mesons on hydrogen and deuterium targets is studied in the HERMES kinematic region $1 < Q^{2} < 7$ GeV$^2$ and $3.0 < W < 6.3$ GeV. Spin density matrix elements and their $Q^{2}$ and $t$ dependences are presented. These data are consistent with s-channel helicity conservation in exclusive $\phi$ meson production. No significant evidence for the contribution of unnatural-parity-exchange amplitudes is found. 
HERMES Collaboration: W. Augustyniak, A. Borissov and S. Manayenkov