Spin-density matrix elements in hard exclusive muoproduction of $\omega$ mesons at COMPASS

Spin density matrix elements (SDMEs) have been determined forexclusive$\omega$ meson production on unpolarized protons, in the COMPASSkinematic region of 1.0 (GeV/c)$^2$ $< Q^{2} <$ 10.0 (GeV/c)$^2$, 5.0GeV/$c^2$ $< W <$ 17.0 GeV/$c^2$ and0.01 (GeV/c)$^2$ $<p^{2}_{T} < $ 0.5 (GeV/c)$^2$ which corresponds to$\left< Q^{2} \right>= 2.1$~(GeV/c)$^2$), $\left< W \right>= 7.6$~GeV/$c^2$and$\left<p^{2}_{T} \right> = 0.16$~(GeV/c)$^2$) mean values.Using extracted SDMEs values the contribution of unnatural parity exchangeamplitudes andthe hypothesis of s-channel helicity conservation (SCHC) werestudied. Certain matrix elements e.g. $ r^{5}_{00}$ corresponding totransition $\gamma^*_T \to V_L$ indicate violation of SCHC in exclusive $\omega $ production.A sizable contribution of unnatural parity exchange amplitudes is found for the exclusive $\omega$ meson muoproduction, and there is aclearindication of its decrease with increasing W.