Exclusive production of $\omega$ mesons was studied at the COMPASS experiment by scattering $160~\mathrm{ GeV}/\mathit{ c}$ muons off transversely polarised protons. Five single-spin and three double-spin azimuthal asymmetries were measured in the range of photon virtuality $1~(\mathrm{ GeV}/\mathit{ c})^2 < Q^2 < 10~(\mathrm{ GeV}/\mathit{ c})^2$, Bjorken scaling variable $0.003 < x_{ \mathit{ Bj}} < 0.3$ and transverse momentum squared of the $\omega$ meson $0.05~(\mathrm{ GeV}/\mathit{ c})^2 < p_{ T}^{ 2} < 0.5~(\mathrm{ GeV}/\mathit{ c})^2$. The measured asymmetries are sensitive to the nucleon helicity-flip Generalised Parton Distributions (GPD) $E$ that are related to the orbital angular momentum of quarks, the chiral-odd GPDs $H_{ T}$ that are related to the transversity Parton Distribution Functions, and the sign of the $\pi\omega$ transition form factor. The results are compared to recent calculations of a GPD-based model.
Categories: hep-ex nucl-ex