Spin density matrix elements in exclusive $$\omega $$ meson muoproduction

Abstract We report on a measurement of Spin Density Matrix Elements (SDMEs) in hard exclusive $$\omega $$ ω meson muoproduction the proton at COMPASS using 160 GeV/ c polarised \mu ^{+}$$ μ + and ^{-}$$ - beams impinging liquid hydrogen target. The covers range 5.0 $$c^2$$ c 2 $$< W<$$ < W 17.0 , with average kinematics $$\langle Q^{2} \rangle =$$ ⟨ Q ⟩ = 2.1 (GeV/ ) $$^2$$ W = 7.6$$ 7.6 p^{2}_{\mathrm{T}} 0.16$$ p T 0.16 . Here, $$Q^2$$ denotes virtuality exchanged photon, mass final hadronic system $$p_T$$ transverse momentum respect to virtual-photon direction. measured non-zero SDMEs for transitions transversely virtual photons longitudinally vector mesons ( $$\gamma ^*_T \rightarrow V_L$$ γ ∗ → V L indicate violation s -channel helicity conservation. Additionally, we observe sizeable contribution unnatural-parity-exchange (UPE) that decreases increasing results provide important input modelling Generalised Parton Distributions (GPDs). In particular, they may allow evaluate model-dependent way UPE assess role parton helicity-flip GPDs production.