Spin density matrix elements in exclusive ω meson muoproduction

We report on a measurement of Spin Density Matrix Elements (SDMEs) in hard exclusive ω meson muoproduction on the proton at COMPASS using 160 GeV/c polarised μ⁺ and μ^- beams impinging on a liquid hydrogen target. The measurement covers the range 5.0 GeV/c²< W< 17.0 GeV/c², with the average kinematics ⟨ Q²⟩ = 2.1 (GeV/c)², ⟨ W⟩ = 7.6 GeV/c², and ⟨pT2⟩=0.16 (GeV/c)². Here, Q² denotes the virtuality of the exchanged photon, W the mass of the final hadronic system and p_T the transverse momentum of the ω meson with respect to the virtual-photon direction. The measured non-zero SDMEs for the transitions of transversely polarised virtual photons to longitudinally polarised vector mesons (γT∗→VL) indicate a violation of s-channel helicity conservation. Additionally, we observe a sizeable contribution of unnatural-parity-exchange (UPE) transitions that decreases with increasing W. The results provide important input for modelling Generalised Parton Distributions (GPDs). In particular, they may allow to evaluate in a model-dependent way the contribution of UPE transitions and assess the role of parton helicity-flip GPDs in exclusive ω production.