Spin density matrix elements in exclusive ω meson muoproduction

We report on a measurement of Spin Density Matrix Elements (SDMEs) in hard exclusive ω ω meson muoproduction on the proton at COMPASS using 160 GeV/c polarised μ ⁺ μ + and μ ⁻ μ - beams impinging on a liquid hydrogen target. The measurement covers the range 5.0 GeV/ c² c 2 $$< W<$$ < W < 17.0 GeV/ c² c 2 , with the average kinematics Q² = ⟨ Q 2 ⟩ = 2.1 (GeV/c) ² 2 , W = 7.6 ⟨ W ⟩ = 7.6 GeV/ c² c 2 , and p²T = 0.16 ⟨ p T 2 ⟩ = 0.16 (GeV/c) ² 2 . Here, Q² Q 2 denotes the virtuality of the exchanged photon, W the mass of the final hadronic system and pT p T the transverse momentum of the ω 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 ( γ ^*T → VL γ T ∗ → V L ) 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 ω production.