Abstract
We report on a measurement of Spin Density Matrix Elements (SDMEs) in hard exclusive omega meson muoproduction on the proton at COMPASS using 160 GeV/c polarised mu+ and mu- beams impinging on a liquid hydrogen target. The measurement covers the range 5.0 GeV/c2=0.16 (GeV/c)2.
Here, Q2 denotes the virtuality of the exchanged photon, W the mass of the final hadronic system and pT the transverse momentum of the omega 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 (gamma T -> 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 omega production.