Spin density matrix elements in exclusive [Formula omitted] meson muoproduction

We report on a measurement of Spin Density Matrix Elements (SDMEs) in hard exclusive [Formula omitted] meson muoproduction at COMPASS using 160 GeV/c polarised [Formula omitted] and [Formula omitted] beams impinging on a liquid hydrogen target. The measurement covers the kinematic range 5.0 GeV/ [Formula omitted] [Formula omitted] 17.0 GeV/ [Formula omitted], 1.0 (GeV/c) [Formula omitted] [Formula omitted] 10.0 (GeV/c) [Formula omitted] and 0.01 (GeV/c) [Formula omitted] [Formula omitted] 0.5 (GeV/c) [Formula omitted]. Here, W denotes the mass of the final hadronic system, [Formula omitted] the virtuality of the exchanged photon, and [Formula omitted] the transverse momentum of the [Formula omitted] 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 ( [Formula omitted]) indicate a violation of s-channel helicity conservation. Additionally, we observe a dominant contribution of natural-parity-exchange transitions and a very small contribution of unnatural-parity-exchange transitions, which is compatible with zero within experimental uncertainties. The results provide important input for modelling Generalised Parton Distributions (GPDs). In particular, they may allow one to evaluate in a model-dependent way the role of parton helicity-flip GPDs in exclusive [Formula omitted] production.