Spin Density Matrix Elements in Exclusive $\ddot{I}{}^0$ Meson Muoproduction

Abstract

We report on a measurement of Spin Density Matrix Elements (SDMEs) in hard exclusive $\ddot{I}{}^0$ meson muoproduction at COMPASS using 160~GeV/$c$ polarised $\hat{I}{¼}^{+}$ and $\hat{I}{¼}^{-}$ beams impinging on a liquid hydrogen target. The measurement covers the kinematic range 5.0~GeV/$c^2$ $<W<$ 17.0~GeV/$c^2$, 1.0 (GeV/$c$)${}^2$ $<Q^2<$ 10.0 (GeV/$c$)${}^2$ and 0.01 (GeV/$c$)${}^2$ $<p_{\mathrm{T}}^2<$ 0.5 (GeV/$c$)${}^2$. Here, $W$ denotes the mass of the final hadronic system, $Q^2$ the virtuality of the exchanged photon, and $p_{\mathrm{T}}$ the transverse momentum of the $\ddot{I}{}^0$ 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 ($γ^*_T \to V^{ }_L$) 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 $\ddot{I}{}^0$ production.