Spin Density Matrix Elements in Exclusive $\ddot{I}‰$ Meson Muoproduction ${}^{\ast }$

Abstract

We report on a measurement of Spin Density Matrix Elements (SDMEs) in hard exclusive $\ddot{I}‰$ meson muoproduction on the proton 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 range 5.0 GeV/$c^2$ $<W<$ 17.0 GeV/$c^2$, with the average kinematics $⟨Q^2⟩=$ 2.1 (GeV/$c$)${}^2$, $⟨W⟩=7.6$ GeV/$c^2$, and $⟨p_{\mathrm{T}}^2⟩=0.16$ (GeV/$c$)${}^2$. Here, $Q^2$ denotes the virtuality of the exchanged photon, $W$ the mass of the final hadronic system and $p_T$ the transverse momentum of the $\ddot{I}‰$ 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 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 $\ddot{I}‰$ production.

31 pages, 12 figures, 1 appendix