Spin density matrix elements in exclusive ρ0 meson muoproduction

Abstract

We report on a measurement of Spin Density Matrix Elements (SDMEs) in hard exclusive ${\rho }^0$ meson muoproduction at COMPASS using 160 GeV/c polarised ${\mu }^{+}$ and ${\mu }^{-}$ 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_{\text{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 $p_{\text{T}}$ the transverse momentum of the ${\rho }^0$ 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 (${\gamma }_T^{\ast }\to V_L^{}$) 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 ${\rho }^0$ production.