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Creators/Authors contains: "Andrieux, V."

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  1. Free, publicly-accessible full text available October 1, 2024
  2. Abstract

    We report on a measurement of Spin Density Matrix Elements (SDMEs) in hard exclusive$$\rho ^0$$ρ0meson muoproduction at COMPASS using 160 GeV/cpolarised$$ \mu ^{+}$$μ+and$$ \mu ^{-}$$μ-beams impinging on a liquid hydrogen target. The measurement covers the kinematic range 5.0 GeV/$$c^2$$c2$$< W<$$<W<17.0 GeV/$$c^2$$c2, 1.0 (GeV/c)$$^2$$2$$< Q^2<$$<Q2<10.0 (GeV/c)$$^2$$2and 0.01 (GeV/c)$$^2$$2$$< p_{\textrm{T}}^2<$$<pT2<0.5 (GeV/c)$$^2$$2. Here,Wdenotes the mass of the final hadronic system,$$Q^2$$Q2the virtuality of the exchanged photon, and$$p_{\textrm{T}}$$pTthe transverse momentum of the$$\rho ^0$$ρ0meson 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 ($$\gamma ^*_T \rightarrow V^{ }_L$$γTVL) indicate a violation ofs-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$$\rho ^0$$ρ0production.

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    Free, publicly-accessible full text available October 1, 2024
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