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  1. Abstract

    We propose a new measurement of the ratio of positron-proton to electron-proton elastic scattering at DESY. The purpose is to determine the contributions beyond single-photon exchange, which are essential for the Quantum Electrodynamic (QED) description of the most fundamental process in hadronic physics. By utilizing a 20 cm long liquid hydrogen target in conjunction with the extracted beam from the DESY synchrotron, we can achieve an average luminosity of$$2.12\times 10^{35}$$2.12×1035 cm$$^{-2}\cdot $$-2·s$$^{-1}$$-1 ($$\approx 200$$200times the luminosity achieved by OLYMPUS). The proposed two-photon exchange experiment (TPEX) entails a commissioning run at a beam energy of 2 GeV, followed by measurements at 3 GeV, thereby providing new data up to$$Q^2=4.6$$Q2=4.6 (GeV/c)$$^2$$2(twice the range of current measurements). We present and discuss the proposed experimental setup, run plan, and expectations.

     
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  2. Free, publicly-accessible full text available October 1, 2024
  3. 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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