Polarization Transfer in Wide-Angle Compton Scattering and Single-Pion Photoproduction from the Proton

Wide-angle exclusive Compton scattering and single-pion photoproduction from the proton have been investigated via measurement of the polarization transfer from a circularly polarized photon beam to the recoil proton. The wide-angle Compton scattering polarization transfer was analyzed at an incident photon energy of 3.7~GeV at a proton scattering angle of \cma$= 70^\circ$. The longitudinal transfer \KLL, measured to be $0.645 \pm 0.059 \pm 0.048$, where the first error is statistical and the second is systematic, has the same sign as predicted for the reaction mechanism in which the photon interacts with a single quark carrying the spin of the proton. However, the observed value is $\sim$3~times larger than predicted by the generalized-parton-distribution-based calculations, which indicates a significant unknown contribution to the scattering amplitude.

Comments: 6 pages, 5 figures, the text has been made consistent with the published

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Affiliations: 1School of Physics, University of the Witwatersrand, Johannesburg 2050, South Africa, 2iThemba Laboratory for Accelerator Based Sciences, PO Box 722, Somerset West 7129, South Africa, 3School of Physics, University of the Witwatersrand, Johannesburg 2050, South Africa, 4School of Earth Sciences, University of the Witwatersrand, Johannesburg 2050, South Africa, 5Department of Physics, University of Cape Town, Rondebosch 7700, South Africa, 6iThemba Laboratory for Accelerator Based Sciences, PO Box 722, Somerset West 7129, South Africa, 7School of Physics, University of the Witwatersrand, Johannesburg 2050, South Africa, 8Laboratory of Theoretical Physics, Joint Institute for Nuclear Research, Dubna, Moscow region, 141980, Russia, 9Institut für Kernphysik, Technische Universität Darmstadt, D-64289, Darmstadt, Germany, 10Institut für Kernphysik, Technische Universität Darmstadt, D-64289, Darmstadt, Germany, 11Institute of Particle and Nuclear Physics, Charles University, CZ-18000, Prague, Czech Republic, 12Department of Physics, University of Stellenbosch, Matieland 7602, South Africa, 13iThemba Laboratory for Accelerator Based Sciences, PO Box 722, Somerset West 7129, South Africa, 14Laboratory of Theoretical Physics, Joint Institute for Nuclear Research, Dubna, Moscow region, 141980, Russia, 15Institut für Kernphysik, Technische Universität Darmstadt, D-64289, Darmstadt, Germany, 16iThemba Laboratory for Accelerator Based Sciences, PO Box 722, Somerset West 7129, South Africa, 17iThemba Laboratory for Accelerator Based Sciences, PO Box 722, Somerset West 7129, South Africa, 18Institut für Theoretische Physik II, Universität Erlangen, D-91058, Erlangen, Germany, 19Institut für Kernphysik, Technische Universität Darmstadt, D-64289, Darmstadt, Germany, 20School of Physics, University of the Witwatersrand, Johannesburg 2050, South Africa, 21iThemba Laboratory for Accelerator Based Sciences, PO Box 722, Somerset West 7129, South Africa, 22iThemba Laboratory for Accelerator Based Sciences, PO Box 722, Somerset West 7129, South Africa, 23Department of Physics, University of Stellenbosch, Matieland 7602, South Africa, 24Research Center for Nuclear Physics, Osaka University, Ibaraki, Osaka 560-0047, Japan, 25School of Physics, University of the Witwatersrand, Johannesburg 2050, South Africa

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