Measurement of the beam-helicity asymmetry I⊙ in the photoproduction of $ \pi^{0}$ $ \pi^{\pm}$ pairs off protons and off neutrons
Oberle, M. ; Ahrens, J. ; Annand, J. ; Arends, H. ; Bantawa, K. ; Bartolome, P. ; Beck, R. ; Bekrenev, V. ; Berghäuser, H. ; Braghieri, A. ; Branford, D. ; Briscoe, W. ; Brudvik, J. ; Cherepnya, S. ; Demissie, B. ; Dieterle, M. ; Downie, E. ; Drexler, P. ; Fil'kov, L. ; Fix, A. ; Glazier, D. ; Heid, E. ; Hornidge, D. ; Howdle, D. ; Huber, G. ; Jahn, O. ; Jaegle, I. ; Jude, T. ; Käser, A. ; Kashevarov, V. ; Keshelashvili, I. ; Kondratiev, R. ; Korolija, M. ; Kruglov, S. ; Krusche, B. ; Kulbardis, A. ; Lisin, V. ; Livingston, K. ; MacGregor, I. ; Maghrbi, Y. ; Mancell, J. ; Manley, D. ; Marinides, Z. ; Martinez, M. ; McGeorge, J. ; McNicoll, E. ; Mekterovic, D. ; Metag, V. ; Micanovic, S. ; Middleton, D. ; Mushkarenkov, A. ; Nefkens, B. ; Nikolaev, A. ; Novotny, R. ; Ostrick, M. ; Oussena, B. ; Pedroni, P. ; Pheron, F. ; Polonski, A. ; Prakhov, S. ; Robinson, J. ; Rosner, G. ; Rostomyan, T. ; Schumann, S. ; Sikora, M. ; Sober, D. ; Starostin, A. ; Supek, I. ; Thiel, M. ; Thomas, A. ; Unverzagt, M. ; Watts, D. ; Werthmüller, D. ; Witthauer, L. ; Zehr, F.
In: The European Physical Journal A, 2014, vol. 50, no. 3, p. 1-18
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- Abstract.: Beam-helicity asymmetries have been measured at the MAMI accelerator in Mainz for the photoproduction of mixed-charge pion pairs in the reactions $ \gamma p\rightarrow n\pi^{0}\pi^{+}$ off free protons and $ \gamma d \rightarrow (p)p\pi^{0}\pi^{-}$ and $ \gamma d\rightarrow (n)n\pi^{0}\pi^{+}$ off quasi-free nucleons bound in the deuteron for incident photon energies up to 1.4GeV. Circularly polarized photons were produced from bremsstrahlung of longitudinally polarized electrons and tagged with the Glasgow-Mainz magnetic spectrometer. The charged pions, recoil protons, recoil neutrons, and decay photons from $ \pi^{0}$ mesons were detected in the $ 4\pi$ electromagnetic calorimeter composed of the Crystal Ball and TAPS detectors. Using a complete kinematic reconstruction of the final state, excellent agreement was found between the results for free and quasi-free protons, suggesting that the quasi-free neutron results are also a close approximation of the free-neutron asymmetries. A comparison of the results to the predictions of the Two-Pion-MAID reaction model shows that the reaction mechanisms are still not well understood, in particular at low incident photon energies in the second nucleon-resonance region.