Measurement of the beam-helicity asymmetry $I^{\odot}$ in the photoproduction of $\pi^0\pi^{\pm}$-pairs off protons and off neutrons

• Main Authors: Oberle, M, Ahrens, J, Annand, J. R. M, Arends, H. J, Bantawa, K, Bartolome, P. A, Beck, R, Bekrenev, V, Berghaeuser, H, Braghieri, A, Branford, D, Briscoe, W. J, Brudvik, J, Cherepnya, S, Demissie, B, Dieterle, M, Downie, E. J, Drexler, P, Fil'kov, L. V, Fix, A, Glazier, D. I, Heid, E, Hornidge, D, Howdle, D, Huber, G. M, Jahn, O, Jaegle, I, Jude, T. C, Kaeser, A, Kashevarov, V. L, Keshelashvili, I, Kondratiev, R, Korolija, M, Kruglov, S. P, Krusche, B, Kulbardis, A, Lisin, V, Livingston, K, MacGregor, I. J. D, Maghrbi, Y, Mancell, J, Manley, D. M, Marinides, Z, Martinez, M, McGeorge, J. C, McNicoll, E, Mekterovic, D, Metag, V, Micanovic, S, Middleton, D. G, Mushkarenkov, A, Nefkens, B. M. K, Nikolaev, A, Novotny, R, Ostrick, M, Oussena, B, Pedroni, P, Pheron, F, Polonski, A, Prakhov, S. N, Robinson, J, Rosner, G, Rostomyan, T, Schumann, S, Sikora, M. H, Sober, D. I, Starostin, A, Supek, I, Thiel, M, Thomas, A, Unverzagt, M, Watts, D. P, Werthmueller, D, Witthauer, L, Zehr, F
• Format: Journal Article
• Language: English
• Published: 08.03.2014
• Subjects: Physics - Nuclear Experiment
• DOI: 10.48550/arxiv.1403.1989

Eur. Phys. J. A50 (2014) 54 Beam-helicity asymmetries have been measured at the MAMI accelerator in Mainz for the photoproduction of mixed-charge pion pairs in the reactions $\boldsymbol{\gamma}p\rightarrow n\pi^0\pi^+$ off free protons and $\boldsymbol{\gamma}d\rightarrow (p)p\pi^0\pi^-$ and $\boldsymbol{\gamma}d\rightarrow (n)n\pi^0\pi^+$ off quasi-free nucleons bound in the deuteron for incident photon energies up to 1.4 GeV. 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.