The isospin structure of photoproduction of pi-eta pairs from the nucleon in the threshold region

• Main Authors: Käser, A, Ahrens, J, Annand, J. R. M, Arends, H. J, Bantawa, K, Bartolome, P. A, Beck, R, Bekrenev, V, Berghäuser, H, Braghieri, A, Branford, D, Briscoe, W. J, Brudvik, J, Cherepnya, S, Costanza, S, Demissie, B, Dieterle, M, Downie, E. J, Drexler, P, Fil'kov, L. V, Fix, A, Glazier, D. I, Hamilton, D, Heid, E, Hornidge, D, Howdle, D, Huber, G. M, Jahn, O, Jaegle, I, Jude, T. C, Kashevarov, V. L, Keshelashvili, I, Kondratiev, R, Korolija, M, Kruglov, S. P, Krusche, B, Lisin, V, Livingston, K, MacGregor, I. J. D, Maghrbi, Y, Mancell, J, Manley, D. M, Marinides, Z, McGeorge, J. C, McNicoll, E, Mekterovic, D, Metag, V, Micanovic, S, Middleton, D. G, Mushkarenkov, A, Nikolaev, A, Novotny, R, Oberle, M, Ostrick, M, Otte, P, 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, Werthmüller, D, Witthauer, L
• Format: Journal Article
• Language: English
• Published: 07.07.2015
• Subjects: Physics - Nuclear Experiment
• DOI: 10.48550/arxiv.1507.01833

Phys. Lett. B 748 (2015) 244 Photoproduction of $\pi\eta$-pairs from nucleons has been investigated from threshold up to incident photon energies of $\approx$~1.4~GeV. The quasi-free reactions $\gamma p\rightarrow p\pi^0\eta$, $\gamma n\rightarrow n\pi^0\eta$, $\gamma p\rightarrow n\pi^+\eta$, and $\gamma n\rightarrow p\pi^-\eta$ were for the first time measured from nucleons bound in the deuteron. The corresponding reactions from a free-proton target were also studied to investigate final-state interaction effects (for neutral pions the free-proton results could be compared to previous measurements; the $\gamma p\rightarrow n\pi^+\eta$ reaction was measured for the first time). For the $\pi^0\eta$ final state coherent production via the $\gamma d\rightarrow d\pi^0\eta$ reaction was also investigated. The experiments were performed at the tagged photon beam of the Mainz MAMI accelerator using an almost $4\pi$ coverage electromagnetic calorimeter composed of the Crystal Ball and TAPS detectors. The total cross sections for the four different final states obey the relation $\sigma(p\pi^0\eta)$ $\approx$ $\sigma(n\pi^0\eta)$ $\approx$ $2\sigma(p\pi^-\eta)$ $\approx$ $2\sigma(n\pi^+\eta)$ as expected for a dominant contribution from a $\Delta^{\star}\rightarrow\eta\Delta(1232)\rightarrow\pi\eta N$ reaction chain, which is also supported by the shapes of the invariant-mass distributions of nucleon-meson and $\pi$-$\eta$ pairs. The experimental results are compared to the predictions from an isobar reaction model.