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

• Published in: arXiv.org 2015-07
• 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: Article
• Language: English
• Subjects: Nucleons; Photon beams; Photoproduction; Pions
• ISSN: 2331-8422

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.