1303.2254 (Y. Maghrbi et al.)

Coherent photoproduction of pi^0- and eta-mesons off 7Li    [PDF]

Y. Maghrbi, B. Krusche, J. Ahrens, J. R. M. Annand, H. J. Arends, R. Beck, V. Bekrenev, B. Boillat, A. Braghieri, D. Branford, W. J. Briscoe, J. Brudvik, S. Cherepnya, R. F. B. Codling, E. J. Downie, P. Drexler, L. V. Fil'kov, A. Fix, D. I. Glazier, R. Gregor, E. Heid, D. Hornidge, I. Jaegle, O. Jahn, V. L. Kashevarov, I. Keshelashvili, A. Knezevic, R. Kondratiev, M. Korolija, D. Krambrich, M. Lang, V. Lisin, K. Livingston, S. Lugert, I. J. D. MacGregor, D. M. Manley, M. Martinez, J. C. McGeorge, D. Mekterovic, V. Metag, B. M. K. Nefkens, A. Nikolaev, R. Novotny, M. Ostrick, P. Pedroni, F. Pheron, A. Polonski, S. Prakhov, J. W. Price, G. Rosner, M. Rost, T. Rostomyan, S. Schadmand, S. Schumann, D. Sober, A. Starostin, I. Supek, C. M. Tarbert, A. Thomas, M. Unverzagt, D. P. Watts, D. Werthmueller, F. Zehr

Coherent photoproduction of $\pi^0$-mesons from threshold ($E_{th} \approx$ 136 MeV) throughout the $\Delta$-resonance region and of $\eta$-mesons close to the production threshold ($E_{th} \approx$ 570 MeV for $\eta$) has been measured for $^7$Li nuclei. The experiment was performed using the tagged-photon beam of the Mainz MAMI accelerator with the Crystal Ball and TAPS detectors combined to give an almost 4$\pi$ solid-angle electromagnetic calorimeter. The reactions were identified by a combined invariant-mass and missing-energy analysis. A comparison of the pion data to plane-wave impulse modelling tests the nuclear mass form factor. So far coherent $\eta$-production had been only identified for the lightest nuclear systems ($^2$H and $^3$He). For $^3$He a large enhancement of the cross section above plane-wave approximations had been reported, indicating the formation of a quasi-bound state. The present Li-data for $\eta$-production agree with a plane-wave approximation. Contrary to $^3$He, neither a threshold enhancement of the total cross section nor a deviation of the angular distributions from the expected form-factor dependence were observed.

View original: http://arxiv.org/abs/1303.2254