1304.1918 (Y. Maghrbi et al.)

Double pion photoproduction off nuclei - are there effects beyond
final-state interaction?    [PDF]

Y. Maghrbi, R. Gregor, S. Lugert, 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. Codling, E. J. Downie, P. Dexler, L. V. Fil'kov, A. Fix, D. I. Glazier, E. Heid, D. Hornidge, O. Jahn, I. Jaegle, V. L. Kashevarov, I. Keshelashvili, A. Knezevic, A. Kondratiev, M. Korolija, M. Kotulla, D. Krambrich, B. Krusche, M. Lang, V. Lisin, K. Livingston, 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. N. Prakhov, J. W. Price, G. Rosner, M. Rost, T. Rostomyan, S. Schadmand, S. Schumann, D. Sober, S. Starostin, I. Supek, A. Thomas, M. Unverzagt, D. P. Watts, D. Werthmueller, F. Zehr

Photoproduction of $\pi^{0}\pi^{0}$ and $\pi^{0}\pi^{\pm}$ pairs from nuclei has been measured over a wide mass range ($^2$H, $^{7}$Li, $^{12}$C, $^{40}$Ca, and $^{\rm nat}$Pb) for photon energies from threshold to 600 MeV. The experiments were performed at the MAMI accelerator in Mainz, using the Glasgow photon tagging spectrometer and a 4$\pi$ electromagnetic calorimeter consisting of the Crystal Ball and TAPS detectors. A shift of the pion-pion invariant mass spectra for heavy nuclei to small invariant masses has been observed for $\pi^0$ pairs but also for the mixed-charge pairs. The precise results allow for the first time a model-independent analysis of the influence of pion final-state interactions. The corresponding effects are found to be large and must be carefully considered in the search for possible in-medium modifications of the $\sigma$-meson. Results from a transport model calculation reproduce the shape of the invariant-mass distributions for the mixed-charge pairs better than for the neutral pairs, but also for the latter differences between model results and experiment are not large, leaving not much room for $\sigma$-in-medium modification.

View original: http://arxiv.org/abs/1304.1918