D. Hornidge, P. Aguar Bartolome, J. R. M. Annand, H. J. Arends, R. Beck, V. Bekrenev, H. Berghaeuser, A. M. Bernstein, A. Braghieri, W. J. Briscoe, S. Cherepnya, M. Dieterle, E. J. Downie, P. Drexler, C. Fernandez-Ramirez, L. V. Filkov, D. I. Glazier, P. Hall Barrientos, E. Heid, M. Hilt, I. Jaegle, O. Jahn, T. C. Jude, V. L. Kashevarov, I. Keshelashvili, R. Kondratiev, M. Korolija, A. Koulbardis, D. Krambrich, S. Kruglov, B. Krusche, A. T. Laffoley, V. Lisin, K. Livingston, I. J. D. MacGregor, J. Mancell, D. M. Manley, E. F. McNicoll, D. Mekterovic, V. Metag, S. Micanovic, D. G. Middleton, K. W. Moores, A. Mushkarenkov, B. M. K. Nefkens, M. Oberle, M. Ostrick, P. B. Otte, B. Oussena, P. Pedroni, F. Pheron, A. Polonski, S. Prakhov, J. Robinson, T. Rostomyan, S. Scherer, S. Schumann, M. H. Sikora, A. Starostin, I. Supek, M. Thiel, A. Thomas, L. Tiator, M. Unverzagt, D. P. Watts, D. Werthmueller, L. Witthauer
A precision measurement of the photon asymmetry $\Sigma$ and differential cross sections $d\sigma/d\Omega$ for the $\gamma p \rightarrow \pi^0p$ reaction in the near-threshold region has been performed with a tagged photon beam and almost $4\pi$ detector at the Mainz Microtron. The Glasgow-Mainz photon tagging facility along with the Crystal Ball/TAPS multi-photon detector system and a cryogenic liquid hydrogen target were used. These data allowed for a precise determination of the energy dependence of the real parts of the $S$- and all three $P$-wave amplitudes for the first time and provide the most stringent test to date of the predictions of Chiral Perturbation Theory and its energy region of convergence. The upper limit of agreement is between 165 and 175 MeV incident photon lab energy, $\simeq25$ MeV above threshold.
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http://arxiv.org/abs/1211.5495
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