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Precise Measurement of the Pion Axial Form Factor in the Pi+ -> e+ nu gamma Decay

E. Frlez·D. Pocanic·V. A. Baranov·W. Bertl·M. Bychkov·N. V. Khomutov·A. S. Korenchenko·S. M. Korenchenko·T. Kozlowski·N. P. Kravchuk·N. A. Kuchinsky·W. Li·R. C. Minehart·D. Mzhavia·B. G. Ritchie·S. Ritt·A. M. Rozhdestvensky·V. V. Sidorkin·L. C. Smith·I. Supek·Z. Tsamalaidze·B. A. VanDevender·E. P. Velicheva·Y. Wang·H. -P. Wirtz·K. O. H. Ziock
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Abstract

We have studied radiative pion decays Pi+ -> e+ nu gamma in three broad kinematic regions using the PIBETA detector and a stopped pion beam. Based on Dalitz distributions of 42,209 events we have evaluated absolute Pi -> e nu gamma branching ratios in the three regions. Minimum chi^2 fits to the integral and differential (E(e+),E(gamma)) distributions result in the axial-to-vector weak form factor ratio of gamma = F_A/F_V = 0.443(15),or F_A = 0.0115(4) with F_V = 0.0259. However, deviations from Standard Model predictions in the high-E(gamma)/low-E(e+) kinematic region indicate the need for further theoretical and experimental work.

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E. Frlez, D. Pocanic, V. A. Baranov, W. Bertl, M. Bychkov, N. V. Khomutov, A. S. Korenchenko, S. M. Korenchenko, T. Kozlowski, N. P. Kravchuk, N. A. Kuchinsky, W. Li, R. C. Minehart, D. Mzhavia, B. G. Ritchie, S. Ritt, A. M. Rozhdestvensky, V. V. Sidorkin, L. C. Smith, I. Supek, Z. Tsamalaidze, B. A. VanDevender, E. P. Velicheva, Y. Wang, H. -P. Wirtz, K. O. H. Ziock. 2003-12-09. Precise Measurement of the Pion Axial Form Factor in the Pi+ -> e+ nu gamma Decay. https://doi.org/10.1103/physrevlett.93.181804

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