D. Pocanic, A. Palladino, L. P. Alonzi, V. A. Baranov, W. Bertl, M. Bychkov, Yu. M. Bystritsky, E. Frlez, V. A. Kalinnikov, N. V. Khomutov, A. S. Korenchenko, S. M. Korenchenko, M. Korolija, T. Kozlowski, N. P. Kravchuk, N. A. Kuchinsky, M. C. Lehman, D. Mekterovic, E. Munyangabe, D. Mzhavia, P. Robmann, A. M. Rozhdestvensky, S. N. Shkarovskiy, U. Straumann, I. Supek, P. Truöl, Z. Tsamalaidze, A. van der Schaaf, E. P. Velicheva, V. P. Volnykh
Building on the rare pion and muon decay results of the PIBETA experiment, the PEN collaboration has undertaken a precise measurement of B_{\pi e2} = R^\pi_{e/\mu}, the \pi^+ -> e^+\nu(\gamma) decay branching ratio, at the Paul Scherrer Institute, to reduce the present 40\times experimental precision lag behind theory to ~ 6-7\times. Because of large helicity suppression, R^\pi_{e/\mu} is uniquely sensitive to contributions from non-(V-A) physics, making this decay a particularly suitable subject of study. Even at current precision, the experimental value of B_{\pi e2} provides the most accurate test of lepton universality available. During runs in 2008-10, PEN has accumulated over 2\times 10^7 \pi_{e2} events; a comprehensive maximum-likelihood analysis is currently under way. The new data will also lead to improved precision of the earlier PIBETA results on radiative \pi and \mu decays.
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http://arxiv.org/abs/1210.5025
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