PEN: a low energy test of lepton universality

• Main Authors: Pocanic, D, Alonzi, L. P, Baranov, V. A, Bertl, W, Bychkov, M, Bystritsky, Yu. M, Frlez, E, Glaser, C. J, Kalinnikov, V. A, Khomutov, N. V, Korenchenko, A. S, Korenchenko, S. M, Korolija, M, Kozlowski, T, Kravchuk, N. P, Kuchinsky, N. A, Lehman, M. C, Mzhavia, D, Palladino, A, Robmann, P, Rozhdestvensky, A. M, Supek, I, Truoel, P, van der Schaaf, A, Velicheva, E. P, Vitz, M. G, Volnykh, V. P
• Format: Journal Article
• Language: English
• Published: 18-01-2017
• Subjects: Physics - High Energy Physics - Experiment; Physics - High Energy Physics - Phenomenology; Physics - Nuclear Experiment
• DOI: 10.48550/arxiv.1701.05254

Allowed charged $\pi$ meson decays are characterized by simple dynamics, few
available decay channels, mainly into leptons, and extremely well controlled
radiative and loop corrections. In that sense, pion decays represent a
veritable triumph of the standard model (SM) of elementary particles and
interactions. This relative theoretical simplicity makes charged pion decays a
sensitive means for testing the underlying symmetries and the universality of
weak fermion couplings, as well as for studying pion structure and chiral
dynamics. Even after considerable recent improvements, experimental precision
is lagging far behind that of the theoretical description for pion decays. We
review the current state of experimental study of the pion electronic decay
$\pi^+ \to e^+\nu_e(\gamma)$, or $\pi_{e2(\gamma)}$, where the $(\gamma)$
indicates inclusion and explicit treatment of radiative decay events. We
briefly review the limits on non-SM processes arising from the present level of
experimental precision in $\pi_{e2(\gamma)}$ decays. Focusing on the PEN
experiment at the Paul Scherrer Institute (PSI), Switzerland, we examine the
prospects for further improvement in the near term.