Fundamental physics in rare pion and muon decays.

Advances in detector techniques, coupled to improved secondary beams at meson factories, have brought about major improvements in the experimental precision of pion and muon rare decay measurements over the last decade. We review recent results, focusing in particular on: π+→π0e+ν (pion beta, πe3, or πβ decay), and π+→e+νγ (radiative pion decay, πe2γ or RPD), as well as the radiative muon decay, μ+→e+ννγ (radiative muon decay, RMD), and their theoretical implications. The PIBETA experiment, with measurements completed at PSI, has brought about an order of magnitude improvement, or better, in the branching ratio precision for these processes. The successor PEN experiment, currently under way at PSI, aims to improve the precision of the π+→e+ν(πe2) decay branching ratio by about an order of magnitude. Each of these results contributes toward a better understanding of the limits on certain particles and interactions not included in the standard model, or toward improving the precision of chiral lagrangian and pion structure parameters. We finally review the near-term prospects for their further improvement. [ABSTRACT FROM AUTHOR]