Pion and muon, the lightest unstable particles, were discovered more than fifty years ago, and have been well studied since. However, over time the Standard Model (SM) of elementary particles and interactions has become so successful that for several key pion and muon properties its predictions are far less uncertain than the best available measurements, primarily those concerning the particles' rare decay modes. Thus, slight deviations from the SM predictions can provide valuable clues to new physics outside of the current SM.
In its first phase, the PIBETA experiment has measured accurately several such rare decays at PSI, the Swiss meson facility. The talk will focus on the motivation, experimental apparatus, method, and the unexpected first results of these measurements.