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  1. As the lightest meson, pion offers unique opportunities for measuring parameters and testing limits of the Standard Model (SM). The PiBeta experiment, carried out at PSI, focused on SM tests accessible through the pion beta, \pi^+ \to \pi^0e^+\nu_e(\gamma) π + → π 0 e + ν e ( γ ) , and electronic radiative, \pi^+ \to e^+\nu_e\gamma π + → e + ν e γ , decay channels. We review the PiBeta experiment, and update the pion beta decay branching ratio B^{\text{exp}}_{\pi\beta}=1.038(6)_{\text{tot}}\times10^{-8} B π β exp = 1.038 ( 6 ) tot × 10 − 8 , along with themore »corresponding derived value of the Cabibbo-Kobayashi-Maskawa matrix element V_{ud} = 0.9738(28) V u d = 0.9738 ( 28 ) .« less
  2. In common with a number of simple processes involving elementaryparticles, charged pion decays are profoundly shaped by applicableStandard Model (SM) symmetries and properties. Given the highly preciseSM theoretical description, pion decays are used as selective probes ofSM parameters, and of possible SM extensions. The PEN experiment at PSIis studying the \pi^+ \to e^+\nu_e(\gamma) π + → e + ν e ( γ ) ,or \pi_{e2(\gamma)} π e 2 ( γ ) decay. The primary goal is to reach the relative precision of 5 \times 10^{-4} 5 × 10 − 4 in R_{e/\mu}^\pi R e / μ π ,the branching ratiomore »for \pi_{e2(\gamma)} π e 2 ( γ ) decay. We review the PEN research program, its present status, andprospects.« less