Fundamental physics in rare pion and muon decays
D. Pocanic 1*
1 Department of Physics and Institute for Nuclear and Particle Physics, University of Virginia
Decays of pions and muons, the two lightest unstable particles, have been studied extensively since their discovery, and have provided invaluable physics insights. For example, the anomalously suppressed decay of the charged pion to the electron (π+ → e+ν, labeled πe2) provided an early signal of the underlying V-A Lorentz structure of the weak interaction. Meanwhile, decades of precise measurements in numerous systems spanning a broad range of energies and masses have motivated painstakingly refined theoretical calculations of all manner of electroweak processes. Consequently, the SU(2)×U(1) electroweak dynamics is arguably the theoretically most precisely understood sector of the standard model (SM) of elementary particles and interactions.
Thanks to the simple dynamics of the pion and muon decays, the simplicity and small number of the available decay channels, extremely well controlled radiative and loop corrections, these decays offer a uniquely sensitive means to explore details and limits of the underlying symmetries. For example, the pion beta decay (π+ → π0e+ν, or πβ) provides a particularly clean means to study the limits of Cabibbo universality, or the conserved vector current (CVC) hypothesis. Similarly, the πe2 decay offers the most sensitive test of lepton universality, the equality of the lepton couplings to the weak boson regardless of the lepton family (generation). On the other hand, radiative decays of the muon (μ → eννbarγ) and pion (π → eνγ, or πe2γ) are sensitive to departures from the basic V-A dynamics of the weak interaction. Currently the experimental precision of all these processes lags significantly, i.e., by an order of magnitude or more, behind the precision of their theoretical description.
We report on a comprehensive program of precise measurements of the pion and muon rare decays at the Paul Scherrer Institute (PSI), Switzerland. The PIBETA experiment  has brought about an order of magnitude improvement of the branching ratio precision of the πβ, πe2γ, and μ → eννbarγ decays. In the process we have confirmed CVC in a meson for the first time well below the radiative corrections level, extracted a new value of the weak u-d quark mixing, greatly improved the precision of the weak pion form factors FA and FV, while setting a new stringent limit on the tensor term FT, and greatly improving the pion polarizability precision. Our muon measurements have resulted in a new limit on the Michel parameter η-bar. The PEN experiment  is currently working toward accomplishing a similar order-of-magnitude improvement in the precision of the measured pie2 decay branching ratio. Physics implications of these results will be discussed in detail.
 Experiment home page: http://pibeta.phys.virginia.edu/
 Experiment home page: http://pen.phys.virginia.edu/