Precision measurements of rare pion decays in the Pioneer experiment
Abstract: Precision measurements of rare decays of charged pions offer important tests of the standard model. The branching ratio of pi+ -> e+ nu to pi+ -> mu+ nu decay provides the best test of electron–muon weak universality. This ratio - due to helicity suppression via the Lorentz structure of the weak current - is also uniquely sensitive to non-V-A exotic currents. Additionally, the rate of pion beta decay pi+ -> pi0 e nu, a weak process analogous to superallowed Fermi transitions in nuclear beta decay, offers theoretically attractive sensitivity to the Vud matrix element of the CKM matrix. In this talk I'll discuss the plans by the Pioneer collaboration for a roughly ten-fold improvement in the branching ratio determinations for the rare pi+ -> e+ nu and pi+ -> pi0 e nu decays. These determinations involve measurements of relative yields from stopped pions of the 70 MeV monoenergetic electrons from pi+ -> e+ nu decay, the coincident photons from pi0's produced in pi+ -> pi0 e nu decay, and the 0-53 MeV Michel electrons from mu decay. The measurements demand both high statistics and careful understanding of systematic effects. To reach these goals we plan to utilize emergent technologies for both electromagnetic calorimetry and 4-dimensional, high-granularity tracking of pions, muons and electrons.