Abstract
In common with a number of simple processes involving elementary particles, charged pion decays are profoundly shaped by applicable Standard Model (SM) symmetries and properties. Given the highly precise SM theoretical description, pion decays are used as selective probes of SM parameters, and of possible SM extensions. The PEN experiment at PSI is studying the \pi^+ \to e^+\nu_e(\gamma)π+→e+νe(γ), or \pi_{e2(\gamma)}πe2(γ) decay. The primary goal is to reach the relative precision of 5 \times 10^{-4}5×10−4 in R_{e/\mu}^\piRe/μπ, the branching ratio for \pi_{e2(\gamma)}πe2(γ) decay. We review the PEN research program, its present status, and prospects.
Highlights
25.1 MotivationFollowing the discovery of the charged pion, its decays presented a puzzle: absence of the direct pion decay to an electron that persisted for over a decade
Given the critical role of the mTPCs in controlling the Reπ/μ systematics, the analysis reported here excludes Run 1, or ∼ 20% of the full PEN data set
The PEN collaboration is on course to improve the experimental precision of the pion electronic decay π+ → e+νe(γ) to a relative precision of ∼ 5 × 10−4
Summary
Following the discovery of the charged pion, its decays presented a puzzle: absence of the direct pion decay to an electron that persisted for over a decade. In the years that followed, πe decay served as an important theory testing ground, providing rapid experimental confirmations [4, 5] for predicted radiative corrections [6, 7]. This close interplay with theory, at the edge of experimentally accessible precision, remains the driving force behind πe measurements today. The strong helicity suppression of the πe decay (squared lepton mass ratio in Eq (25.1)) makes this decay uniquely sensitive to a class of pseudoscalar (P), or P-loop-coupled, non-(V−A) contributions, arising from new,. Sensitive to non-(V−A) contributions, are not discussed here; neither is τπ+
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