Search for the rare decays $\pi^+ \to \mu^+\nu_\mu\nu\bar\nu$ and $\pi^+ \to e^+\nu_e\nu\bar\nu$

TL;DR

This paper reports on the search for rare pion decay modes involving neutrinos, setting upper limits on their branching ratios, using data from the PIENU experiment, and improving previous measurements.

Contribution

The study provides the first experimental upper limit for the decay ${ m extpi}^+ o m  u_ u ar{ u}$ and an improved limit for ${ m extpi}^+ o e^+ u_e u ar{ u}$, advancing understanding of rare pion decays.

Findings

Upper limit for ${ m extpi}^+ o m  u_ u ar{ u}$ decay rate.

Improved upper limit for ${ m extpi}^+ o e^+ u_e u ar{ u}$ decay rate.

Data from the PIENU experiment constrains rare decay branching ratios.

Abstract

The rare pion decays ${\pi}^+{\rightarrow}{\mu}^+{\nu}_{\mu}{\nu}\bar{\nu}$ and ${\pi}^+{\rightarrow}e^+{\nu}_{e}{\nu}\bar{\nu}$ are allowed in the Standard Model but highly suppressed. These decays were searched for using data from the PIENU experiment. A first result for ${\Gamma}({\pi}^+{\rightarrow}{\mu}^+{\nu}_{\mu}{\nu}\bar{\nu})/{\Gamma}({\pi}^+{\rightarrow}{\mu}^+{\nu}_{\mu})<8.6{\times}10^{-6}$, and an improved measurement ${\Gamma}({\pi}^+{\rightarrow}{e}^+{\nu}_{e}{\nu}\bar{\nu})/{\Gamma}({\pi}^+{\rightarrow}{\mu}^+{\nu}_{\mu})<1.6{\times}10^{-7}$ were obtained.