Search for three body pion decays ${\pi}^+{\to}l^+{\nu}X$

TL;DR

This study used data from the PIENU experiment to search for rare three-body pion decays involving a neutral boson, setting new limits on their occurrence and constraining theoretical models like the Majoron-neutrino coupling.

Contribution

It provides the first experimental limits on certain pion decay modes involving a neutral boson and improves existing bounds on these rare processes.

Findings

Improved upper limits on ${ m rac{ au}{ au_{total}}}$ for ${ m rac{ m o e u X}{ o m e u}}$ decays.

First constraints on ${ m rac{ m o m u X}{ m o m u}}$ decays in specified mass range.

Constraints on Majoron-neutrino coupling from pion decay data.

Abstract

The three body pion decays ${\pi}^+{\rightarrow}l^+{\nu}X~(l=e,{\mu})$, where $X$ is a weakly interacting neutral boson, were searched for using the full data set from the PIENU experiment. An improved limit on ${\Gamma}({\pi}^+{\to}e^+{\nu}X)/{\Gamma}({\pi}^+{\to}{\mu}^+{\nu}_{\mu})$ in the mass range $0<m_X<120$ MeV/$c^2$ and a first result for ${\Gamma}({\pi}^+{\to}{\mu}^+{\nu}X)/{\Gamma}({\pi}^+{\to}{\mu}^+{\nu}_{\mu})$ in the region $0<m_X<33.9$ MeV/$c^2$ were obtained. The Majoron-neutrino coupling model was also constrained using the current experimental result of the ${\pi}^+{\to}e^+{\nu}_e({\gamma})$ branching ratio.