Search for three body pion decays <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msup><mml:mi>π</mml:mi><mml:mo>+</mml:mo></mml:msup><mml:mo stretchy="false">→</mml:mo><mml:msup><mml:mi>l</mml:mi><mml:mo>+</mml:mo></mml:msup><mml:mi>ν</mml:mi><mml:mi>X</mml:mi></mml:math>
A. A. Aguilar-Arevalo, M. Aoki, M. Blecher, D. Britton, Dorothea Vom Bruch, D. Bryman, Song Xi Chen, J. Comfort, S. Cuen-Rochin, L. Doria, P. Gumplinger, A. Hussein, Y. Igarashi, S. Ito, S. H. Kettell, L. L. Kurchaninov, L. Littenberg, C. Malbrunot, R. E. Mischke, T. Numao, D. Protopopescu, A. Sher, T. Sullivan, D. V. Vavilov
Abstract
The three body pion decays ${\ensuremath{\pi}}^{+}\ensuremath{\rightarrow}{l}^{+}\ensuremath{\nu}X(l=e,\ensuremath{\mu})$, where $X$ is a weakly interacting neutral boson, were searched for using the full data set from the PIENU experiment. An improved limit on $\mathrm{\ensuremath{\Gamma}}({\ensuremath{\pi}}^{+}\ensuremath{\rightarrow}{e}^{+}\ensuremath{\nu}X)/\mathrm{\ensuremath{\Gamma}}({\ensuremath{\pi}}^{+}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\nu}}_{\ensuremath{\mu}})$ in the mass range $0<{m}_{X}<120\text{ }\text{ }\mathrm{MeV}/{c}^{2}$ and a first result for $\mathrm{\ensuremath{\Gamma}}({\ensuremath{\pi}}^{+}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}\ensuremath{\nu}X)/\mathrm{\ensuremath{\Gamma}}({\ensuremath{\pi}}^{+}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\nu}}_{\ensuremath{\mu}})$ in the region $0<{m}_{X}<33.9\text{ }\text{ }\mathrm{MeV}/{c}^{2}$ were obtained. The Majoron-neutrino coupling model was also constrained using the current experimental result of the ${\ensuremath{\pi}}^{+}\ensuremath{\rightarrow}{e}^{+}{\ensuremath{\nu}}_{e}(\ensuremath{\gamma})$ branching ratio.