Search for three body pion decays $\ddot{I}{€}^{+}\to l^{+}\hat{I}½X$

Abstract

The three body pion decays $\ddot{I}{€}^{+}\to l^{+}\hat{I}½X(l=e,\hat{I}¼)$, where $X$ is a weakly interacting neutral boson, were searched for using the full data set from the PIENU experiment. An improved limit on $\hat{I}“(\ddot{I}{€}^{+}\to e^{+}\hat{I}½X)/\hat{I}“(\ddot{I}{€}^{+}\to \hat{I}{¼}^{+}\hat{I}{½}_{\hat{I}}¼)$ in the mass range $0<m_X<120$ MeV/$c^2$ and a first result for $\hat{I}“(\ddot{I}{€}^{+}\to \hat{I}{¼}^{+}\hat{I}½X)/\hat{I}“(\ddot{I}{€}^{+}\to \hat{I}{¼}^{+}\hat{I}{½}_{\hat{I}}¼)$ 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 $\ddot{I}{€}^{+}\to e^{+}\hat{I}{½}_e({\hat{I}}^3)$ branching ratio.

8 pages, 7 figures. arXiv admin note: text overlap with arXiv:2006.00389