Search for three body pion decays π+→l+νX

The three body pion decays π+→l+νX(l=e,μ), where X is a weakly interacting neutral boson, were searched for using the full data set from the PIENU experiment. An improved limit on Γ(π+→e+νX)/Γ(π+→μ+νμ) in the mass range 0<mX<120 MeV/c2 and a first result for Γ(π+→μ+νX)/Γ(π+→μ+νμ) in the region 0<mX<33.9 MeV/c2 were obtained. The Majoron-neutrino coupling model was also constrained using the current experimental result of the π+→e+νe(γ) branching ratio.