Search For Three Body Pion Decays Pi(+) -> L(+) Nu X

The three body pion decays pi(+) -> l(+)nu X(1= 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(+) -> e(+)nu X)/Gamma(pi(+) -> mu(+)nu(mu)) in the mass range 0 < m(X) < 120 MeV/c(2) and a first result for Gamma(pi(+) -> mu(+)nu X) /Gamma(pi(+) -> mu(+)v(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(+) -> e(+) nu(e) (gamma) branching ratio.