Improved search for heavy neutrinos in the decay π→eν:
Using the complete data set, PIENU has improved the exclusion limits for heavy neutral leptons coupled to electrons by an order of magnitude for masses less than the pion mass (139.6 MeV). As reported in Physical Review D (2018), so far only three neutrinos have been found, corresponding to the three electron-type particles. The tiny values of the neutrino masses have stimulated theoretical speculation that additional heavy neutral leptons may exist which, if verified, could have important consequences for the origin and composition of the universe. However the mass range for those new particles is relatively unconstrained. PIENU could directly observe evidence for heavy neutral leptons such as sterile neutrinos in the 2-body decays of pions π+→e+νh where νh is a massive neutrino, by observing an extra peak in the positron energy spectrum. After suppressing backgrounds by five orders of magnitude, the search came up empty allowing new more sensitive limits to be obtained; these results give the best limits in any mass region so far studied. In future, PIENU expects to have new results on heavy neutrinos coupled to muons by studying π+→µ+νh. In addition, the comparison of the measured and predicted π+→e+ν branching ratio also constrains the presence of non-SM neutrinos in the lowest energy region.