A search for massive neutrinos has been made in the decay π→e+ν. No evidence was found for extra peaks in the positron energy spectrum indicative of pion decays involving massive neutrinos (π→e+νh). Upper limits (90 \% C.L.) on the neutrino mixing matrix element ∣Uei∣2 in the neutrino mass region 60--135 MeV/c2 were set, which are %representing an order of magnitude improvement over previous results.
@article{arxiv.1712.03275,
title = {Improved Search for Heavy Neutrinos in the Decay $\pi\rightarrow e\nu$},
author = {A. Aguilar-Arevalo and M. Aoki and M. Blecher and D. I. Britton and D. vom Bruch and D. A. Bryman and S. Chen and J. Comfort and S. Cuen-Rochin and L. Doria and P. Gumplinger and A. Hussein and Y. Igarashi and S. Ito and S. Kettell and L. Kurchaninov and L. S. Littenberg and C. Malbrunot and R. E. Mischke and T. Numao and D. Protopopescu and A. Sher and T. Sullivan and D. Vavilov},
journal= {arXiv preprint arXiv:1712.03275},
year = {2018}
}