Search for Massive Neutrinos in the Decay pi --> e nu
  Evidence of massive neutrinos in the pi --> e nu decay spectrum was sought with the background pi --> mu --> e decay chain highly suppressed. Upper limits (90% C.L.) on the neutrino mixing matrix element |U_ei|^2 in the neutrino mass region 60--129 MeV/c^2 were set at the level of 10^-8. 
PIENU Collaboration: M. Aoki, M. Blecher, D.A. Bryman, S. Chen, M.   Ding, L. Doria, P. Gumplinger, C. Hurst, A. Hussein, Y....
hep-ex