Limits on the Mixing of Tau Neutrino to Heavy Neutrinos
  Limits at 90 % c.l. on the square of the mixing strength |U_tau4|^2 between nu_tau and a mostly isosinglet heavy neutrino with mass in the range 10-290 MeV/c^2 are reported. The results were derived using the negative result of a search for neutral particles decaying into two electrons conducted by the CHARM collaboration in a neutrino beam dump experiment. Upper limits ~10^-4 were obtained for neutrino masses larger than 160 MeV/c^2. 
J. Orloff, A. Rozanov and C. Santoni