Limits on the mixing of tau neutrino to heavy neutrinos

Limits at 90% c.l. on the square of the mixing strength vertical bar Uτ4 vertical bar2 between ντ and a mostly isosinglet heavy neutrino with mass in the range 10-290 MeV/c2 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 congruent with 10-4 were obtained for neutrino masses larger than 160 MeV/c2