We have located 3886 neutrino interactions in the fiducial volume of a hybrid emulsion spectrometer installed in the Fermilab wide-band neutrino beam. A search for ${\tau }^{-}$ decays yielded no candidate, resulting in an upper limit of 0.002 (0.073) for direct coupling of ${\nu }_{\mu } \left({\nu }_e\right)$ to ${\tau }^{-}$. The ${\nu }_{\mu } \left({\nu }_e\right)$ to ${\nu }_{\tau }$ limits to mass differences and mixing angles ($\alpha$) between the neutrinos are at maximum mixing $\Delta M^2<0.9\mathrm{} \mathrm{}\left(9.0\right)\mathrm{}$ ${\mathrm{eV}}^2$, and at maximum sensitivity ${sin}^2\left(2\mathrm{}\alpha \right)<\mathrm{}0.004\mathrm{} \mathrm{}\left(0.12\right)\mathrm{}$. The direct-coupling limits are also used to show that most ${\tau }^{-}$ decays must contain a neutral lepton other than ${\nu }_{\mu }$ or ${\nu }_e$.

• Received 19 August 1986

DOI:https://doi.org/10.1103/PhysRevLett.57.2897