Search for a Heavy Neutrino in τ Decays at Belle

We report on a search for a heavy Majorana neutrino in the decays τ^-→π^-ν_h , ν_h→π^±ℓ^∓, ℓ=e, μ . The results are obtained using the full data sample of 988 ^{fb -1} collected with the Belle detector at the KEKB asymmetric energy e⁺e^- collider, which contains 912 ×10⁶ τ τ pairs. We observe no significant signal and set 95% CL upper limits on the couplings of the heavy right-handed neutrinos to the conventional standard model left-handed neutrinos in the mass range 0.2 - 1.6 GeV /c² . This is the first study of a mixed couplings of heavy neutrinos to τ leptons and light-flavor leptons.