We report on a search for a heavy Majorana neutrino in the decays ${\tau }^{-}\to {\pi }^{-}{\nu }_h$, ${\nu }_h\to {\pi }^{\pm }{\ell }^{\mp }$, $\ell =e$, $\mu$. The results are obtained using the full data sample of $988{\mathrm{fb}}^{-1}$ collected with the Belle detector at the KEKB asymmetric energy $e^{+}{e}^{-}$ collider, which contains $912\times {10}^6\tau \tau$ 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 $0.2–1.6\mathrm{GeV}/{\mathrm{c}}^2$. This is the first study of a mixed couplings of heavy neutrinos to $\tau$ leptons and light-flavor leptons.

• Received 25 December 2022
• Revised 13 July 2023
• Accepted 13 October 2023

DOI:https://doi.org/10.1103/PhysRevLett.131.211802

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Published by the American Physical Society