Search for the lepton flavour violating decays $B^{+}\to K^{+}{\tau }^{\pm }{\mathrm{\ell }}^{\mp }$ ($\mathrm{\ell }=e,\mu$) at Belle

We present a search for the lepton-flavour-violating decays $B^{+}\to K^{+}{\tau }^{\pm }{\mathrm{\ell }}^{\mp }$, with $\mathrm{\ell }=(e,\mu )$, using the full data sample of $772\times {10}^6$ $B\bar{B}$ pairs recorded by the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider. We use events in which one $B$ meson is fully reconstructed in a hadronic decay mode. We find no evidence for $B^{\pm }\to K^{\pm }\tau \mathrm{\ell }$ decays and set upper limits on their branching fractions at the 90% confidence level in the $(1$-$3)\times {10}^{-5}$ range. The obtained limits are the world’s best results.