Search for $B^{+}\to e^{+}\nu$ and $B^{+}\to {\mu }^{+}\nu$ decays using hadronic tagging

We present a search for the rare leptonic decays $B^{+}\to e^{+}{\nu }_e$ and $B^+ \to \mu^+ \nu_\mu$,usingthefull$\Upsilon(4S)$datasampleof$772\times 10^6$ $B\bar{B}$ pairs collected with the Belle detector at the $\rm {\small KEKB}$asymmetric-energy$e^+ e^-$collider\mathrm{.}Oneofthe$B$mesonsfromthe$\Upsilon(4S) \to B\bar{B}$ decay is fully reconstructed in a hadronic mode, while the recoiling side is analyzed for the signal decay. We find no evidence of a signal in any of the decay modes. Upper limits of the corresponding branching fractions are determined as ${\cal B}(B^+ \to e^+ \nu_e)<3.5\times 10^{-6}$and${\cal B}(B^+ \to \mu^+ \nu_\mu)<2.7 \times 10^{-6}$at90$%$ confidence level.