Search for $B^+ \to ��^+\, ��_��$ and $B^+ \to ��^+\, N$ with inclusive tagging

We report the result for a search for the leptonic decay of $B^+ \to ��^+ \, ��_��$ using the full Belle data set of 711 fb${}^{-1}$ of integrated luminosity at the $��(4S)$ resonance. In the Standard Model leptonic $B$-meson decays are helicity and CKM suppressed. To maximize sensitivity an inclusive tagging approach is used to reconstruct the second $B$ meson produced in the collision. The directional information from this second $B$ meson is used to boost the observed $��$ into the signal $B$ meson rest-frame, in which the $��$ has a monochromatic momentum spectrum. Though its momentum is smeared by the experimental resolution, this technique improves the analysis sensitivity considerably. Analyzing the $��$ momentum spectrum in this frame we find $\mathcal{B}(B^+ \to ��^+ \, ��_��) = \left( 5.3 \pm 2.0 \pm 0.9 \right) \times 10^{-7}$ with a one-sided significance of 2.8 standard deviations over the background-only hypothesis. This translates to a frequentist upper limit of $\mathcal{B}(B^+ \to ��^+ \, ��_��) < 8.6 \times 10^{-7}$ at 90% CL. The experimental spectrum is then used to search for a massive sterile neutrino, $B^+ \to ��^+ \, N$, but no evidence is observed for a sterile neutrino with a mass in a range of 0 - 1.5 GeV. The determined $B^+ \to ��^+ \, ��_��$ branching fraction limit is further used to constrain the mass and coupling space of the type II and type III two-Higgs-doublet models.