Search for $B^{+}\to {\mu }^{+}{\nu }_{\mu }$ and $B^{+}\to {\mu }^{+}N$ with inclusive tagging

Abstract

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

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