Search for $B^+ \to \mu^+\, \nu_\mu$ and $B^+ \to \mu^+\, N$ with inclusive tagging

Physical review / D 101(3), 032007 (2020). doi:10.1103/PhysRevD.101.032007
We report the result for a search for the leptonic decay of $B^+ \to \mu^+\, \nu_\mu$ using the full Belle dataset of 711 fb$^{-1}$ of integrated luminosity at the $ϒ(4S)$ resonance. In the Standard Model leptonic $B$-meson decays are helicity and Cabibbo-Kobayashi-Maskawa 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 μ into the signal B-meson rest frame, in which the μ has a monochromatic momentum spectrum. Though its momentum is smeared by the experimental resolution, this technique improves the analysis sensitivity considerably. Analyzing the $μ$ momentum spectrum in this frame we find $\mathcal{B}$($B^+ \to \mu^+\, \nu_\mu)=(5.3±2.0±0.9) × 10^{-7}$ with a one-sided significance of 2.8 standard deviations over the background-only hypothesis. This translates to a frequentist upper limit of $\mathcal{B}$($B^+ \to \mu^+\, \nu_\mu) < 8.6 × 10^{-7}$ at 90% confidence level. 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.
Published by Inst.302363, Melville, NY