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 $\Upsilon(4S)$ 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.
Highlights
Mm2B2l 2 f2B jV ub j2 τB ; ð1Þ with GF denoting Fermi’s constant, mB and ml the B meson and lepton masses, respectively, and jVubj the relevant CKM matrix element of the process
To maximize sensitivity an inclusive tagging approach is used to reconstruct the second B meson produced in the collision
Events containing a high-momentum muon candidate are identified as potential signal events, and the additional charged particles and neutral energy depositions in the rest of the event (ROE) are used to reconstruct the second B meson produced in the collision process
Summary
Precision measurements of leptonic decays of B mesons offer a unique tool to test the validity of the Standard Model (SM) of particle physics. Events containing a high-momentum muon candidate are identified as potential signal events, and the additional charged particles and neutral energy depositions in the rest of the event (ROE) are used to reconstruct the second B meson produced in the collision process With such an inclusive reconstruction one reduces the background due to nonresonant eþe− → qq (q 1⁄4 u; d; s; c) continuum processes, and, after a dedicated calibration, it is possible to deduce the direction of the signal B meson. The experimental resolution on the boost vector reconstructed from ROE information broadens this signal signature The use of this frame, which enhances the expected sensitivity of the search, is the main improvement over the preceding analysis, published in Ref.
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