Eiichi Nakano

23 September 2019
2019 Lattice X Workshop

Abstract: The search for the leptonic decay of B+ → μ+ νμ using the full Belle data set of 711 fb−1 of integrated luminosity near the Υ(4S) resonance is described. In the Standard Model leptonic B meson decays are helicity and CKM suppressed and to maximize sensitivity an inclusive tagging approach is used in this measurement to reconstruct the second B meson produced in the collision. The directional information is used after a dedicated calibration to boost the signal-side μ into the frame, where the signal B meson is at rest. This improves the sensitivity of the analysis considerably as in this frame the signal μ possesses a monochromatic momentum, which is smeared by the experimental resolution. Analyzing the μ momentum spectrum in this frame we find B(B+ → μ+ νμ) = (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 B(B+ → μ+ νμ) < 8.6 × 10−7 at 90% CL. The experimental spectrum is then used to search for a massive sterile neutrino, B+ → μ+ N, finding no evidence for such a process for the range of 0 - 1.5 GeV of the sterile neutrino mass. The determined B+ → μ+ νμ branching fraction limit is further used to constrain the mass and coupling space of the type II and type III two Higgs doublet model.

Note: 30 min