Study of B+ → μ+νμ decays at Belle and Belle II

This thesis presents a study of the leptonic B+ → μ+νμ decay using data collected at the Belle and Belle II experiments. The branching fraction of the B+ → μ+νμ decay is determined by examining the muon momentum in the rest frame of the B meson. A data set corresponding to an integrated luminosity of 365 fb−1, collected by the Belle II experiment at a center-of-mass energy of 10.58 GeV, is analyzed and then combined with the data samples previously used by the Belle collaboration in an earlier analysis of this decay analyzing a data set corresponding to an integrated luminosity of 711 fb−1 (M. T. Prim et al., Phys. Rev. D 101 (2020) 032007). An untagged strategy is used to reconstruct the B+ → μ+νμ decay. The kinematics of the signal-side B meson is inferred from the remaining particles in the event, after applying necessary corrections. This allows for the determination of the B meson kinematics and the boost of the muon momentum into the B meson rest frame, where a monochromatic momentum peak is expected. Backgrounds, predominantly arising from continuum processes, are corrected in a data driven way and suppressed using multivariate analysis techniques, and specific parts of the analysis are validated with a dedicated hadronic control channel. The B+ → μ+νμ branching fraction is obtained by fitting the muon momentum distribution in the B meson rest frame, simultaneously, in multiple categories that include both Belle and Belle II data. The resulting branching fraction is measured to be ℬ(B+ → μ+νμ) = (4.4 ± 1.9(stat.) ± 1.0(sys.)) × 10−7, with an observed significance of 2.4 standard deviations relative to the background-only hypothesis. At the 90% confidence and credibility level, Frequentist and Bayesian upper limits are set as ℬ(B+ → μ+νμ) < 6.7 × 10−7 and ℬ(B+ → μ+νμ) < 7.2 × 10−7, respectively. Implications for new physics scenarios, including type II and type III two-Higgs-doublet models and sterile neutrinos, are explored. In addition, the partial branching fraction of inclusive charmless semileptonic decays B → Xuℓνℓ with a muon momentum in the B meson rest frame greater than 2.2 GeV is measured and found to be Δℬ(B → Xuℓνℓ) = (2.72 ± 0.05(stat.) ± 0.29(sys.)) × 10−4. A model-dependent investigation of B → Xuℓνℓ decays via weak annihilation decays is presented and their influence on the measurement of the B+ → μ+νμ branching fraction evaluated.