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

We report a measurement of the branching fraction for the leptonic decay B+ -> mu(+)nu(mu). This work presents the first B+ -> mu(+)nu(mu) result using Belle II data, an updated Belle measurement that supersedes the previous result, and their combination, which yields the most precise search to date. The analysis is based on 1076 fb(-1) of e(+)e(-) collision data collected at a center-of-mass energy of 10.58 GeV with the Belle and Belle II detectors at the KEKB and SuperKEKB colliders, respectively. We measure B(B+ -> mu(+)nu(mu)) = (4.4 +/- 1.9 +/- 1.0) x 10(-7), where the first uncertainty is statistical and the second systematic. The observed significance relative to the background-only hypothesis is 2.4 standard deviations. We set a 90% confidence level upper limit of B(B+ -> mu(+)nu(mu)) < 6.7 x 10(-7) using a frequentist approach and a 90% credibility level upper limit of B(B+ -> mu(+)nu(mu)) < 7.2 x 10(-7) using a Bayesian approach. These are the most stringent limits to date. The result is interpreted as an exclusion region in the parameter space of type II and type III two-Higgs-doublet models. We search for stable sterile neutrinos with masses m(N) is an element of [0, 1.5] GeV. No signal is observed, and the resulting exclusion on the squared mixing parameter vertical bar U-mu N vertical bar(2) provides improvement over previous limits. We report a measurement of the partial branching fraction of semileptonic B -> X(u)l nu(l) decays with p(mu)(B) > 2.2 GeV, obtaining Delta B(B -> X(u)l nu(l)) = (2.72 +/- 0.05 +/- 0.29) x 10(-4). We present a model-dependent study of weak annihilation decays using the muon momentum spectrum. We observe a signal of 2.4 standard deviations above the background-only hypothesis in regions where the distribution resembles that of B -> X(u)l nu(l) decays.