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

Le résumé fourni par la source

We report a measurement of the branching fraction for the leptonic decay B + → μ + ν μ . This work presents the first B + → μ + ν μ 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 + → μ + ν μ ) = ( 4.4 ± 1.9 ± 1.0 ) × 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 + → μ + ν μ ) < 6.7 × 10 − 7 using a frequentist approach and a 90% credibility level upper limit of B ( B + → μ + ν μ ) < 7.2 × 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 ∈ [ 0 , 1.5 ] GeV . No signal is observed, and the resulting exclusion on the squared mixing parameter | U μ N | 2 provides improvement over previous limits. We report a measurement of the partial branching fraction of semileptonic B → X u ℓ ν ℓ decays with p μ B > 2.2 GeV , obtaining Δ B ( B →

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