The B+ -> K+ nu nu decay is mediated by a flavor-changing neutral current, which makes this decay quite rare in the Standard Model, happening about 6 times every million B+ decays, according to the theory. Moreover, the presence of two undetected neutrinos in the final state and of only one visible charged track makes searching for this decay particularly challenging.
We perform an analysis using electron-positron collisions recorded by the Belle II experiment between the years 2019 and 2022, using the properties of the accompanying B meson in the event to suppress background from other decays of the signal B candidate and light-quark pair production. We determine the branching fraction of the decay to be 2.7 sigma above the Standard Model expectation, providing the first evidence for this decay with a significance of 3.6 standard deviations.
