Implications of an enhanced B → K ν ν ¯ branching ratio

Rare decays mediated by b → s ν ν ¯ transitions have been reported by the Belle II experiment. The branching ratio of the decay B + → K + ν ν ¯ is found to be enhanced with respect to the standard model value. If taken at face value, the implications are profound; either lepton flavor universality is violated at the (multi)-TeV-scale, or light new physics is involved. This holds in general if B ( B + → K + ν ν ¯ ) exceeds 1.2 × 10 − 5 ( 1.3 × 10 − 5 ) at 1 σ ( 2 σ ), which tightens with a decreasing upper limit on B ( B → K * ν ν ¯ ) , that is in reach of the Belle II experiment. In view of the strong constraints on electron-muon universality violation in | Δ b | = | Δ s | = 1 processes, viable explanations are heavy, (5–10)-TeV tree-level new physics mediators that couple only to tau-flavors, or lepton flavor violating ones. In addition, couplings of similar size to both left- and right-handed quarks are generically required, implying nonminimal beyond the standard model sectors which are carefully balanced against flavor constraints. The decay B s 0 → invisibles can shed light on whether new physics is light or heavy. In the former case, branching ratios can be as large as 10 − 5 . Published by the American Physical Society 2024