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Fakultät Physik

Implications of an enhanced B→Kνν¯ branching ratio

  • Hiller
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Rare decays mediated by b→sνν¯ transitions have been reported by the Belle II experiment. The branching ratio of the decay B+→K+νν¯ 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+→K+νν¯) exceeds 1.2⋅10−5(1.3⋅10−5) at 1σ (2σ), which tightens with a decreasing upper limit on (B→K∗νν¯), that is in reach of the Belle II experiment. In view of the strong constraints on electron-muon universality violation in |Δb|=|Δs|=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 required, implying non-minimal BSM sectors which are carefully balanced against flavor constraints. The decay Bs→invisibles can shed light on whether new physics is light or heavy. In the former case, branching ratios can be as large as a permille.

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