2023/08/31 by Bause, Rigo, Gisbert, Hector, Hiller, Gudrun
#FOS: Physical sciences #High Energy Physics - Phenomenology (hep-ph)
paper · doi:10.48550/arxiv.2309.00075
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 |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 generically required, implying non-minimal BSM sectors which are carefully balanced against flavor constraints. The decay Bs0 → 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.