Measurements of electroweak penguin $B$ decays with missing energy at Belle and Belle II

Abstract

Electroweak penguin $B$ decays with neutrinos or $τ$ leptons in the final state are sensitive probes of physics beyond the Standard Model. Using the full Belle and Run 1 Belle II datasets, the first search for $B^0 \to K^0_Sτ^+τ^-$ sets the upper limit on the branching fraction to $\mathcal{B} < 8.3 \times 10^{-4}$, the search for $B^+ \to K^+τ^+τ^-$ sets $\mathcal{B} < 5.6 \times 10^{-4}$, and their isospin combination gives $\mathcal{B}(B \to Kτ^+τ^-) < 5.4 \times 10^{-4}$, all at 90% confidence level (CL$_s$). The first search for inclusive $B \to X_sν\barν$ decays at Belle II sets $\mathcal{B} < 3.3 \times 10^{-4}$. Finally, the Belle II $B^+ \to K^+ν\barν$ result is reinterpreted in the Weak Effective Theory using its published likelihood. The data prefer an enhanced vector contribution and mildly favour a nonzero tensor contribution.