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

Abstract

Electroweak penguin $B$ decays with neutrinos or $\tau$ 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 $B^0\to K_S^0{\tau }^{+}{\tau }^{-}$ sets the upper limit on the branching fraction to $\mathcal{B}<8.3\times 10^{-4}$, the search for $B^{+}\to K^{+}{\tau }^{+}{\tau }^{-}$ sets $\mathcal{B}<5.6\times 10^{-4}$, and their isospin combination gives $\mathcal{B}(B\to K{\tau }^{+}{\tau }^{-})<5.4\times 10^{-4}$, all at 90% confidence level (CL${}_s$). The first search for inclusive $B\to X_s\nu \bar{\nu }$ decays at Belle II sets $\mathcal{B}<3.3\times 10^{-4}$. Finally, the Belle II $B^{+}\to K^{+}\nu \bar{\nu }$ 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.

arXiv comment6 pages, 4 figures. Contribution to the 43rd International Conference on High Energy Physics (ICHEP2026), 30 July - 5 August 2026, Natal, Brazil