Measurements of electroweak penguin and lepton-flavor violating B decays to final states with missing energy at Belle and Belle II

Abstract

The Belle and Belle II experiments have collected a 1.2 ab${}^{-1}$ sample of collisions at a center-of-mass energy corresponding to the ${\rm Y}(4S)$ resonance. These datasets, with low particle multiplicity and constrained initial state kinematics, are an ideal environment to search for rare electroweak penguin $B$ decays and lepton-flavor-violating $B$ decays to final states with missing energy from neutrinos. Results from $b\to s\nu \bar{\nu }$ processes and their interpretation are presented. In addition, we provide an overview of the search for the $B\to K^{\ast 0}{\tau }^{+}{\tau }^{-}$ decays and the lepton-flavor violating decays $B^0\to K^{(\ast )0}{\tau }^{\pm }{\ell }^{\mp }$, where $\ell$ is an electron or a muon.

Presented at the 32nd International Symposium on Lepton Photon Interactions at High Energies (Lepton Photon 2025), Madison, Wisconsin, USA, 25-29 August 2025