Abstract

Rare B-meson decays with invisible signatures are sensitive probes of physics beyond the Standard Model. The flavor-changing neutral current decay B⁺ → K⁺ νν̄ is theoretically clean, with a predicted branching fraction of ~5×10−6 in the Standard Model. Various New Physics scenarios, including those involving dark matter candidates, could significantly enhance this rate. The most recent Belle II measurement reports a 4.6-fold enhancement over the Standard Model with compatibility of only 2.7σ, providing an intriguing hint of New Physics. This experimentally challenging measurement is presented, highlighting innovative methods for background suppression and improved signal efficiency. A novel framework for reinterpretation of this measurement under alternative signal kinematics is also introduced, enabling robust tests of a broad class of New Physics models. Finally, the future prospects for this golden channel and related B-meson decays with similar signatures are discussed. In that context, recent improvements in graph neural network–based methods for displaced track and vertex reconstruction are highlighted. These innovations not only advance studies of B-meson decays at Belle II, but also provide broadly applicable tools for LHCb and beyond.