Evidence for a Rare B Decay with Two Invisible Neutrinos at Belle II

by Slavomira Stefkova

Thursday 7 Sept 2023, 16:00 → 17:00 Australia/Sydney

The rate of the flavour-changing neutral current decay $B^+\to K^+\nu\bar\nu$ is predicted with high precision within the Standard Model. Simultaneously, several New Physics models predict a significant enhancement for this rate. $B^+\to K^+\nu\bar\nu$ decays are, however, experimentally challenging as they are not only rare but also contain two neutrinos, leaving no signature in the detector. In this seminar, I will present a new measurement of the branching fraction for $B^+\to K^+\nu\bar\nu$ decays. This measurement is based on 362 fb${}^{-1}$ of SuperKEKB electron-positron collision data collected at the Upsilon(4S) resonance by the Belle II experiment in Tsukuba, Japan. Using two different reconstruction techniques, which are combined at the likelihood level, we achieve the most precise measurement of the rate of the $B^+\to K^+\nu\bar\nu$ process to date. At the end, I will also outline Belle II's future prospects for $B\to K\nu\bar\nu$ decays and similar processes with invisible signatures.