Particle Physics Seminar

Search for rare B+→K+νν̄ decays using an inclusive tagging method at Belle II

by Slavomira Stefkova

Nußallee 12/1-1.049 - Conference Room II (PI)

Nußallee 12/1-1.049 - Conference Room II



The Belle II detector located at the SuperKEKB electron-positron collider in Tsukuba, Japan, has been collecting the data since 2019 with the final aim of accumulating a dataset corresponding to 50 ab-1 of integrated luminosity. In this talk I will present a search for the flavor-changing neutral current decay B+→K+νν̄ with the first 63 fb-1 of the collision data collected at the Υ(4S) resonance by the Belle II experiment. B+→K+νν̄ decays are of paramount interest as they belong to the family of b→sll transitions, where several anomalies with hints for lepton flavor universality violation have been reported in the past few years. At first glance, searching for this rare decay with such a small dataset may seem to be a step too far given that only one out of about 200000 B-mesons is expected to decay like this and that it contains two neutrinos which do not leave any signature in the detector. But the novel reconstruction approach (inclusive tagging method), good performance of the Belle II detector, and usage of advanced machine learning algorithms result in setting a competitive limit even if earlier experiments had roughly 10 times more data at hand. In this talk I will discuss how this first published B-physics result from the Belle II experiment sets a new precedent for precision tests of Standard Model predictions for the B+→K+νν̄ decays. I will also summarize the prospects for future measurements in this and similar decay channels using the growing Belle II dataset.