$B^+\to K^+\nu\bar\nu$ and other recent Belle II results

by Eldar Ganiev (DESY)

Monday 25 Nov 2024, 11:00 → 12:00 Europe/Zurich

BSP 626 / Zoom

In this seminar, we will begin with an overview of selected recent physics results from the Belle II experiment at the SuperKEKB collider in Japan. Following this, we will delve into our measurement of the decay $B^+\to K^+\nu\bar\nu$. This process occurs via a flavor-changing neutral current. According to the Standard Model, the expected branching fraction is approximately $5×10^{−6}$, while potential enhancements are anticipated in scenarios involving New Physics.

We search for this rare decay in a sample of electron-positron collisions collected at the $\Upsilon(4S)$ resonance and corresponding to 362 fb$^{-1}$ of integrated luminosity. We use the inclusive properties of the accompanying $B$ meson in $\Upsilon(4S) \to B\bar{B}$ events to suppress background from other decays of the signal \B candidate and light-quark pair production.

We validate the measurement with an auxiliary analysis based on a conventional hadronic reconstruction of the accompanying $B$ meson. Our main and auxiliary analyses yield consistent results. Combining them, we determine the branching fraction of the decay $B^+\to K^+\nu\bar\nu$ to be $[2.3 \pm 0.5({\rm stat})^{+0.5}_{-0.4}({\rm syst})]\times 10^{-5}$, providing the first evidence for this decay, with a significance of 3.5 standard deviations.

BelleII@EPFL.pdf