B+ -> K+ nu nubar and other highlights from Belle II

About

In this seminar, we will first survey selected first physics highlights from the Belle II experiment at the SuperKEKB collider in Japan, before going in depth on our recent measurement of $B^+\rightarrow K^+ \nu\bar{\nu}$. The decay $B^+\rightarrow K^+ \nu\bar{\nu}$ is mediated by a flavor-changing neutral current. In the Standard Model, the predicted branching fraction is about $6\times 10^{-5}$, while enhancements are foreseen in New Physics scenarios.

We search for this rare decay in a 362 fb$^{-1}$ sample of electron-positron collisions at the $\Upsilon(4S)$ resonance. We use the inclusive properties of the accompanying B meson in $\Upsilon(4S)\rightarrow B\bar{B}$ 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 inclusive and hadronic tagged analyses yield consistent results. Combining them, we determine the branching fraction of the decay $B^+\rightarrow K^+ \nu\bar{\nu}$ to be $2.4 \pm 0.5(stat)^{+0.5}_{−0.4}(syst) \times 10^{−5}$, providing the first evidence for this decay, with a significance of 3.6 standard deviations.