Search for $B^{+}\to K^{+}\nu \overline{\nu }$ Decays Using an Inclusive Tagging Method at Belle II

A search for the flavor-changing neutral-current decay $B^{+}\to K^{+}\nu \overline{\nu }$ is performed at the Belle II experiment at the SuperKEKB asymmetric energy electron-positron collider. The data sample corresponds to an integrated luminosity of $63{\mathrm{fb}}^{-1}$ collected at the $\mathrm{\Upsilon }(4S)$ resonance and a sample of $9{\mathrm{fb}}^{-1}$ collected at an energy 60 MeV below the resonance. Because the measurable decay signature involves only a single charged kaon, a novel measurement approach is used that exploits not only the properties of the $B^{+}\to K^{+}\nu \overline{\nu }$ decay, but also the inclusive properties of the other $B$ meson in the $\mathrm{\Upsilon }(4S)\to B\overline{B}$ event, to suppress the background from other $B$ meson decays and light-quark pair production. This inclusive tagging approach offers a higher signal efficiency compared to previous searches. No significant signal is observed. An upper limit on the branching fraction of $B^{+}\to K^{+}\nu \overline{\nu }$ of $4.1\times {10}^{-5}$ is set at the 90% confidence level.

Figure 1

The lowest-order quark-level diagrams for the $b\to s\nu \overline{\nu }$ transition in the SM are either of the penguin (a) or the box (b) type.

Figure 2

Distribution of the classifier output ${\mathrm{BDT}}_1$ (main figure) and ${\mathrm{BDT}}_2$ for ${\mathrm{BDT}}_1>0.9$ (inset). The distributions are shown before (${J/\psi }_{\to {\mu }^{+}{\mu }^{-}}$) and after (${J/\psi }_{\to {\cancel{\mu }}^{+}{\cancel{\mu }}^{-}}$) the muon removal and update of the kaon-candidate momentum of selected $B^{+}\to K^{+}J/\psi$ events in simulation (MC) and data. As a reference, the classifier outputs directly obtained from simulated $B^{+}\to K^{+}\nu \overline{\nu }$ signal events are overlaid. The simulation histograms are scaled to the total number of $B^{+}\to K^{+}J/\psi$ events selected in data.

Figure 3

Yields in on-resonance data and as predicted by the simultaneous fit to the on- and off-resonance data, corresponding to an integrated luminosity of 63 and $9{\mathrm{fb}}^{-1}$, respectively. The predicted yields are shown individually for charged and neutral $B$ meson decays and the sum of the five continuum categories. The leftmost three bins belong to CR1 with ${\mathrm{BDT}}_2\in [0.93,0.95]$ and the other nine bins correspond to the SR, three for each range of ${\mathrm{BDT}}_2\in [0.95,0.97,0.99,1.0]$. Each set of three bins is defined by $p_T(K^{+})\in [0.5,2.0,2.4,3.5]\mathrm{GeV}/c$. All yields in the rightmost three bins are scaled by a factor of 2.