Search for the rare decay $B\to K\nu \overline{\nu }$

We present a search for the rare decays $B^{+}\to K^{+}\nu \overline{\nu }$ and $B^0\to K^0\nu \overline{\nu }$ using $459\times {10}^6$ $B\overline{B}$ pairs collected with the BABAR detector at the SLAC National Accelerator Laboratory. Flavor-changing neutral-current decays such as these are forbidden at tree level but can occur through one-loop diagrams in the standard model (SM), with possible contributions from new physics at the same order. The presence of two neutrinos in the final state makes identification of signal events challenging, so reconstruction in the semileptonic decay channels $B\to D^{(*)}l\nu$ of the $B$ meson recoiling from the signal $B$ is used to suppress backgrounds. We set an upper limit at the 90% confidence level (CL) of $1.3\times {10}^{-5}$ on the total branching fraction for $B^{+}\to K^{+}\nu \overline{\nu }$, and $5.6\times {10}^{-5}$ for $B^0\to K^0\nu \overline{\nu }$. We additionally report 90% CL upper limits on partial branching fractions in two ranges of dineutrino mass squared for $B^{+}\to K^{+}\nu \overline{\nu }$.

Figure 1

Lowest-order Feynman diagrams for $B\to K\nu \overline{\nu }$, with the $W$ box on the left and $Z$ penguin on the right.

Figure 2

(a) $K^{+}$ and (b) $K^0$ BDT output for data (diamonds), background MC (solid), and signal MC (dotted) events. For each plot, the scale for the data and background events is on the left axis, and the scale for the signal events is on the right axis. The distribution of signal MC events is normalized to unit area.

Figure 3

BDT selection efficiency in the signal region versus $p_K^{*}$ for (a) $K^{+}$, (b) $K_S^0$, and (c) high-$q^2$ $K^{+}$ simulated signal events, considering only the effect of the BDT output cut.

Figure 4

Averaged BDT signal-region output for (a) $K^{+}$, (b) $K_S^0$, and (c) high-$q^2$ $K^{+}$ data, with expected signal and background contributions. The signal estimate assumes a branching fraction of $3.8\times {10}^{-6}$.

Figure 5

Averaged $p_K^{*}$ signal-region output for (a) $K^{+}$, (b) $K_S^0$, and (c) high-$q^2$ $K^{+}$ data, with expected signal and background contributions. The signal estimate assumes a branching fraction of $3.8\times {10}^{-6}$.

Figure 6

Integrated numbers of observed (red triangles), expected background (black circles), and excess events (blue squares) for data for each BDT: (a) $K^{+}$, (b) $K_S^0$, and (c) high-$q^2$ $K^{+}$. The individual uncertainties are purely statistical and assume no correlation between data sets. The horizontal dashed lines show the sum of the statistical and systematic uncertainties on the mean number of excess events.