Search for $B\to K^{\mathbf{(}*\mathbf{)}}\nu \overline{\nu }$ and invisible quarkonium decays

We search for the flavor-changing neutral-current decays $B\to K^{(*)}\nu \overline{\nu }$, and the invisible decays $J/\psi \to \nu \overline{\nu }$ and $\psi (2S)\to \nu \overline{\nu }$ via $B\to K^{(*)}J/\psi$ and $B\to K^{(*)}\psi (2S)$, respectively, using a data sample of $471\times {10}^6$ $B\overline{B}$ pairs collected by the BABAR experiment. We fully reconstruct the hadronic decay of one of the $B$ mesons in the ${\rm Y}(4S)\to B\overline{B}$ decay, and search for the $B\to K^{(*)}\nu \overline{\nu }$ decay in the rest of the event. We observe no significant excess of signal decays over background and report branching fraction upper limits of $\mathcal{B}(B^{+}\to K^{+}\nu \overline{\nu })<3.7\times {10}^{-5}$, $\mathcal{B}(B^0\to K^0\nu \overline{\nu })<8.1\times {10}^{-5}$, $\mathcal{B}(B^{+}\to K^{*+}\nu \overline{\nu })<11.6\times {10}^{-5}$, $\mathcal{B}(B^0\to K^{*0}\nu \overline{\nu })<9.3\times {10}^{-5}$, and combined upper limits of $\mathcal{B}(B\to K\nu \overline{\nu })<3.2\times {10}^{-5}$ and $\mathcal{B}(B\to K^{*}\nu \overline{\nu })<7.9\times {10}^{-5}$, all at the 90% confidence level. For the invisible quarkonium decays, we report branching fraction upper limits of $\mathcal{B}(J/\psi \to \nu \overline{\nu })<3.9\times {10}^{-3}$ and $\mathcal{B}(\psi (2S)\to \nu \overline{\nu })<15.5\times {10}^{-3}$ at the 90% confidence level. Using the improved kinematic resolution achieved from hadronic reconstruction, we also provide partial branching fraction limits for the $B\to K^{(*)}\nu \overline{\nu }$ decays over the full kinematic spectrum.

Figure 1

Lowest-order SM Feynman diagrams for $b\to s\nu \overline{\nu }$ transitions. The virtual top quark provides the dominant contribution in each case.

Figure 2

Lowest-order Feynman diagrams of (from left to right) the SM decay $c\overline{c}\to \nu \overline{\nu }$, the SUSY decay $c\overline{c}$ into a pair of goldstinos ($\tilde{g}$) via a $c$-squark in the t-channel, and the SUSY decay $c\overline{c}\to \tilde{g}\overline{\tilde{g}}$ via a virtual $Z^0$ in the s-channel.

Figure 3

The$m_{\mathrm{ES}}$ distribution for the $B_{\mathrm{tag}}$ candidates in data (points) and in the expected combinatorial background as predicted by the MC (shaded). This distribution includes only the charged and neutral $B_{\mathrm{tag}}$ candidates that pass the purity restrictions, the multivariate continuum suppression, and a requirement of one to three signal-side tracks. The data within the $m_{\mathrm{ES}}$ sideband region is used to extrapolate the expected number of combinatorial background events within the signal region.

Figure 4

The $E_{\mathrm{extra}}$ distribution over the full $s_B$ spectrum in the (from top to bottom) $K^{+}$, $K^0$, $K^{*+}$, and $K^{*0}$ channels after applying all other signal selection criteria. The expected combinatorial (shaded) plus $m_{\mathrm{ES}}$-peaking (solid) background contributions are overlaid on the data (points). The $B\to K^{(*)}\nu \overline{\nu }$ signal MC distributions (dashed) have arbitrary normalization. Both the $B\to K^{(*)}\nu \overline{\nu }$ and $c\overline{c}\to \nu \overline{\nu }$ searches select events to the left of the vertical line that corresponds to the $E_i$ value of that channel, as given in Table 1.

Figure 5

The $s_B$ distribution for (from top to bottom) $B^{+}\to K^{+}\nu \overline{\nu }$, $B^0\to K^0\nu \overline{\nu }$, $B^{+}\to K^{*+}\nu \overline{\nu }$, and $B^0\to K^{*0}\nu \overline{\nu }$ events after applying the full signal selection. The expected combinatorial (shaded) plus $m_{\mathrm{ES}}$-peaking (solid) background contributions are overlaid on the data (points). The signal MC distributions (dashed) are normalized to branching fractions of $20\times {10}^{-5}$ for $B^{+}\to K^{+}\nu \overline{\nu }$ and $50\times {10}^{-5}$ for the other channels. Events to the left of the vertical lines are selected to obtain SM-sensitive limits, while the full spectra are used to determine partial branching fractions.

Figure 6

The central values (points with $1\sigma$ error bars) of the partial branching fractions $\Delta {\mathcal{B}}_i$ versus $s_B$, for (a) $B^{+}\to K^{+}\nu \overline{\nu }$, (b) $B^0\to K^0\nu \overline{\nu }$, (c) $B^{+}\to K^{*+}\nu \overline{\nu }$, and (d) $B^0\to K^{*0}\nu \overline{\nu }$. The subplots show the distribution of the final signal efficiencies within each $s_B$ interval (histogram with error bars) and over the full $s_B$ spectra (dotted line). The partial branching fractions are provided only within the intervals that are unaffected by the kinematic limit at large $s_B$.

Figure 7

The constraints at 90% CL on $ϵ$ and $\eta$ of Eq. (6) for sensitivity to new physics with right-handed currents. The $B\to K\nu \overline{\nu }$ (diagonal shading) and $B\to K^{*}\nu \overline{\nu }$ (grey shading) excluded areas are determined from the upper and lower limits of this $B\to K^{(*)}\nu \overline{\nu }$ analysis (solid curves) and from the most-stringent upper limits from previous semileptonic-tag analyses [15, 16] (dashed curves). The dot shows the expected SM value.

Figure 8

The $m_{\nu \overline{\nu }}\equiv \sqrt{s_B{m}_B^2}$ distribution for (from top to bottom) $B^{+}\to K^{+}c\overline{c}$, $B^0\to K^{0}c\overline{c}$, $B^{+}\to K^{*+}c\overline{c}$, and $B^0\to K^{*0}c\overline{c}$ events after applying the full signal selection. The expected combinatorial (shaded) plus $m_{\mathrm{ES}}$-peaking (solid) background contributions are overlaid on the data (points). The signal MC distributions (dashed) are normalized to $\mathcal{B}(c\overline{c}\to \nu \overline{\nu })$ values of 2% for the $K^{+}$ channel, 10% for the $K^0$ channel, and 5% for the $K^{*}$ channels.