A Search for the Decay $B^{-}\to K^{-} ν\barν$

TL;DR

This paper reports a search for the rare FCNC decay $B^{-}\rightarrow K^{-} \nu \overline{\nu}$ using hadronic tagging of the accompanying $B^+$ to suppress backgrounds from two neutrinos. The BABAR analysis combines a hadronic-tag approach with stringent kinematic and event-shape selections, achieving a preliminary limit of $\mathcal{B}(B^{-}\rightarrow K^{-} \nu \overline{\nu})<1.05\times10^{-4}$ at 90% CL and, when merged with a previous independent BABAR result, a combined limit of $<7.0\times10^{-5}$ at 90% CL. The result remains below the SM expectations and provides a progressively tighter constraint on new physics scenarios that could enhance this FCNC decay. The measurement demonstrates the effectiveness of hadronic tagging in controlling backgrounds for channels with two neutrinos in the final state.

Abstract

We present a search for the rare flavour-changing neutral-current decay $B^{-}\to K^{-} ν\barν$ based on a sample of $(86.9 \pm 1.0) \times 10^{6}$ $Υ(4S) \to $B\bar{B}$ events collected by the BABAR experiment at the SLAC B-factory. Signal candidate events are selected by fully reconstructing a $B^+ \to \bar{D}^{0} X^+$ decay, where $X^+$ represents a combination of up to three charged pions or kaons and up to two $π^0$ candidates. The charged tracks and calorimeter clusters not used in the $B^+$ reconstruction are required to be compatible with a $B^{-}\to K^{-} ν\barν$ decay. We observe a total of three signal candidate events with an expected background of $2.7 \pm 0.8$, resulting in a preliminary limit of $\mathcal{B}(B^{-}\to K^{-} ν\barν) < 1.05 \times 10^{-4}$ at the 90% confidence level. This search is combined with the results of a previous and statistically independent preliminary BABAR search for $B^{-}\to K^{-} ν\barν$ to give a limit of $\mathcal{B}(B^{-}\to K^{-} ν\barν) < 7.0 \times 10^{-5}$ at the 90% confidence level.

Figures (7)

• Figure 1: Electroweak penguin (left) and box (right) Feynman diagrams for the process $\hbox{$b \rightarrow\xspace s \nu \overline{\nu}$}$ predicted by the SM. In both cases the amplitudes are expected to be dominated by the heavy $t$ quark contribution.
• Figure 2: The distribution $\hbox{$m_{\rm ES}$}\xspace$ for $B^+ \rightarrow\xspace \hbox{$\overline{D}^{0}$} X^+$ candidates in the $\hbox{$B^{-}\rightarrow\xspace K^{-} \nu \overline{\nu}$}$ signal MC (top), and in the data (bottom). The bottom plot also shows the expected contributions from continuum and $B\bar{B}$ MC. No signal-side selection cuts have been applied. Signal MC is shown scaled to the data luminosity assuming $\hbox{$\mathcal{B}(\hbox{$B^{-}\rightarrow\xspace K^{-} \nu \overline{\nu}$}) = 4 \times 10^{-6}$}$; inclusive background MC and offpeak data are shown scaled to the onpeak data luminosity of $\hbox{$80.06$ fb$^{-1}$}$.
• Figure 3: The distribution of the number, $\hbox{$N_{\rm tracks}$}$, of signal-side charged tracks is plotted for the $\hbox{$B^{-}\rightarrow\xspace K^{-} \nu \overline{\nu}$}$ signal MC (top), and for onpeak data and generic MC (bottom) for events which pass the tag $B$ selection. No signal-side selection cuts have been applied. Signal MC is shown scaled to the data luminosity assuming $\hbox{$\mathcal{B}(\hbox{$B^{-}\rightarrow\xspace K^{-} \nu \overline{\nu}$}) = 4 \times 10^{-6}$}$, and generic MC and offpeak data are shown scaled to the onpeak data luminosity of $\hbox{$80.06$ fb$^{-1}$}$.
• Figure 4: The CM momentum distribution of signal-side kaons candidates is plotted for the $\hbox{$B^{-}\rightarrow\xspace K^{-} \nu \overline{\nu}$}$ signal MC (top), and for onpeak data and generic MC (bottom). Plotted events are required to possess exactly one signal-side track satisfying kaon identification criteria and having a charge opposite that of the tag $B$. Signal MC is shown scaled to the data luminosity assuming $\hbox{$\mathcal{B}(\hbox{$B^{-}\rightarrow\xspace K^{-} \nu \overline{\nu}$}) = 4 \times 10^{-6}$}$, and generic MC and offpeak data are shown scaled to the onpeak data luminosity of $\hbox{$80.06$ fb$^{-1}$}$.
• Figure 5: The total extra neutral energy distribution, $\hbox{$E_{\rm extra}$}$ is plotted for the $\hbox{$B^{-}\rightarrow\xspace K^{-} \nu \overline{\nu}$}$ signal MC (top), and for onpeak data and generic MC (bottom). Plotted events are required to possess exactly one signal-side track satisfying kaon identification criteria and having a charge opposite that of the tag $B$. Signal MC is shown scaled to the data luminosity assuming $\hbox{$\mathcal{B}(\hbox{$B^{-}\rightarrow\xspace K^{-} \nu \overline{\nu}$}) = 4 \times 10^{-6}$}$, and generic MC and offpeak data are shown scaled to the onpeak data luminosity of $\hbox{$80.06$ fb$^{-1}$}$.