Search for Invisible Decays of the $\mathit{{\rm Y}}(1S)$

We search for invisible decays of the $\mathit{{\rm Y}}(1S)$ meson using a sample of $91.4\times {10}^6$ $\mathit{{\rm Y}}(3S)$ mesons collected at the BABAR/PEP-II $B$ factory. We select events containing the decay $\mathit{{\rm Y}}(3S)\to {\pi }^{+}{\pi }^{-}\mathit{{\rm Y}}(1S)$ and search for evidence of an undetectable $\mathit{{\rm Y}}(1S)$ decay recoiling against the dipion system. We set an upper limit on the branching fraction $\mathcal{B}\mathbf{(}\mathit{{\rm Y}}(1S)\to \mathrm{\text{invisible}}\mathbf{)}<3.0\times {10}^{-4}$ at the 90% confidence level.

Figure 1

The maximum likelihood fit to the dipion recoil mass for data in the invisible subsample. The components of the fit are the nonpeaking background (dash-dotted line) and the peaking component (solid filled line). The total fit function is also shown (solid line). Comparing the fit to the binned data, ${\chi }^2/(\mathrm{\text{degree of freedom}})=0.973$.

Figure 2

The distribution of $\cos (\theta )$, where $\theta$ is the laboratory-frame polar angle of (a) the positively charged high-momentum track in the four-track subsample and (b) the single high-momentum track in the three-track subsample. The normalization correction from the four-track subsample has been applied to the MC yields in both cases.