Search for Production of Invisible Final States in Single-Photon Decays of $\mathit{{\rm Y}}(1S)$

We search for single-photon decays of the $\mathit{{\rm Y}}(1S)$ resonance, $\mathit{{\rm Y}}\to \gamma +\mathrm{\text{invisible}}$, where the invisible state is either a particle of definite mass, such as a light Higgs boson $A^0$, or a pair of dark matter particles, $\chi \overline{\chi }$. Both $A^0$ and $\chi$ are assumed to have zero spin. We tag $\mathit{{\rm Y}}(1S)$ decays with a dipion transition $\mathit{{\rm Y}}(2S)\to {\pi }^{+}{\pi }^{-}\mathit{{\rm Y}}(1S)$ and look for events with a single energetic photon and significant missing energy. We find no evidence for such processes in the mass range $m_{A^0}\le 9.2\mathrm{GeV}$ and $m_{\chi }\le 4.5\mathrm{GeV}$ in the sample of $98\times {10}^6$ $\mathit{{\rm Y}}(2S)$ decays collected with the BABAR detector and set stringent limits on new physics models that contain light dark matter states.

Figure 1

Projection plots from the fit with $N_{\mathrm{sig}}=0$ onto (a,c,e) $M_{\mathrm{recoil}}$ and (b,d,f) $M_X^2$. (a,b) Low-mass region with IFR veto, (c,d) low-mass region without IFR veto, (e,f) high-mass region. Overlaid is the fit with $N_{\mathrm{sig}}=0$ (solid blue line), continuum background (black dashed line), radiative leptonic $\mathit{{\rm Y}}(1S)$ decays (green dash-dotted line), and (c,d) radiative hadronic $\mathit{{\rm Y}}(1S)$ decays or (e,f) ${\eta }^{\prime }$ background (magenta dotted line).

Figure 2

Ninety percent C.L. upper limits for $\mathcal{B}\mathbf{(}\mathit{{\rm Y}}(1S)\to \gamma A^0\mathbf{)}\times \mathcal{B}(A^0\to \mathrm{\text{invisible}})$.

Figure 3

Ninety percent C.L. upper limits for $\mathcal{B}\mathbf{(}\mathit{{\rm Y}}(1S)\to \gamma \chi \overline{\chi }\mathbf{)}$.