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

We report results of a search for the invisible decay of the ${\rm Y}(1S)$ via the ${\rm Y}(3S)\to {\pi }^{+}{\pi }^{-}{\rm Y}(1S)$ transition using a data sample of $2.9{\mathrm{fb}}^{-1}$ at the ${\rm Y}(3S)$ resonance. The data were collected with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider. No signal is found, and an upper limit for the branching fraction at the 90% confidence level is determined to be $\mathcal{B}\mathbf{(}{\rm Y}(1S)\to \mathrm{\text{invisible}}\mathbf{)}<2.5\times {10}^{-3}$.

Figure 1

Mass difference $\Delta M=M_{{\mu }^{+}{\mu }^{-}{\pi }^{+}{\pi }^{-}}-M_{{\mu }^{+}{\mu }^{-}}$ distribution where $M_{{\mu }^{+}{\mu }^{-}}$ lies in the ${\rm Y}(1S)$ mass region. The solid curve shows the fits to signal plus background distribution.

Figure 2

Invariant mass distribution of the two cascade pions for the ${\rm Y}(3S)\to {\pi }^{+}{\pi }^{-}{\rm Y}(1S)$, ${\rm Y}(1S)\to {\mu }^{+}{\mu }^{-}$ cascade decay candidates. The open histogram shows the same distribution for MC calculations events.

Figure 3

$M_{{\pi }^{+}{\pi }^{-}}^{\mathrm{recoil}}$ distribution for the control sample ${\rm Y}(3S)\to {\pi }^{+}{\pi }^{-}{\rm Y}(1S)$, ${\rm Y}(1S)\to {\mu }^{+}{\mu }^{-}$ decay candidates. The solid curve shows the fit results.

Figure 4

Recoil mass distribution against two pions, $M_{{\pi }^{+}{\pi }^{-}}^{\mathrm{recoil}}$. The solid curve shows the result of the fit to signal plus background distributions, the shaded area shows the total background contribution, the dashed line shows the combinatorial background contribution, and the dot-dashed line shows the expected signal for $\mathcal{B}\mathbf{(}{\rm Y}(1S)\to \chi \chi \mathbf{)}=6\times {10}^{-3}$.