Study of $\mathit{{\rm Y}}(3S,2S)\to \eta \mathit{{\rm Y}}(1S)$ and $\mathit{{\rm Y}}(3S,2S)\to {\pi }^{+}{\pi }^{-}\mathit{{\rm Y}}(1S)$ hadronic transitions

We study the $\mathit{{\rm Y}}(3S,2S)\to \eta \mathit{{\rm Y}}(1S)$ and $\mathit{{\rm Y}}(3S,2S)\to {\pi }^{+}{\pi }^{-}\mathit{{\rm Y}}(1S)$ transitions with $122\times {10}^6\mathit{{\rm Y}}(3S)$ and $100\times {10}^6\mathit{{\rm Y}}(2S)$ mesons collected by the BABAR detector at the PEP-II asymmetric-energy $e^{+}{e}^{-}$ collider. We measure $\mathcal{B}[\mathit{{\rm Y}}(2S)\to \eta \mathit{{\rm Y}}(1S)]=(2.39\pm 0.31(\mathrm{stat}.)\pm 0.14(\mathrm{syst}.))\times {10}^{-4}$ and $\Gamma [\mathit{{\rm Y}}(2S)\to \eta \mathit{{\rm Y}}(1S)]/\Gamma [\mathit{{\rm Y}}(2S)\to {\pi }^{+}{\pi }^{-}\mathit{{\rm Y}}(1S)]=(1.35\pm 0.17(\mathrm{stat}.)\pm 0.08(\mathrm{syst}.))\times {10}^{-3}$. We find no evidence for $\mathit{{\rm Y}}(3S)\to \eta \mathit{{\rm Y}}(1S)$ and obtain $\mathcal{B}[\mathit{{\rm Y}}(3S)\to \eta \mathit{{\rm Y}}(1S)]<1.0\times {10}^{-4}$ and $\Gamma [\mathit{{\rm Y}}(3S)\to \eta \mathit{{\rm Y}}(1S)]/\Gamma [\mathit{{\rm Y}}(3S)\to {\pi }^{+}{\pi }^{-}\mathit{{\rm Y}}(1S)]<2.3\times {10}^{-3}$ as upper limits at the 90% confidence level. We also provide improved measurements of the $\mathit{{\rm Y}}(2S)-\mathit{{\rm Y}}(1S)$ and $\mathit{{\rm Y}}(3S)-\mathit{{\rm Y}}(1S)$ mass differences, $562.170\pm 0.007(\mathrm{stat}.)\pm 0.088(\mathrm{syst}.)\mathrm{MeV}/c^2$ and $893.813\pm 0.015(\mathrm{stat}.)\pm 0.107(\mathrm{syst}.)\mathrm{MeV}/c^2$, respectively.

Figure 1

$\Delta M_{\pi \pi }$ and $M_{\ell \ell }$ distributions for (a,b) $\mathit{{\rm Y}}(2S)\to {\pi }^{+}{\pi }^{-}\mathit{{\rm Y}}(1S)\to {\pi }^{+}{\pi }^{-}{\mu }^{+}{\mu }^{-}$ candidates, (c,d) $\mathit{{\rm Y}}(3S)\to {\pi }^{+}{\pi }^{-}\mathit{{\rm Y}}(1S)\to {\pi }^{+}{\pi }^{-}{\mu }^{+}{\mu }^{-}$ candidates, and (e,f) $\mathit{{\rm Y}}(3S)\to {\pi }^{+}{\pi }^{-}\mathit{{\rm Y}}(1S)\to {\pi }^{+}{\pi }^{-}{e}^{+}{e}^{-}$ candidates. Data are represented by dots, the fit results as solid curves, and the background components by the dashed curves.

Figure 2

$\Delta M_{\eta }$ and $M_{\pi \pi \gamma \gamma }$ distributions for (a,b) $\mathit{{\rm Y}}(2S)\to \eta \mathit{{\rm Y}}(1S)\to {\pi }^{+}{\pi }^{-}\gamma \gamma {\mu }^{+}{\mu }^{-}$ candidates, (c,d) $\mathit{{\rm Y}}(3S)\to \eta \mathit{{\rm Y}}(1S)\to {\pi }^{+}{\pi }^{-}\gamma \gamma {\mu }^{+}{\mu }^{-}$ candidates, and (e,f) $\mathit{{\rm Y}}(3S)\to \eta \mathit{{\rm Y}}(1S)\to {\pi }^{+}{\pi }^{-}\gamma \gamma e^{+}{e}^{-}$ candidates. Data are represented by dots, the fit results as solid curves, and the background components by the dashed curves.

Figure 3

$\Delta M_{\eta }^{\prime }$ and $M_{\gamma \gamma }$ distributions for (a,b) $\mathit{{\rm Y}}(2S)\to \eta \mathit{{\rm Y}}(1S)\to \gamma \gamma {\mu }^{+}{\mu }^{-}$ candidates, and (c,d) $\mathit{{\rm Y}}(3S)\to \eta \mathit{{\rm Y}}(1S)\to \gamma \gamma {\mu }^{+}{\mu }^{-}$ candidates. Data are represented by dots, the fit results as solid curves, and the background components by the dashed curves.