Study of the hadronic transitions ${\rm Y}(2\mathrm{S})\to (\eta ,{\pi }^0){\rm Y}(1\mathrm{S})$ at Belle

We study the rare hadronic transitions ${\rm Y}(2\mathrm{S})\to {\rm Y}(1\mathrm{S})\eta$ and ${\rm Y}(2\mathrm{S})\to {\rm Y}(1\mathrm{S}){\pi }^0$ using a sample of $158\times {10}^6{\rm Y}(2\mathrm{S})$ decays collected with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider. We measure the ratios of branching fractions $(\mathcal{B})\frac{\mathcal{B}({\rm Y}(2\mathrm{S})\to {\rm Y}(1\mathrm{S})\eta )}{\mathcal{B}({\rm Y}(2\mathrm{S})\to {\rm Y}(1\mathrm{S}){\pi }^{+}{\pi }^{-})}=(1.99\pm 0.14(\mathrm{stat})\pm 0.11(\mathrm{syst}))\times {10}^{-3}$ and $\frac{\mathcal{B}({\rm Y}(2\mathrm{S})\to {\rm Y}(1\mathrm{S}){\pi }^0)}{\mathcal{B}({\rm Y}(2\mathrm{S})\to {\rm Y}(1\mathrm{S}){\pi }^{+}{\pi }^{-})}<2.3\times {10}^{-4}$ at the 90% confidence level (CL). Assuming the value $\mathcal{B}({\rm Y}(2\mathrm{S})\to {\rm Y}(1\mathrm{S}){\pi }^{-}{\pi }^{+})=(17.92\pm 0.26)\%$, we obtain $\mathcal{B}({\rm Y}(2\mathrm{S})\to {\rm Y}(1\mathrm{S})\eta )=(3.57\pm 0.25(\mathrm{stat})\pm 0.21(\mathrm{syst}))\times {10}^{-4}$ and $\mathcal{B}({\rm Y}(2\mathrm{S})\to {\rm Y}(1\mathrm{S}){\pi }^0)<4.1\times {10}^{-5}(90\%\mathrm{CL})$.

Figure 1

Mass distribution of the $\eta$ candidates in $\gamma \gamma {\mu }^{+}{\mu }^{-}$ (top), $\gamma \gamma e^{+}{e}^{-}$ (middle), ${\pi }^{+}{\pi }^{-}{\pi }^0{\mu }^{+}{\mu }^{-}$, and ${\pi }^{+}{\pi }^{-}{\pi }^0{e}^{+}{e}^{-}$ (bottom) final states. The fit function in blue, solid represents the simultaneous fit of the four channels; the red dashed curve shows the best-fit background component.

Figure 2

$\gamma \gamma /{\pi }^{+}{\pi }^{-}{\pi }^0$ invariant mass for ${\rm Y}(2\mathrm{S})\to {\rm Y}(1\mathrm{S})\eta$ candidates, summing all the four final states. The fit function in blue, solid represents the best fit with the red dashed curve showing its the background component.

Figure 3

Final fit to the $\gamma \gamma$ pair invariant mass for ${\rm Y}(2\mathrm{S})\to {\rm Y}(1\mathrm{S}){\pi }^0$ candidates. The fit function in blue, solid represents the best fit; the red dashed curve shows its background component.