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

Abstract

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

arXiv comment6 pages, 6 figures, submitted to PRD(RC)

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• Experimental review of the ${\rm Y}(1S,2S,3S)$ physics at $e^{+}{e}^{-}$ colliders and the LHC