Observation of ${\rm Y}(4S)decaysto$π^+pi^-Υ(1S)$and$π^+pi^-Υ(2S)

Abstract

We present the first measurement of ${\rm Y}(4S)$ decays to ${\pi }^{+}p{i}^{-}{\rm Y}(1S)$ and ${\pi }^{+}p{i}^{-}{\rm Y}(2S)$ based on a sample of 230$\times 10^6$ ${\rm Y}(4S)$ mesons collected with the BABAR detector. We measure the product branching fractions $\mathcal{B}({\rm Y}(4S)\to {\pi }^{+}p{i}^{-}{\rm Y}(1S))\times \mathcal{B}({\rm Y}(1S)\to {\mu }^{+}{\mu }^{-})=(2.23\pm 0.25_{stat}\pm 0.27_{sys})\times 10^{-6}$ and $\mathcal{B}({\rm Y}(4S)\to {\pi }^{+}p{i}^{-}{\rm Y}(2S))\times \mathcal{B}({\rm Y}(2S)\to {\mu }^{+}{\mu }^{-})=(1.69\pm 0.26_{stat}\pm 0.20_{sys})\times 10^{-6}$, from which we derive the partial widths $\Gamma ({\rm Y}(4S)\to {\pi }^{+}p{i}^{-}{\rm Y}(1S))=(1.8\pm 0.4)$~keV and $\Gamma ({\rm Y}(4S)\to {\pi }^{+}p{i}^{-}{\rm Y}(2S))=(2.7\pm 0.8)$~keV.

7 pages,, 3 figures,submitted to Physical Review Letters

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