Observation of $Υ(4S) decays to$π^+pi^-Υ(1S)$and$π^+pi^-Υ(2S)

We present the first measurement of $Υ(4S)$ decays to $π^+pi^-Υ(1S)$ and $π^+pi^-Υ(2S)$ based on a sample of 230$\times10^6$ $Υ(4S)$ mesons collected with the BABAR detector. We measure the product branching fractions ${\cal B}(Υ(4S)\to π^+pi^-Υ(1S))\times{\cal B}(Υ(1S)\toμ^+μ^-)=(2.23\pm0.25_{stat} \pm0.27_{sys})\times 10^{-6}$ and ${\cal B}(Υ(4S)\to π^+pi^-Υ(2S))\times{\cal B}(Υ(2S)\toμ^+μ^-)=(1.69\pm0.26_{stat}\pm0.20_{sys})\times 10^{-6}$, from which we derive the partial widths $Γ(Υ(4S)\to π^+pi^-Υ(1S))=(1.8\pm0.4)$~keV and $Γ(Υ(4S)\to π^+pi^-Υ(2S))=(2.7\pm0.8)$~keV.

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