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

physical review letters

Observation of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>Υ</mml:mi><mml:mo stretchy="false">(</mml:mo><mml:mn>4</mml:mn><mml:mi>S</mml:mi><mml:mo stretchy="false">)</mml:mo></mml:math>Decays to<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msup><mml:mi>π</mml:mi><mml:mo>+</mml:mo></mml:msup><mml:msup><mml:mi>π</mml:mi><mml:mo>−</mml:mo></mml:msup><mml:mi>Υ</mml:mi><mml:mo stretchy="false">(</mml:mo><mml:mn>1</mml:mn><mml:mi>…

Observation ofΥ(4S)Decays toπ+π−Υ(1…