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Using a data sample of $\Upsilon(2S)$ events collected with the CLEO IIdetector at the Cornell Electron Storage Ring, we have investigated thehadronic transitions between the $\Upsilon(2S)$ and the $\Upsilon(1S)$. Thecharged dipion transition $\Upsilon(2S)\to \Upsilon(1S)\pi^+\pi^-$ was studiedusing two different analysis techniques. Selecting events in which$\Upsilon(1S)$ decays into a $\mu\mu$ or $ee$ lepton pair(``exclusive''analysis), we measured the branching fraction of B=0.189+-0.004+-0.009, whileusing a method allowing $\Upsilon(1S)$ to decay into anything (``inclusive''analysis) we found B=0.0196+-0.002+-0.010. The appropriate average of the twomeasurements gives B=0.0192+-0.002+-0.010. Combining the exclusive andinclusive results we derive the $\Upsilon(1S)$ leptonic branching ratiosB_{ee}=0.0229+-0.0008+-0.0011 and B_{\mu\mu}=0.0249+-0.0008+-0.0013 We alsostudied $\Upsilon(2S)\to \Upsilon(1S)\pi^0\pi^0$ and obtained the branchingfraction of B=0.092+-0.006+-0.007. Parameters of the $\pi\pi$ system (dipioninvariant mass spectra, angular distributions) were analyzed and found to beconsistent with current theoretical models. Lastly, we searched for the $\eta$and single $\pi^0$ transitions and obtained the upper limits of 0.0028 and0.0011 respectively.

physical review. d. particles, fields, gravitation, and cosmology/physical review. d. particles and fields

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Hadronic transitionsΥ(2S)→Υ(1S)