The Hadronic Transitions $Upsilon(2S)\to Upsilon(1S)$

Abstract

Using a data sample of ${\rm Y}(2S)$ events collected with the CLEO II detector at the Cornell Electron Storage Ring, we have investigated the hadronic transitions between the ${\rm Y}(2S)$ and the ${\rm Y}(1S)$. The charged dipion transition ${\rm Y}(2S)\to {\rm Y}(1S){\pi }^{+}{\pi }^{-}$ was studied using two different analysis techniques. Selecting events in which ${\rm Y}(1S)$ decays into a $\mu \mu$ or $ee$ lepton pair(``exclusive'' analysis), we measured the branching fraction of B=0.189+-0.004+-0.009, while using a method allowing ${\rm Y}(1S)$ to decay into anything (``inclusive'' analysis) we found B=0.0196+-0.002+-0.010. The appropriate average of the two measurements gives B=0.0192+-0.002+-0.010. Combining the exclusive and inclusive results we derive the ${\rm Y}(1S)$ leptonic branching ratios B_{ee}=0.0229+-0.0008+-0.0011 and B_{μμ}=0.0249+-0.0008+-0.0013 We also studied ${\rm Y}(2S)\to {\rm Y}(1S){\pi }^0{\pi }^0$ and obtained the branching fraction of B=0.092+-0.006+-0.007. Parameters of the $\pi \pi$ system (dipion invariant mass spectra, angular distributions) were analyzed and found to be consistent with current theoretical models. Lastly, we searched for the $\eta$ and single ${\pi }^0$ transitions and obtained the upper limits of 0.0028 and 0.0011 respectively.

29 page postscript file, postscript file also available through http://w4.lns.cornell.edu/public/CLNS

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