Hadronic transitionsΥ(2S)→Υ(1S)

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Abstract

Using a $73.6{\mathrm{pb}}^{\mathrm{\ensuremath{-}}1}$ data sample of $\ensuremath{\Upsilon}(2S)$ events collected with the CLEO II detector at the Cornell Electron Storage Ring, we have investigated the hadronic transitions between the $\ensuremath{\Upsilon}(2S)$ and the $\ensuremath{\Upsilon}(1S).$ The dipion transition $\ensuremath{\Upsilon}(2S)\ensuremath{\rightarrow}\ensuremath{\Upsilon}(1S){\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$ was studied using two different analysis techniques. Selecting events in which $\ensuremath{\Upsilon}(1S)\ensuremath{\rightarrow}{e}^{+}{e}^{\ensuremath{-}},{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$ (``exclusive'' analysis), and using the $\ensuremath{\Upsilon}(1S)$ leptonic branching fractions world averages from the PDG review, we obtained $\mathcal{B}(\ensuremath{\Upsilon}(2S)\ensuremath{\rightarrow}\ensuremath{\Upsilon}(1S){\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}})=0.189\ifmmode\pm\else\textpm\fi{}0.004\ifmmode\pm\else\textpm\fi{}0.010,$ while using a method allowing $\ensuremath{\Upsilon}(1S)\ensuremath{\rightarrow}\mathrm{anything}$ (``inclusive'' analysis) we obtained $\mathcal{B}(\ensuremath{\Upsilon}(2S)\ensuremath{\rightarrow}\ensuremath{\Upsilon}(1S){\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}})=0.196\ifmmode\pm\else\textpm\fi{}0.002\ifmmode\pm\else\textpm\fi{}0.010.$ The appropriate weighted average of the two measurements gives $\mathcal{B}(\ensuremath{\Upsilon}(2S)\ensuremath{\rightarrow}\ensuremath{\Upsilon}(1S){\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}})=0.192\ifmmode\pm\else\textpm\fi{}0.002\ifmmode\pm\else\textpm\fi{}0.010.$ Combining the exclusive and inclusive results we derive the $\ensuremath{\Upsilon}(1S)$ leptonic branching fractions ${\mathcal{B}}_{\mathrm{ee}}=0.0229\ifmmode\pm\else\textpm\fi{}0.0008\ifmmode\pm\else\textpm\fi{}0.0011$ and ${\mathcal{B}}_{\ensuremath{\mu}\ensuremath{\mu}}=0.0249\ifmmode\pm\else\textpm\fi{}0.0008\ifmmode\pm\else\textpm\fi{}0.0013.$ We also studied $\ensuremath{\Upsilon}(2S)\ensuremath{\rightarrow}\ensuremath{\Upsilon}(1S){\ensuremath{\pi}}^{0}{\ensuremath{\pi}}^{0}$ and obtained $\mathcal{B}(\ensuremath{\Upsilon}(2S)\ensuremath{\rightarrow}\ensuremath{\Upsilon}(1S){\ensuremath{\pi}}^{0}{\ensuremath{\pi}}^{0})=0.092\ifmmode\pm\else\textpm\fi{}0.006\ifmmode\pm\else\textpm\fi{}0.008.$ Parameters of the \ensuremath{\pi}\ensuremath{\pi} system (dipion invariant mass spectra, angular distributions) were analyzed and found to be consistent with current theoretical models. Lastly, we searched for the \ensuremath{\eta} and single ${\ensuremath{\pi}}^{0}$ transitions and obtained the 90% confidence level upper limits $\mathcal{B}(\ensuremath{\Upsilon}(2S)\ensuremath{\rightarrow}\ensuremath{\Upsilon}(1S)\ensuremath{\eta})<0.0028$ and $\mathcal{B}(\ensuremath{\Upsilon}(2S)\ensuremath{\rightarrow}\ensuremath{\Upsilon}(1S){\ensuremath{\pi}}^{0})<0.0011.$

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