Study of hadronic transitions between Υ states and observation of Υ(4S)→ηΥ(1S) decay

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We present a study of hadronic transitions between $\mathit{\ensuremath{\Upsilon}}(mS)$ ($m=4$, 3, 2) and $\mathit{\ensuremath{\Upsilon}}(nS)$ ($n=2$, 1) resonances based on $347.5\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ of data taken with the BABAR detector at the PEP-II storage rings. We report the first observation of $\mathit{\ensuremath{\Upsilon}}(4S)\ensuremath{\rightarrow}\ensuremath{\eta}\mathit{\ensuremath{\Upsilon}}(1S)$ decay with a branching fraction $\mathcal{B}(\mathit{\ensuremath{\Upsilon}}(4S)\ensuremath{\rightarrow}\ensuremath{\eta}\mathit{\ensuremath{\Upsilon}}(1S))=(1.96\ifmmode\pm\else\textpm\fi{}{0.06}_{\mathrm{stat}}\ifmmode\pm\else\textpm\fi{}{0.09}_{\mathrm{syst}})\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ and measure the ratio of partial widths $\ensuremath{\Gamma}(\mathit{\ensuremath{\Upsilon}}(4S)\ensuremath{\rightarrow}\ensuremath{\eta}\mathit{\ensuremath{\Upsilon}}(1S))/\ensuremath{\Gamma}(\mathit{\ensuremath{\Upsilon}}(4S)\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}\mathit{\ensuremath{\Upsilon}}(1S))=2.41\ifmmode\pm\else\textpm\fi{}{0.40}_{\mathrm{stat}}\ifmmode\pm\else\textpm\fi{}{0.12}_{\mathrm{syst}}$. We set 90% CL upper limits on the ratios $\ensuremath{\Gamma}(\mathit{\ensuremath{\Upsilon}}(2S)\ensuremath{\rightarrow}\ensuremath{\eta}\mathit{\ensuremath{\Upsilon}}(1S))/\ensuremath{\Gamma}(\mathit{\ensuremath{\Upsilon}}(2S)\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}\mathit{\ensuremath{\Upsilon}}(1S))<5.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ and $\ensuremath{\Gamma}(\mathit{\ensuremath{\Upsilon}}(3S)\ensuremath{\rightarrow}\ensuremath{\eta}\mathit{\ensuremath{\Upsilon}}(1S))/\ensuremath{\Gamma}(\mathit{\ensuremath{\Upsilon}}(3S)\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}\mathit{\ensuremath{\Upsilon}}(1S))<1.9\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}$. We also present new measurements of the ratios $\ensuremath{\Gamma}(\mathit{\ensuremath{\Upsilon}}(4S)\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}\mathit{\ensuremath{\Upsilon}}(2S))/\ensuremath{\Gamma}(\mathit{\ensuremath{\Upsilon}}(4S)\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}\mathit{\ensuremath{\Upsilon}}(1S))=1.16\ifmmode\pm\else\textpm\fi{}{0.16}_{\mathrm{stat}}\ifmmode\pm\else\textpm\fi{}{0.14}_{\mathrm{syst}}$ and $\ensuremath{\Gamma}(\mathit{\ensuremath{\Upsilon}}(3S)\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}\mathit{\ensuremath{\Upsilon}}(2S))/\ensuremath{\Gamma}(\mathit{\ensuremath{\Upsilon}}(3S)\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}\mathit{\ensuremath{\Upsilon}}(1S))=0.577\ifmmode\pm\else\textpm\fi{}{0.026}_{\mathrm{stat}}\ifmmode\pm\else\textpm\fi{}{0.060}_{\mathrm{syst}}$.

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