The $\chi(b$) States in Exclusive Radiative Decay of the $\Upsilon$ (2s)

The Crystal Ball detector at the DESY storage ring DORIS II has been used to study radiative decays of the \ensuremath{\Upsilon}(2S) resonance. We report on the analysis of the exclusive channel \ensuremath{\Upsilon}(2S)\ensuremath{\rightarrow}\ensuremath{\gamma}${\ensuremath{\chi}}_{b}$, ${\ensuremath{\chi}}_{b}$\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\Upsilon}(1S)\ensuremath{\rightarrow}${e}^{+}$${e}^{\mathrm{\ensuremath{-}}}$ or ${\ensuremath{\mu}}^{+}$${\ensuremath{\mu}}^{\mathrm{\ensuremath{-}}}$. We detect two ${\ensuremath{\chi}}_{b}$ states by observing two monochromatic photon lines at energies (107.0\ifmmode\pm\else\textpm\fi{}1.1\ifmmode\pm\else\textpm\fi{}1.3) MeV and (131.7\ifmmode\pm\else\textpm\fi{}0.9\ifmmode\pm\else\textpm\fi{}1.3) MeV, respectively. The product branching ratios B[\ensuremath{\Upsilon}(2S)\ensuremath{\rightarrow}\ensuremath{\gamma}${\ensuremath{\chi}}_{b}$]\ifmmode\times\else\texttimes\fi{}B[${\ensuremath{\chi}}_{b}$\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\Upsilon}(1S) ] are (1.6\ifmmode\pm\else\textpm\fi{}0.3\ifmmode\pm\else\textpm\fi{}0.2)% for the first state and (2.1\ifmmode\pm\else\textpm\fi{}0.3\ifmmode\pm\else\textpm\fi{}0.3)% for the second. For the product branching ratio of the third ${\ensuremath{\chi}}_{b}$ state, which has been observed in inclusive measurements, we find an upper limit of 0.2% (90% C.L.). Combining our results with inclusive measurements, branching ratios for ${\ensuremath{\chi}}_{b}$\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\Upsilon}(1S) are derived. Using theoretical estimates for the radiative widths of the ${\ensuremath{\chi}}_{b}$ states, we determine their hadronic widths; the results are compared with QCD predictions.