Bb¯spectroscopy from theϒ(3S)state

Abstract

Using the CUSB-II detector at CESR we have observed electric dipole transitions between the $\ensuremath{\Upsilon}$ and ${\ensuremath{\chi}}_{b}$ states, both in exclusive and inclusive channels. We have measured their branching ratios and find $B(\ensuremath{\Upsilon}(3S)\ensuremath{\rightarrow}{\ensuremath{\chi}}_{b}(2{P}_{2,1,0})\ensuremath{\gamma})=(11.1\ifmmode\pm\else\textpm\fi{}0.5\ifmmode\pm\else\textpm\fi{}0.4)%,(11.5\ifmmode\pm\else\textpm\fi{}0.5\ifmmode\pm\else\textpm\fi{}0.5)%,(6.0\ifmmode\pm\else\textpm\fi{}0.4\ifmmode\pm\else\textpm\fi{}0.6)%$. We have measured the center of gravity of the ${\ensuremath{\chi}}_{b}(2P)$ states to be (10259.5\ifmmode\pm\else\textpm\fi{}0.4\ifmmode\pm\else\textpm\fi{}1.0) MeV and their fine-structure mass splittings to be (13.5\ifmmode\pm\else\textpm\fi{}0.4\ifmmode\pm\else\textpm\fi{}0.5) MeV between the $J=2$ and $J=1$ states and (23.5\ifmmode\pm\else\textpm\fi{}0.7\ifmmode\pm\else\textpm\fi{}0.7) MeV between $J=1$ and $J=0$, leading to a fine-structure ratio of 0.574\ifmmode\pm\else\textpm\fi{}0.024\ifmmode\pm\else\textpm\fi{}0.02. The measured fine-structure splitting implies that the spin-orbit interaction dominates over the tensor interaction and that the long-range confining potential is due to an effective scalar exchange. From the measured branching ratios we infer the hadronic widths of the ${\ensuremath{\chi}}_{b}(2P)$ states and find them to be consistent with QCD predictions. We use them to derive values of ${\ensuremath{\alpha}}_{s}$. We have observed the suppressed transition $\ensuremath{\Upsilon}(3S)\ensuremath{\rightarrow}{\ensuremath{\chi}}_{b}(1P)\ensuremath{\gamma}$ and the ${\ensuremath{\pi}}^{0}{\ensuremath{\pi}}^{0}$ transitions from the $\ensuremath{\Upsilon}(3S)$. We find $B(\ensuremath{\Upsilon}(3S)\ensuremath{\rightarrow}\ensuremath{\Upsilon}(1S){\ensuremath{\pi}}^{0}{\ensuremath{\pi}}^{0})=(2.2\ifmmode\pm\else\textpm\fi{}0.4\ifmmode\pm\else\textpm\fi{}0.3)%$ and $B(\ensuremath{\Upsilon}(3S)\ensuremath{\rightarrow}\ensuremath{\Upsilon}(2S){\ensuremath{\pi}}^{0}{\ensuremath{\pi}}^{0})=(1.7\ifmmode\pm\else\textpm\fi{}0.5\ifmmode\pm\else\textpm\fi{}0.2)%$. We also present a measurement of the dipion invariantmass spectrum from the transition $\ensuremath{\Upsilon}(3S)\ensuremath{\rightarrow}\ensuremath{\Upsilon}(1S){\ensuremath{\pi}}^{0}{\ensuremath{\pi}}^{0}$.