First observation of aΥ(1D)state

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Abstract

We present the first evidence for the production of $\ensuremath{\Upsilon}(1D)$ states in the four-photon cascade, $\ensuremath{\Upsilon}(3S)\ensuremath{\rightarrow}\ensuremath{\gamma}{\ensuremath{\chi}}_{b}(2P),$ ${\ensuremath{\chi}}_{b}(2P)\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\Upsilon}(1D),$ $\ensuremath{\Upsilon}(1D)\ensuremath{\rightarrow}\ensuremath{\gamma}{\ensuremath{\chi}}_{b}(1P),$ ${\ensuremath{\chi}}_{b}(1P)\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\Upsilon}(1S),$ followed by the $\ensuremath{\Upsilon}(1S)$ annihilation into ${e}^{+}{e}^{\ensuremath{-}}$ or ${\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}.$ The signal has a significance of 10.2 standard deviations. The measured product branching ratio for these five decays, $(2.5\ifmmode\pm\else\textpm\fi{}0.5\ifmmode\pm\else\textpm\fi{}0.5)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5},$ is consistent with the theoretical estimates. The data are dominated by the production of one $\ensuremath{\Upsilon}(1D)$ state consistent with the $J=2$ assignment. Its mass is determined to be $(10161.1\ifmmode\pm\else\textpm\fi{}0.6\ifmmode\pm\else\textpm\fi{}1.6)$ MeV, which is consistent with the predictions from potential models and lattice QCD calculations. We also searched for $\ensuremath{\Upsilon}(3S)\ensuremath{\rightarrow}\ensuremath{\gamma}{\ensuremath{\chi}}_{b}(2P),$ ${\ensuremath{\chi}}_{b}(2P)\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\Upsilon}(1D),$ followed by either $\ensuremath{\Upsilon}(1D)\ensuremath{\rightarrow}\ensuremath{\eta}\ensuremath{\Upsilon}(1S)$ or $\ensuremath{\Upsilon}(1D)\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}\ensuremath{\Upsilon}(1S).$ We find no evidence for such decays and set upper limits on the product branching ratios.

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