First observation of a ${\rm Y}\mathrm{}\left(1D\right)\mathrm{}$ state

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