First observation of a Υ(1D) state

We present the first evidence for the production of Υ(1D) states in the four-photon cascade, Υ(3S)→γχ_b(2P), χ_b(2P)→γΥ(1D), Υ(1D)→γχ_b(1P), χ_b(1P)→γΥ(1S), followed by the Υ(1S) annihilation into e⁺e^- or μ⁺μ^-. The signal has a significance of 10.2 standard deviations. The measured product branching ratio for these five decays, (2.5±0.5±0.5)×10^-5, is consistent with the theoretical estimates. The data are dominated by the production of one Υ(1D) state consistent with the J=2 assignment. Its mass is determined to be (10161.1±0.6±1.6) MeV, which is consistent with the predictions from potential models and lattice QCD calculations. We also searched for Υ(3S)→γχ_b(2P), χ_b(2P)→γΥ(1D), followed by either Υ(1D)→ηΥ(1S) or Υ(1D)→π⁺π^-Υ(1S). We find no evidence for such decays and set upper limits on the product branching ratios.