χbstates in exclusive radiative decay of the Υ(2S)

Abstract

The Crystal Ball detector at the DESY storage ring DORIS II has been used to study radiative decays of the Υ(2S) resonance. We report on the analysis of the exclusive channel Υ(2S)→γ${\chi }_b$, ${\chi }_b$→γΥ(1S)→$e^{+}$ $e^{\mathrm{-}}$ or ${\mu }^{+}$ ${\mu }^{\mathrm{-}}$. We detect two ${\chi }_b$ states by observing two monochromatic photon lines at energies (107.0±1.1±1.3) MeV and (131.7±0.9±1.3) MeV, respectively. The product branching ratios B[Υ(2S)→γ${\chi }_b$]×B[${\chi }_b$→γΥ(1S) ] are (1.6±0.3±0.2)% for the first state and (2.1±0.3±0.3)% for the second. For the product branching ratio of the third ${\chi }_b$ 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 ${\chi }_b$→γΥ(1S) are derived. Using theoretical estimates for the radiative widths of the ${\chi }_b$ states, we determine their hadronic widths; the results are compared with QCD predictions.