Observation of Υ(2S)→ηΥ(1S) and Search for Related Transitions

We report the first observation of Υ(2S)→ηΥ(1S), with a branching fraction B=(2.1_-0.6^+0.7(stat)±0.3(syst))×10^-4 and a statistical significance 5.3σ. Data were acquired with the CLEO III detector at the CESR e⁺e^- symmetric collider. This is the first process observed involving a b-quark spin flip. For related transitions, 90% confidence limits in units of 10^-4 are B(Υ(2S)→π⁰Υ(1S))0Υ(1S))0Υ(2S))<5.1.