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We report the first observation of the transition Upsilon(2S) > etaUpsilon(1S), with branching fraction B=(2.1+0.7-0.6(stat.)+-0.3(syst.)) x10^{-4} and statistical significance 5.3 sigma. Data were acquired with theCLEO detector at the CESR e+ e- symmetric collider. This is the first processobserved involving a b-quark spin flip. Upper limits at 90% confidence levelfor related processes, in units of 10^{-4}, are B[Upsilon(2S) -> pi0Upsilon(1S)] < 1.8, B[Upsilon(3S) -> eta Upsilon(1S)] < 1.8, B[Upsilon(3S) ->pi0 Upsilon(1S)] < 0.7, and B[Upsilon(3S) -> pi0 Upsilon(2S)] < 5.1.

Observation of Upsilon(2S) -> eta Upsilon(1S) and search for related transitions