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

Understand

We report the first observation of the transition Upsilon(2S) > eta Upsilon(1S), with branching fraction B=(2.1+0.7-0.6(stat.)+-0.3(syst.)) x 10^{-4} and statistical significance 5.3 sigma.

• Data were acquired with the CLEO detector at the CESR e+ e- symmetric collider.
• This is the first process observed involving a b-quark spin flip.
• Upper limits at 90% confidence level for related processes, in units of 10^{-4}, are B[Upsilon(2S) -> pi0 Upsilon(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.