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

TL;DR

This paper reports the first observation of the Upsilon(2S) to eta Upsilon(1S) transition, measuring its branching fraction and significance, and sets upper limits for related transitions involving bottomonium states.

Contribution

First observation of the Upsilon(2S) to eta Upsilon(1S) transition, including measurement of its branching fraction and significance, and establishing upper limits for related processes.

Findings

Branching fraction B=(2.1+0.7-0.6(stat.)+-0.3(syst.)) x 10^{-4}

Statistical significance of 5.3 sigma

Upper limits for related transitions at 90% confidence level

Abstract

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.