First Observation of $\Upsilon(1S)\to \gamma\pi\pi$

TL;DR

This paper reports the first observation of radiative decay modes of the Upsilon(1S) into gamma plus two pions, providing measurements of branching ratios and evidence for the intermediate f2(1270) resonance.

Contribution

First experimental observation of Upsilon(1S) radiative decays to gamma pi pi and gamma pi0 pi0, with measured branching ratios and resonance identification.

Findings

Branching ratio for Upsilon(1S)->gamma pi+pi- is approximately 6.3 x 10^(-5).

Branching ratio for Upsilon(1S)->gamma pi0pi0 is approximately 1.7 x 10^(-5).

Data are consistent with decay proceeding via Upsilon(1S)->gamma f2(1270).

Abstract

We report on a study of exclusive radiative decays of the Upsilon(1S) resonance collected with the CLEO-II detector operating at CESR. We present the first observation of the radiative decays Upsilon(1S)->gamma pi+pi- and Upsilon(1S)->gamma pi0pi0. For the dipion mass regime m(pipi)>1.0 GeV, we obtain Br(Upsilon(1S)->gamma pi+pi-=(6.3+/-1.2+/-1.3) x 10^(-5), and Br(Upsilon(1S)->gamma pi0pi0=(1.7+/-0.6+/-0.3) x 10^(-5). The observed gamma pipi events are consistent with the hypothesis Upsilon(1S)->gamma f2(1270).