Exclusive Radiative Decays of Upsilon in SCET

Abstract

We study exclusive radiative decays of the ${\rm Y}$ using soft-collinear effective theory and non-relativistic QCD. In contrast to inclusive radiative decays at the endpoint we find that color-octet contributions are power suppressed in exclusive decays, and can safely be neglected, greatly simplifying the analysis. We determine the complete set of Lorentz structures that can appear in the SCET Wilson coefficients and match onto them using results from a previous calculation. We run these coefficients from the scale $\text{mups}$ to the scale $\Lambda \sim 1\text{GeV}$, thereby summing large logarithms. Finally we use our results to predict the ratio of branching fractions $B({\rm Y}\to \gamma f_2)/B(J/\psi \to \gamma f_2)$, $B(J/\psi \to \gamma f_2)/B({\psi }^{\prime }\to \gamma f_2)$, and the partial rate for ${\rm Y}\to \gamma \pi \pi$.

Comments: 17 pages, 2 figures. Updated to reflect published version