Inclusive Radiative $J/\psi$ Decays

TL;DR

This study measures the direct photon spectrum in J/psi decays using CLEO-c data, finding a ratio of radiative to three-gluon decay that aligns with quark-charge scaling but exceeds expectations from strong coupling running.

Contribution

First measurement of the inclusive direct photon branching ratio in J/psi decays with detailed spectrum analysis and comparison to Upsilon(1S) decays.

Findings

The ratio R=0.137 with uncertainties was determined.

Photon energy spectrum shape is similar to Upsilon(1S) decays.

The measured R is higher than predicted by strong coupling evolution.

Abstract

Using data taken with the CLEO-c detector at the Cornell Electron Storage Ring, we have investigated the direct photon momentum spectrum in the decay J/psi->gamma+gluon+gluon, via the 'tagged' process: e+e- -> psi(2S); psi(2S)->J/psi pi+pi-; J/psi->photon + X. Including contributions from two-body radiative decay processes, we find the ratio of the inclusive direct photon branching fraction to that of the dominant three-gluon branching fraction to be R=0.137+/-0.001+/-0.016+/-0.004, where the errors shown are statistical, systematic, and the model-dependent uncertainty related to the extrapolation to zero photon energy. The shape of the scaled photon energy spectrum in J/psi->gg gamma is observed to be very similar to that of Upsilon(1S)->gg gamma. The R value obtained is roughly consistent with that expected by a simple quark-charge scaling of the value determined at the Upsilon(1S), but somewhat higher than the value expected from the running of the strong coupling constant.