Relativistic and Binding Energy Corrections to Direct Photon Production   In Upsilon Decay
  A systematic gauge-invariant method is used to calculate the rate for an upsilon meson to decay inclusively into a prompt photon. An expansion is made in the quark relative velocity v, which is a small natural parameter for heavy quark systems. Inclusion of these O(v^2) corrections tends to increase the photon rate in the middle z range and to lower it for larger z, a feature supported by the data. 
Mohammad Ali Yusuf, Pervez Hoodbhoy