The Confining Interaction and Radiative Decays of Heavy Quarkonia
  The radiative spin-flip transition rates of heavy quarkonium states depend sensitively on the matrix elements of the effective confining interaction through the associated two-quark exchange current operator. The Hamiltonian model based on a scalar linear confining interaction with a single gluon exchange hyperfine term is shown to provide an adequate description of the $J/\psi\to \eta_c\gamma$ and $\psi(2S)\to \eta_c\gamma$ decay widths once the relativistic single quark magnetic moment operator is treated without approximation and the exchange current is taken into account. Predictions are given for the radiative spin-flip decay widths of the 1S,2S and 3S states of the $c\bar c$, $b\bar b$ and $B_c^+$ systems. In the B_c^+ system the gluon exchange current also contributes to the spin-flip transition rates. 
T.A. L\"ahde, C.J. Nyf\"alt, D.O. Riska