Two-pion transitions in quarkonium revisited
  Two-pion transitions in charmonium and bottomonium are considered with the leading relativistic effects taken into account. The contribution of the chromo- magnetic interaction of the charmed quarks to the amplitude of the decay $\psi(2S) \to \pi \pi J/\psi$ is estimated from the available data. This contribution is enhanced by a factor of three in the decay $\eta_c(2S) \to \pi \pi \eta_c$ and should produce a noticeable modification of the rate and the spectrum in the latter decay. It is argued that the peculiar observed spectrum in the decay $\Upsilon(3S) \to \pi \pi \Upsilon$ arises due to a dynamical suppression of the leading nonrelativistic quarkonium amplitude and thus enhanced prominence of the relativistic terms. Also discussed are the effects of the final state interaction between the pions. 
M.B. Voloshin