Two-pion transitions in quarkonium revisited

Two-pion transitions in charmonium and bottomonium are considered with the leading relativistic effects taken into account. It is estimated that the contribution of the chromo-magnetic interaction of the charmed quarks to the amplitude of the decay psi(2S)->pi pi J/psi is at the level of (15-20)%. This contribution is enhanced by a factor of 3 in the decay eta(c)(2S)->pi pi eta(c), producing a significant modification of the rate and the spectrum in the latter decay. It is also suggested that the peculiar observed spectrum in the decay Upsilon(3S)->pi pi Upsilon can be explained by a relative prominence of the relativistic terms, if the leading nonrelativistic quarkonium amplitude is dynamically suppressed. Also discussed are the effects of the final state interaction between the pions.