A new $G$-parity violating amplitude in the $J/\psi$ decay?
  The $J/\psi$ meson has negative $G$-parity so that, in the limit of isospin conservation, its decay into $\pi^+\pi^-$ should be purely electromagnetic. However, the measured branching fraction $\mathcal{B}(J/\psi\to\pi^+\pi^-)$ exceeds by more than 3.9 standard deviations the expectation computed according to BaBar data on the $e^+e^-\to\pi^+\pi^-$ cross section. The possibility that the two-gluon plus one-photon decay mechanism is not suppressed by $G$-parity conservation is discussed, even by considering other multi-pion decay channels. As also obtained by phenomenological computation, such a decay mechanism could be responsible for the observed discrepancy. Finally, we notice that the BESIII experiment, having the potential to perform an accurate measurement of the $e^+e^-\to\pi^+\pi^-$ cross section in the 3 GeV energy region, can definitely prove or disprove this strong $G$-parity-violating mechanism by confirming or confuting the BaBar data. 
R. Baldini Ferroli, F. De Mori, M. Destefanis, M. Maggiora, S.   Pacetti, L. Yan, M. Bertani, A. Calcaterra, G. Felici, P. P...
hep-ph