A new $G$-parity violating amplitude in the $J/\psi$ decay?

TL;DR

This paper investigates an unexpected decay rate of the $J/\psi$ meson into pions, suggesting a possible new $G$-parity violating mechanism involving two-gluon plus one-photon decay, which could challenge existing electromagnetic decay expectations.

Contribution

It proposes a novel $G$-parity violating decay mechanism for $J/\psi$ involving two-gluon plus one-photon processes, explaining the observed excess in decay rate beyond electromagnetic predictions.

Findings

Measured branching fraction exceeds electromagnetic expectation by 3.9 standard deviations.

Two-gluon plus one-photon decay mechanism could account for the discrepancy.

Future BESIII measurements can confirm or refute this $G$-parity violation hypothesis.

Abstract

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.