Anomalous decays of eta' and eta into four pions

TL;DR

This paper predicts branching ratios for eta' and eta decays into four pions using chiral perturbation theory and vector-meson dominance, highlighting potential signals of CP violation.

Contribution

It provides the first theoretical estimates for these rare decay modes, including CP-violating effects, based on a combined theoretical framework.

Findings

Predicted branching ratios for eta' decays into four pions.

Identified decay modes as P-wave or D-wave dominated.

Suggested that eta --> 4pi0 decay could signal CP violation.

Abstract

We calculate the branching ratios of the yet unmeasured eta' decays into four pions, based on a combination of chiral perturbation theory and vector-meson dominance. The decays eta' --> 2(pi+ pi-) and eta' --> pi+ pi- 2pi0 are P-wave dominated and can largely be thought to proceed via two rho resonances; we predict branching fractions of (1.0+-0.3)*10^-4 and (2.4+-0.7)*10^-4, respectively, not much lower than the current experimental upper limits. The decays eta' --> 4pi0 and eta --> 4pi0, in contrast, are D-wave driven as long as conservation of CP symmetry is assumed, and are significantly further suppressed; any experimental evidence for the decay eta --> 4pi0 could almost certainly be interpreted as a signal of CP violation. We also calculate the CP-violating amplitudes for eta' --> 4pi0 and eta --> 4pi0 induced by the QCD theta-term.