Measurement of the double Dalitz decay $\hat{I}\cdot \to e^{+}{e}^{-}{e}^{+}{e}^{-}$

BESIII Collaboration, M. Ablikim, M. N. Achasov +689

The BESIII experiment measured the rare decay of the eta meson into two electron-positron pairs, determining its branching fraction to be about 2.6 × 10⁻⁵.