Kaon Decay into Three Photons Revisited

We take another look at the rare kaon decay into three photons. Specifically, after imposing the requirements of gauge invariance and Bose symmetry, we derive a general form of the decay amplitude, including both parity-conserving and parity-violating contributions. Subsequently, we adopt a chiral-Lagrangian approach in conjunction with dimensional analysis arguments to estimate the branching ratios of K_(L,S) → 3γ in the standard model, obtaining values as large as ℬ(K_L → 3γ) ≃ 7 × 10^(−17) and ℬ(K_S → 3γ) ≃ 1 × 10^(−19), which exceed those found previously by a few orders of magnitude. Measurements of ℬ(K_(L,S) → 3γ) substantially bigger than these numbers would likely hint at the presence of new physics beyond the standard model.