Complete Leading Order Analysis in Chiral Perturbation Theory of the Decays $K_L\to\gamma\gamma$ and $K_L\to\ell^+\el^-\gamma$

Abstract

The decays $K_L \to \gamma\gamma$ and $K_L \to \ell^+ \ell^- \gamma$ are studied at the leading order $p^6$ in Chiral Perturbation Theory. One-loop contributions stemming from the odd intrinsic parity $|\Delta S| = 1$ effective Lagrangian of order $p^4$ are included and shown to be of possible relevance. They affect the decay $K_L \to \gamma\gamma$ adding to the usual pole terms a piece free of counterterm uncertainties. In the case of the $K_L \to \ell^+ \ell^- \gamma$ decays the dependence of the form factor on the dilepton invariant mass requires a counterterm. The form factor may receive a sizeable contribution from chiral logarithms. Including considerations from the $K_L \to \pi^+ \pi^- \gamma$ direct emission amplitude, we obtain two consistent scenarios. In one scenario the long distance contributions from the one-loop terms are important, while in the other they are marginal. In both cases the counterterm is shown to be significant.