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

The decays $K_L \to γγ$ and $K_L \to \ell^+ \ell^- γ$ are studied at the leading order $p^6$ in Chiral Perturbation Theory. One-loop contributions stemming from the odd intrinsic parity $|ΔS| = 1$ effective Lagrangian of order $p^4$ are included and shown to be of possible relevance. They affect the decay $K_L \to γγ$ adding to the usual pole terms a piece free of counterterm uncertainties. In the case of the $K_L \to \ell^+ \ell^- γ$ 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 $K_L \to π^+ π^- γ$ 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.