Long-distance contributions to the K_L --> mu+ mu- decay width

The dispersive two-photon contribution to the K_L --> mu+ mu- decay amplitude is analyzed, using chiral perturbation theory techniques and large-N_C considerations. A consistent description of the decays Pi^0 --> e+ e-, Eta --> mu+ mu- and K_L --> mu+ mu- is obtained. As a byproduct, one predicts Br(Eta –> e+ e-) = (5.8±0.2) x 10^-9 and Br(K_L --> e+ e-) = (9.0±0.4) x 10^-12.