The Decays K → πℓ + ℓ − beyond Leading Order in the Chiral Expansion*

We present a model–independent analysis of K + → π + ℓ + ℓ − and KS → π 0 ℓ + ℓ − decays, including K → 3π unitarity corrections and a general decomposition of the dispersive amplitude. From the existing data on K + → π + e + e − we predict the ratio R = B(K + → π +µ+µ −)/B(K+ → π + e + e − ) to be larger than 0.23, in slight disagreement with the recent measurement R = 0.167 ± 0.036. Consequences for the K ± → π ± e + e − charge asymmetries and for the KL → π 0 e + e − mode are also discussed. * Work supported in part by TMR, EC–Contract No. ERBFMRX–CT980169 (EURODAΦNE