The s-d transition element and the ΔI = 1/2 rule

It is confirmed that the sum of s-d self-energy graphs is gauge-parameter independent on the quark mass shells for finiteRξ gauges. It is argued that the result, of orderO(M _w ^-2 ), has physical significance and, to avoid confusion, should not be rotated away by rediagonalization. In particular, the ensuing s π|H _W,eff|K s transition becomes the origin of the ΔI = 1/2 rule for kaon decays, consistent with observation, both in magnitude and in sign.