Standard Model estimate of K+ →π+4e branching ratio

The branching ratio of the K+→π+e+e-e+e- (K+→π+4e) decay is calculated at leading order in the Standard Model. The dominance of the neutral-pion pole determines the overall branching ratio to be B(K+→π+4e)=B(K+→π+π0)B(π0→4e)≈7.0(3)×10-6. The significance of this contribution is very much concentrated in the context of the whole available phase space, throughout most of which the one-photon-exchange topology is prevalent in turn. It is thus interesting to present branching ratios for only parts of the allowed kinematical region. We find, for instance, B(K+→π+4e,m4e>150 MeV)=6.0(6)×10-11.