KL → π0νν beyond the standard model

We analyze the decay KL → π0νν in a model independent way. If lepton flavor is conserved the final state is (to a good approximation) purely CP even. In that case this decay mode goes mainly through CP violating interference between mixing and decay. Consequently, a theoretically clean relation between the measured rate and electroweak parameters holds in any given model. Specificaly, Γ(KL → π0νν)Γ(K+ → π+νν) = sin2θ (up to known isospin corrections), where θ is the relative CP violating phase between the K - K mixing amplitude and the s → dνν decay amplitude. The experimental bound on BR(K+ → π+νν) provides a model independent upper bound: BR(KL → π0νν) < 1.1 × 10−8. In models with lepton flavor violation, the final state is not necessarily a CP eigenstate. Then CP conserving contributions can dominate the decay rate.