K-L->pi(0)nu(nu)over-bar beyond the Standard Model

We analyze the decay K-L --> pi(0) nu<(nu)over bar> 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. Specifically, Gamma(K-L --> pi(0) nu<(nu)over bar>)/Gamma(K+ --> pi(+)nu<(nu)over bar>) = sin(2) theta (up to known isospin corrections), where theta is the relative CP violating phase between the K - (K) over bar mixing amplitude and the s --> d nu<(nu)over bar> decay amplitude. The experimental bound on BR(K+ --> pi(+)nu<(nu)over bar>) provides a model independent upper bound: BR(K-L --> pi(0) nu<(nu)over bar>) < 1.1 x 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. (C) 1997 Published by Elsevier Science B.V.