Rare Decays as Window to New Physics

Abstract

Rare decays of K mesons are reviewed from the perspective of testing the ``ones'' and ``zeros'' of the standard model. Decays K^+ -> pi^+ nu anti-nu and K_L -> pi^0 nu anti-nu probe the one-loop effective Hamiltonian for s -> d nu anti-nu, and can constrain the rho, eta coordinates of the unitarity triangle. Decays such as K_L -> pi^0 l^+ l^-, K_L -> mu^+ mu^-, K^+ -> pi^+ l^+ l^- and K_L -> pi^+ pi^- e^+ e^- involve short-distance effects, as well as long-distance photon-induced contributions. Some comments are added on curious features of electroweak amplitudes in the ``gaugeless'' limit, and in the chiral electron limit m_e -> 0.