In recent years the study of the rare decays of kaons has had three primary motivations. The first is the search for physics beyond the Standard Model (BSM). Virtually all attempts to redress the theoretical shortcomings of the Standard Model (SM) predict some degree of lepton flavor violation (LFV). Decays such as K{sub L} → [mu]{sup {+-}}e{sup {-+}} have very good experimental signatures and can consequently be pursued to remarkable sensitivities. These sensitivities correspond to extremely high energy scales in models where the only subpression is that of the mass of the exchanged field. There are also theories that predict new particles created in kaon decay or the violation of symmetries other than lepton flavor. The second is the potential of decays that are allowed but that are extremely suppressed in the SM. In several of these, the leading component is a G.I.M.-suppressed [1] one-loop process that is quite sensitive to fundamental SM parameters such as V{sub td}. These decays are also potentially very sensitive to BSM physics. Finally there are a number of long-distance-dominated decays which can test theoretical techniques such as chiral Lagrangians that purport to explain the low-energy behavior of QCD. Knowledge of some of these decays is also needed to extract more fundamental information from certain of the one-loop processes. This field is quite active as indicated by Table 1 that lists the decays for which results have been forthcoming in the last couple of years as well as those that are under analysis. Thus in a short review such as this, one must be quite selective. Recent results on rare kaon decays are reviewed and prospects for future experiments are discussed.

The authors study on the rare decay K{sub L} 2![pi]°[gamma][gamma], measure a{sub V}, and branching ratio by analyzing 96, 97 and 99 data. The measurements were taken by KTeV at Fermi National Accelerator Laboratory. After all cuts, they have 1982 K{sub L} 2![pi]°[gamma][gamma] candidate. The background level is estimated as 30%. K{sub L} 2![pi]°[gamma][gamma] branching ratio is measured as (1.29 ± 0.03(stat) ± 0.04(sys)) x 10−6. By using D'Ambrosio's theory to fit a{sub V}, a{sub V} = -( -0.31 ± 0.05(stat) ± 0.07(sys)).

The authors report on the evidence for the decays B° → D{sub s}?− and B° → D{sub s}− K+ and the results of a search for the decays B° → D*{sub s}+?− and B° → D*{sub s}− K+ from a sample of 84 million?(4S) decays into B meson pairs collected with the BaBar detector at the PEP II asymmetric-energy e+e− collider. The measured B° → D{sub s}+?− yield has a probability of less than 10−3 to be a fluctuation of the background and they measure the branching fraction?(B° → D{sub s}+?−) = (3.2 ± 0.9(stat.) ± 1.0 (syst.)) x 10−5. The measured B° → D{sub s}− K+ yield has a probability of less than 5 x 10−4 to be a fluctuation of the background and they measure the branching fraction?(B° → D{sub s}− K+) = (3.2 ± 1.0(stat.) ± 1.0(syst.)) x 10−5. They also set 90% C.L. limits?(B° → D*{sub s}+?−)