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Weakly interacting K --> pi X^0 emission with m_{X^0} simeq m_{pi^0} is out of sight of the current K^+ --> pi^+ nu nubar study, but it can be sensed by the K_L --> pi^0 nu nubar search. This evades the usual Grossman-Nir bound of B(K_L --> pi^0 nu nubar) < 1.4 x 10^-9, thus the KOTO experiment is already starting to probe New Physics. An intriguing possibility is the Z gauge boson of a weak leptonic force that couples to L_mu - L_tau (the difference between the muon and tauon numbers), which may explain the long-standing muon g-2 anomaly, but is constrained by nu_mu N --> nu_mu N mu^+ mu^- scattering to m_{Z} lesssim 400 MeV. An explicit model for K --> pi Z is given, which illustrates the link between rare kaon and B --> K mu^+ mu^-, K^{(*)} nu nubar decays. Complementary to these searches and future lepton experiments, the LHC might discover the scalar boson phi responsible for light m_{Z} generation via phi --> Z Z --> 2(mu^+ mu^-).
A search for additional evidence for the rare kaon decay K^+ --> pi^+ nu nubar has been made with a new data set comparable in sensitivity to the previous exposure that produced a single event.
The first search for the decay K^+ -> pi^+ pi^0 nu nubar has been performed with the E787 detector at BNL. Based on zero events observed in the kinematical search region defined by 90 MeV/c < P_{pi^+} < 188 MeV/c and 135 MeV < E_{pi^0} < 180 MeV, an
Three events for the decay K+ => pi+,nu,nubar have been observed in the pion momentum region below the K+ => pi+,pi0 peak, 140 < P_pi < 199 MeV/c, with an estimated background of 0.93+-0.17(stat.)+0.32-0.24(syst.) events. Combining this observation w
The results and goals of experiments E787, E949, CKM and KOPIO on the measurement of the branching fractions of K+ -> pi+,nu,nubar and K0L -> pi0,nu,nubar are presented.
We estimate B(K -> pi nu nubar) in the context of the Standard Model by fitting for lambda_t = Vtd x V*ts of the `kaon unitarity triangle relation. We fit data from epsilon_K, the CP-violating parameter describing K-mixing, and a_{psi K}, the CP-viol