Measurement of Re()
The CP-violating decay and the semileptonic charge asymmetry in Ke3 and Kmu3 decays arise primarily from one mechanism: an asymmetry in the rate of particle-antiparticle transitions. This effect parametrized by epsilon, corresponds to transitions with delta S=2. A Superweak model maintains that the fundamental interaction responsible for CP violation is a delta S=2 coupling, and that there are no delta S=1 manifestations of the effect. That is to say, that the CP-odd eigenstate, , never decays to , and that the does so only because it includes an admixture of the CP-even eigenstate, . By contrast, the Standard Model predicts the existence of ``direct'' CP violation: a delta S=1 transition in which the decays to two pions. This transition is described in part by strong and electroweak penguin diagrams. The relative importance of the various diagrams, and the extent of the constructive or destructive interference between them, depends upon the mass of the top quark.
The strength of this transition is conventionally expressed by the means of the parameter . Assuming CPT invariance, one can express the ratio of to decay amplitudes to and as
From these relationships one can determine the real part of , by measuring the double ratio,