Wolfenstein, L.; Violation of CP Invariance and the Possibility of Very Weak Interactions
Phys. Rev. Lett. 13 (1964) 562;

Reprinted in CP violation, editor L. Wolfenstein, p. 117-119..
D. B. Lichtenberg (ed.) and S. P. Rosen (ed.), Developments in the Quark Theory of Hadrons, Vol.1, 227-229..

Motivation
The observation (J. H. Christenson et al. Phys. Rev. Lett. 13 (1964) 138) of the decay K_{L} 2 provides evidence that the weak interactions are not invariant with respect to the operation CP. The required magnitude of this CP-violating term in the weak-interaction Hamiltonian and the implications for other possible observations depend upon the theoretical model for explaining the K_{L} 2 decay. Sachs has provided such a model (R. G. Sachs, Phys. Rev. Lett. 13 (1964) 286) in which (1) the CP-violating term is maximal in the sense that the Q = - S decay interaction is 90^{°} out of phase with Q = + S interaction, (2) CP
violation would not occur in nonleptonic decays or in leptonic decays other than that of K^{o}, and (3) the strength of the CP-conserving term is comparable to the CP-violating term. In this note we wish to raise some possible objections to the model of Sachs and to suggest an alternative which essentially satisfies conditions (1) and (2) above but requires that CP-violating weak interactions are weaker than the CP-conserving ones by a factor 10^{-8} to 10^{-7}.
(Extracted from the introductory part of the paper.).

Related references See also R. G. Sachs, Phys. Rev. Lett. 13 (1964) 286;
W. J. Willis et al., Phys. Rev. Lett. 13 (1964) 291;
B. L. Ioffe, Zh. Eksp. Teor. Fiz. 42 (1962) 1411;
B. L. Ioffe, JETP 15 (1962) 978;
Analyse data from J. H. Christenson, J. W. Cronin, V. L. Fitch, and R. Turlay, Phys. Rev. Lett. 13 (1964) 138;
R. W. Birge et al., Phys. Rev. Lett. 11 (1963) 35;

Record comments
Invention of the superweak theory for CP-violation in weak interactions.