Asymmetry in e+ + e- → K+ + K- and eta → π0 + γ + γ

The final-state interaction for the off-shell-photon process γ + γ → K+ + K- is discussed in terms of an ''intermediate representation'' where one of the photons is on shell. This allows the evaluation of the absorptive part and the soft-photon contribution for e+ + e- → K+ + K- from the two-photon intermediate state. The asymmetry due to the interference between the one-photon and the two-photon amplitudes for this process, near the f'-resonance position, is found to be quite large, but goes through a zero. Generalization of the analysis of the final-state interaction in the s-wave state to SU(3) allows us to predict a decay width GAMMA (eta → π0 + γ + γ) approx. = 0.13 keV, which is in good agreement with the experimental number