Asymmetry ine+e¯→K++K−andη→π0+γ+γ

The final-state interaction for the off-shell-photon process $\gamma +\gamma \to K^{+}+K^{-}$ 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 $e+\overline{e}\to K^{+}+K^{-}$ 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 $f^{\prime }$-resonance position, is found to be quite large, but goes through 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 $\Gamma (\eta \to {\pi }^0+\gamma +\gamma )\approx 0.05$ keV, which is in good agreement with the experimental number.