Pion pair production fromγγcollisions at the SLACe+e−storage ring PEP

We have studied several features of the production of charged-hardon pairs by $\gamma \gamma$ collisions. We have measured the $f^0$ partial width ${\Gamma }_{f^0\to \gamma \gamma }\left(Q^2\right)$ for $Q^2$ in the range $0<\mathrm{}{Q}^2<1.4\mathrm{}$ Ge${\mathrm{V}}^2$/${\mathit{c}}^2$, and obtained ${\Gamma }_{f^0\to \gamma \gamma }=2.52\mathrm{}\pm 0.13\pm 0.38$ keV at $Q^2\approx 0$. The measured $Q^2$ dependence is in agreement with the generalized vector-dominance model. The cross section for $\gamma \gamma \to ({\pi }^{+}{\pi }^{-}+K^{+}{K}^{-})$ in the mass region $1.6\le M_{\pi \pi }\le 2.5$ GeV/${\mathit{c}}^2$ has also been measured and the result compared with that expected from the QCD continuum.