Measurement of the total hadronic cross section in taggedγγreactions

We present a measurement of the total cross section for $\gamma \gamma \to \mathrm{hadrons}$, with one photon quasireal and the other a spacelike photon of mass squared $-Q^2$. Results are presented as a function of $Q^2$ and the $\gamma \gamma$ center-of-mass energy $W$, with the $Q^2$ range extending from 0.2 to 60 Ge${\mathrm{V}}^2$, and $W$ in the range from 2 to 10 GeV. The data were taken with the TPC/Two-Gamma facility at the SLAC $e^{+}{e}^{-}$ storage ring PEP, which was operated at a beam energy of 14.5 GeV. The cross section exhibits a gentle falloff with increasing $W$. Its $Q^2$ dependence is shown to be well described by an incoherent sum of vector-meson and pointlike scattering over most of the observed $W$ range. Agreement at high $Q^2$ is improved if a minimum-$p_T$ cutoff (motivated by QCD) is imposed on the pointlike contribution.