A study of photon-nucleus collisions at high transverse energy

We have measured, for the first time, the atomic number dependence of photon-nucleus collisions at high transverse energy. The measurements were made at FNAL by using a large acceptance calorimeter with 50-400 GeV photons incident on LH$\sb2,$ LD$\sb2,$ Be, C, Al, Cu, Sn and Pb targets. The cross section is parameterized as $\sigma\sb{\gamma A} = \sigma\sb{\gamma p}A\sp\alpha,$ where $\alpha$ is found to increase from 0.9 to 1.1 when the total transverse energy $E\sb{T}$ is increased from roughly 6 GeV to 12 GeV; $\alpha$ is also found to increase with the photon energy at fixed $x\sb{T}\equiv E\sb{T}/\sqrt{s},$ but depends only weakly on the photon energy at fixed $E\sb{T}.$ The increase of $\alpha$ with total transverse energy is qualitatively consistent with previous measurements from the hadron-nucleus collisions, but our $\alpha$ values are somewhat lower. At high transverse energy, photon induced collisions are found to be more jet-like than pion induced collisions, revealing the point-like nature of the photons at high $E\sb{T}.$ The mean planarity in $\gamma A$ collisions is independent of A in the kinematic range covered by this experiment