Total Hadronic Photoabsorption Cross Sections on Hydrogen and Complex Nuclei from 4 to 18 GeV

Abstract

Final total cross sections are given for a counter experiment at SLAC on hadronic photon absorption in hydrogen, deuterium, carbon, copper, and lead at incident energies from 3.7 to 18.3 GeV. Some of the nucleon cross sections have been revised and the C, Cu, and Pb data from 3.7 to 7.4 GeV have not been reported previously. The cross sections for complex nuclei vary approximately as ${A}^{0.9}$ in our energy range, indicating that the photon interacts, at least partially, as a strongly interacting particle. The energy dependences of the proton and neutron cross sections are also similar to those of hadron-nucleon cross sections and hence may be fitted by a typical Regge parametrization, yielding ${\ensuremath{\sigma}}_{T}(\ensuremath{\gamma}p)=(98.7\ifmmode\pm\else\textpm\fi{}3.6)+(65.0\ifmmode\pm\else\textpm\fi{}10.1){\ensuremath{\nu}}^{\ensuremath{-}\frac{1}{2}}$ \ensuremath{\mu}b and ${\ensuremath{\sigma}}_{T}(\ensuremath{\gamma}n)=(103.4\ifmmode\pm\else\textpm\fi{}6.7)+(33.1\ifmmode\pm\else\textpm\fi{}19.4){\ensuremath{\nu}}^{\ensuremath{-}\frac{1}{2}}$ \ensuremath{\mu}b, where $\ensuremath{\nu}$ is the photon energy in GeV. These extrapolate to the same value at infinite energy, consistent with Pomeranchukon exchange, and the energy-dependent part yields an isovector-to-isoscalar-exchange ratio of 0.18 \ifmmode\pm\else\textpm\fi{} 0.06. While these observations are qualitatively consistent with vector meson dominance, quantitatively vector dominance fails in relating our results to $\ensuremath{\rho}$ photo-production on hydrogen or to experiments determining the $\ensuremath{\rho}$-nucleon cross section. Vector dominance cannot be rescued by assuming that the $\ensuremath{\rho}$-photon coupling constant depends on the photon mass. Instead, an additional short-range interaction is apparently required, possibly due to a heavy (\ensuremath{\gtrsim} 2 GeV / ${\mathit{c}}^{2}$) vector meson or to a bare-photon interaction. The additional interaction accounts for approximately 20% of the total photoabsorption cross section.