Photon-Proton and Photon-Photon Scattering from Nucleon-Nucleon Forward Amplitudes

Abstract

We show that the data on $\gamma p$ and $\gamma\gamma$ interactions can be derived from the $pp$ and $\bar p p$ forward scattering amplitudes using vector meson dominance and the additive quark model. The nucleon-nucleon data are parameterized using a model where high energy cross sections rise with energy as a consequence of the increasing numbers of soft partons populating the colliding particles. We present detailed descriptions of the data on the total and elastic cross sections, the ratio of the real to imaginary part of the forward scattering amplitude, and on the slope of the differential cross sections for $pp$, $\bar p p$, $\gamma p$, $\gamma\gamma$, $\gamma p \to \gamma V$ and $\gamma\gamma \to V_i V_j$ reactions, where $V= \rho, \omega, \phi$. We make a wide range of predictions for future HERA and LHC experiments and for $\gamma\gamma$ measurements at LEP.