The Regge theory we have described in the previous chapters applies equally well when either or both of the initial hadrons is replaced with a current, for example the electromagnetic current. In particular, we may replace one of the hadrons with a real photon. This is obviously true if we may use the vector-meson dominance model, in which the photon is assumed to behave just like an on-shell ρ or another vector particle. But the vector-dominance assumption is not necessary: the applicability of Regge theory does not depend on it. Most reactions discussed in this chapter are ones involving pomeron exchange. However in section 5.6 we discuss pion photoproduction with two objectives: one is to compare and contrast π0 photoproduction with π–p charge exchange; and the other is to look at the role of pion exchange in charged pion photoproduction.

Photon-proton and photon-photon total cross sections

The photon-proton total cross section should be fitted by forms similar to the hadron-hadron total cross sections in figures 3.1 and 3.2. This is verified in figure 5.1. Extending the fit to low energies confirms the two-component duality hypothesis, as is seen in figure 5.2. We discuss in section 7.5 whether the fit should include an additional component, the hard pomeron.

The γγ total cross section can be predicted from the known pp, p̅p, pn, p̅n and γp total cross sections. We need to determine the C = +1 exchange contributions.