In the preceding chapter, we focused on some interesting total cross-sections. That is, we were concerned with the behaviour of the (imaginary part of the) scattering amplitudes in the forward direction (i.e. t = 0). It is now time to turn our attention to processes which involve the square of the scattering amplitude. Since in the Regge limit the centre-of-mass energy is much larger than the momentum transferred from the incoming particles to any of the outgoing particles such processes must produce a rapidity gap (see Section 1.10) in the final state.

After a brief word regarding elastic scattering at t = 0 we continue by looking at processes at large t. Of course we will find a high energy behaviour which is driven by the leading eigenvalue of the BFKL kernel. In addition, we demonstrate that large t is a good way of keeping the dynamics perturbative (recall that the impact factors were the only way to ensure this in the t = 0 case) and that the dominant contributions are characterized by the physics of diffusion in the transverse plane. After demonstrating these important points, we go on to discuss the specific example of vector meson production in two-photon collisions, i.e. γγ → VV where V denotes a vector meson.

The second part of this chapter will be concerned with the physics of diffraction dissociation. In particular, we look in some detail at the particular process of photon dissociation in deep inelastic scattering.