Introduction

In our discussion of S-matrix theory in chapter 1, and in the development of Regge theory in chapter 2, we have for simplicity ignored the possibility that the external particles entering or leaving a given process may have intrinsic spin. Only the internal Reggeons have been permitted non-zero angular momentum. Since most hadronic scattering experiments use the spin = ½ nucleon as the target, with beams of spin = 0 (π or K), spin = ½ (p, n, p, ∧ etc.) or spin = 1(γ), and since the particles produced in the final state may have any integer or halfinteger spin, it is essential to rectify this deficiency before we can confront the predictions of Regge theory with the real world.

There are three important points to bear in mind while doing this. First, an experiment may include in the initial state particles whose spin orientations have been predetermined (polarization experiments), or may involve detection of the spin direction of some of the final-state particles, by secondary scattering or by observing their subsequent decay. So there are further experimental observables (in addition to σtot and dσ/dt) which show how the scattering probability depends on these spin directions. Secondly, the dependence of the scattering process on the spin vectors means that the Lorentz invariance and crossing properties of the scattering amplitudes will generally be more complicated than those for spinless particles.