Introduction

One of the most important conclusions of chapters 2 and 4 was that whenever a Regge trajectory, α(t), passes through a right-signature integral value of J - v a t-plane pole will occur in the scattering amplitude because of the vanishing of the factor sin[π(α(t) + λ′)] in (4.6.2). And, as we found in section 1.5, such poles correspond to physical particles; to a particle which is stable against strong-interaction decays if the pole occurs below the t-channel threshold, or to a resonance which can decay into other lighter hadrons if it occurs above threshold. If a given trajectory passes through several such integers it will contain several particles of increasing spin, and so it is possible to classify the observed particles and resonances into families, each family lying on a given Regge trajectory. Some examples are given in figs. 5.5 and 5.6 below.

This chapter is mainly devoted to presenting the evidence for this Regge classification, but as there will be a different trajectory for each different set of internal quantum numbers such as B, I, S, etc. it will be useful for us first to examine briefly the way in which the particles have been classified according to their internal quantum numbers using SU(3) symmetry and the quark model. Readers requiring a more complete discussion than we have space for here will find the books by Carruthers (1966), Gourdin (1967), and Kokkedee (1969) very helpful.