Introductory remarks concerning duality

At the time when this Discussion Meeting was proposed, it was clear that for many systems, such as the pion-nucleon system or the pion-pion system, there exist many resonance states, perhaps even increasing exponentially in number as the mass range explored moves to higher values (Barash-Schmidt et al. 1969). It was also clear that many high-energy reaction processes which are peripheral in character are mediated by simple processes of reggeon exchange. An outstanding example was the process of pion-nucleon charge-exchange, π - + p → π 0 + n, which Hohler, Baacke, Schlaile & Sonderegger (1966) found to be well described over the momentum range p lab = 4 to 18 GeV/ c as due to exchange of a reggeon of the ρ trajectory, and from which they determined the parameters of the ρ trajectory over the range 0 to 1 (GeV/ c ) 2 for the momentum transfer variable - t . It had generally been conventional to analyse the differential cross-section and polarization data on meson-baryon scattering in terms of independent partial wave amplitudes. This was certainly appropriate for those partial waves for which there occurred resonance states in the energy range considered, and for which the partial wave amplitudes were therefore rapidly varying; scattering in the other partial waves (as well as the non-resonant scattering in the resonating partial waves) was then termed ‘background scattering’.