- Many pions = multi-pion resonances: \( \sigma(600) \), \( \rho(770) \), \( \omega(782) \) etc. One-Boson-Exchange Model.
- Refined Meson Theories
- Sophisticated models for two-pion exchange:
- Paris Potential (Lacombe et al., Phys. Rev. C 21, 861 (1980))
- Bonn potential (Machleidt et al., Phys. Rep. 149, 1 (1987))
*Quark cluster models. Begin of effective field theory studies.