Spectroscopy originally meant the use of a prism to disperse visible light and analyze a complex wave form as a function of its wavelengths. The concept was expanded to include any measurement of observables as a function of wavelength or oscillation frequency. As the frequency is equivalent to energy in quantum mechanics, collecting and sorting of energy levels in any phenomenon is also called spectroscopy. It is an indispensable procedure in science to elucidate the underlying basic rules from observation of seemingly complex phenomena. The role of the periodic table in understanding atomic structure and radiation spectra in clarifying the nature of quantum mechanics come to mind immediately. In a similar vein, collecting the masses of hadrons, including both elementary particles and resonances, which were thought of as dynamic processes of interacting elementary particles, and determining their spin, parity and other quantum numbers were essential procedures in reaching the quark structure of the hadrons. Later on, we will deal with level structures of quark composites, but here we follow history to learn a heuristic way to reach the truth. The first step is to collect facts. The second step is to sort them out, find a regularity and see if they can be reproduced by a simple model. The third step is to elucidate the underlying law that governs the dynamics and reach the truth. 1)
Discoveries in the pre-accelerator era were made using isotopes and cosmic rays. That was the age of hunting particles, exciting but very much dependent on luck. With the advent of large accelerators - Cosmotron (3.3 GeV, 1953-1968), Bevatron (6.2 GeV, 1954-1971), PS (28 GeV, 1959-) and AGS (33 GeV, 1960-) -the age of harvesting particles had arrived. It became possible to plan and prepare experiments in an organized way, controlling the particle's conditions.
In this chapter, we describe how to extract information on hadrons (ฯ, nucleon, K, , ฯ, etc.) and resonances and determine their properties. This is the first step described above. We start from the basic structure of the interactions between pions and nucleons, which are representatives of the meson and baryon families of the hadron. Analysis of hadron interactions exemplified by ฯ-N scattering is essential in order to understand their dynamics. By analyzing their behavior we can 1) This is Sakata's syllogism that prevailed in his school, whose pupils proposed IOO symmetry [212] (flavor S U(3) of the Sakata model [339]), a predecessor of the quark model, lepton-baryon symmetry, the fourth quark [271] and the Kobayashi-Maskawa model [239].