the fundamental questions is that “what the matter is composed of?” In 1897,
atoms are known as the basic building blocks of matter. In the year 1911,
Ernest Rutherford demonstrated that when alpha particles are scattered on a
thin gold foil that the atom is composed of mostly empty space with a dense
core at its center which is called the nucleus. Thereafter, protons and
neutrons were discovered. In 1956, McAllister and Hofstadter published
experimental results of elastic scattering of the electrons from a hydrogen
target which revealed that the proton has an internal structure. In 1964,
Gell-Mann (and independently) Zweig proposed that nucleons are composed of
point-like particles which are called quarks. These quarks are postulated to
have spin-1/2, fractional electric charge. Combinations of different flavors of
quarks yield protons and neutrons which belong to the type of particles called
baryons (built up from three quarks) and mesons as (quark and an antiquark).
These two groups of particles are categorized as hadrons. The quarks showed
further decay properties which suggested that they have a substructure.
Terazawa, H., Chikashige, Y. and Akama, K. (1977) Unified Model of the Nambu-Jona-Lasinio Type for All Elementary-Particle Forces. Physical Review D, 15, 480-487. https://doi.org/10.1103/PhysRevD.15.480