Yukawa developed the first quantum field theory of the strong force, with newly discovered particles known as ‘mesons’ acting as the force carrying virtual particles.
Russian physicist Lev Okun coined the term ‘hadron’ in 1962, in order to describe particles that experience the strong nuclear force, like protons, neutrons, pions, and kaons.
All hadrons contain quarks, and interact via the strong nuclear force. All leptons are elementary particles, they do not contain quarks, and do not experience the strong force. Only the weak force can change the flavour of a quark. Gross, Wilczek, and Politzer showed that the strong force between quarks increases as they move apart.
In 1964, American physicists James Bjørken and Sheldon Lee Glashow predicted that another quark with a charge of +2/3e must exist in order to better describe how quarks decay via the weak nuclear force.