In 1960 particle physicists believed that the nucleon and pion were elementary Like the electron and the photon Nuclei were bound by a pion-exchange force Like atoms were bound by a photon-exchange force Today we know that hadrons are made of quarks which we believe are elementary Hadrons are bound by a gluon exchange force.

The journey from nucleons and pions to quarks and gluons First stage – there were too many hadrons 1. Gell-Mann and Ne’eman classified the nucleon and pion into SU(3) octets.

2. No reason for the number 3 in SU(3); No reason for octets 3. No reason why mesons and baryons should both be in octets 4. Mesons and baryons were very different, like electron and photon.