The pion, the mediator of the nuclear force proposed in 1935 by Yukawa. The first particle discovered in the cosmic rays looked like the pion, but was later found to be a lepton, the muon. Experiments at high altitudes on cosmic rays led finally to the discovery of the pion. More experiments soon showed other surprises, the strange particles.
How the properties of the charged pion have been measured.
The discoveries of the charged leptons and of the neutrinos.
How, in 1928, A. M. Dirac found the fundamental relativistic wave equation and the Dirac Lagrangian. Dirac’s fundamental predictions of the existence for each fermion of an antiparticle with the same mass but opposite ‘charges’. How the positron and the antiproton were discovered. The important concepts of helicity and chirality.
The Majorana equation for completely neutral fermions.