Body face
- hadrons made of a quark and an antiquark, bound by the strong interaction
- integer spin → bosons
- baryon number 0 → none are stable
- lightest: pions
- pion exchange: Yukawa’s (1935) model of the nuclear force between protons and neutrons, range ≈ 1.4 fm
- kaons: contain a strange quark, where CP violation was first seen
- charmonium (charm-anticharm) and bottomonium (bottom-antibottom): heavy pairs that behave like a hydrogen atom