Flavour Conservation Laws

Strangeness the classic example of a "flavour" conservation law. These conservation laws are simply a reflection of the existence of the fundamental fermions.

The strong and electromagnetic interactions conserve the fermion flavours, e.g. the only way to get rid of an electron is to annihilate it with an anti-electron (a positron). Similarly, the only way for the strong or electromagnetic to get rid of a strange quark is to annihilate it with an anti-strange quark. In this sense we actually have 12 conservation laws, but some are meaningless (i.e. neutrinos don't interact strongly or electromagnetically) or obscured by other symmetries.

Similarly, baryon and lepton number conservation are automatic if flavour conservation is exact.

Isospin

The up and down quarks have negligible mass compared to their strong binding energies in any hadron. As a result, replacing an up quark with a down quark makes almost no difference. For example a proton (uud) is essentially the same as a neutron (udd), except for a tiny mass difference (~0.1%) and their electric charges.

This symmetry is an accident, since there is no obvious reason why the up and down quarks should have the masses they do, but it is very powerful in understanding hadronic systems and their interactions.