Classification of particles

The family tree

Twentieth-century experiments turned up far more particles than anyone wanted, and the zoo only made sense once it was sorted by which interactions each particle feels. Hadrons are the particles subject to the strong interaction. Leptons are deaf to it. They are subject to the weak interaction, and the charged ones feel the electromagnetic force as well. The lepton list here is short, being the electron, the muon, their neutrinos of electron type and muon type, and the antiparticles of all of them. So far as every experiment can tell, leptons are genuinely fundamental.

Hadrons split again. The baryons take the proton and neutron as their specified examples, along with the antibaryons, the antiproton and the antineutron. The mesons on the syllabus are the pion and the kaon. A pion holds a job title as well, being the exchange particle of the strong nuclear force between nucleons. A kaon is chiefly famous for what it does next, which is to decay into pions, a decay the quarks lesson takes apart quark by quark.

The bookkeeping numbers

Each family gets a counting number. Baryon number B runs +1 for a baryon, −1 for an antibaryon and 0 for mesons and leptons. B is conserved in every interaction, and the proton is the lightest baryon there is, so the proton is the only stable free baryon. Every other baryon eventually decays down to a proton in steps, because that baryon number has to end up somewhere and there is nowhere lighter to put it.

The word free is doing quiet work there. A neutron on its own lasts about a quarter of an hour, while a neutron bound inside a stable nucleus never decays at all, since the binding-energy accounts leave it nothing to gain. Write the spec sentence in the exam, and keep the reason for the nuclei around you in your back pocket.

Lepton number L works the same way, +1 for leptons and −1 for antileptons, but it is conserved separately for the electron family and the muon family. An interaction must balance electron-type lepton number and muon-type lepton number individually. The muon itself is unstable and decays into an electron, with the neutrinos required to keep both family ledgers straight.

Strange particles

Kaons misbehave instructively. They are produced by the strong interaction, copiously and fast, and yet they decay by the weak interaction, slowly. Resolving that needs a third quantum number, strangeness, symbol S.

Strange particles are always created in pairs, one carrying S = +1 and the other S = −1, because the strong interaction conserves strangeness and can therefore make it only in cancelling pairs. The weak interaction keeps looser books. In a weak process strangeness may change by 0, +1 or −1, which is how a lone kaon decays at all, and why it has to wait for the weak interaction to get round to it. One cultural note belongs here. The discoveries in this zoo came out of large collaborations of scientists and engineers, and their collective checking is how a new particle becomes accepted knowledge.