Classification of particles

Hadrons and leptons

Particles are classified as hadrons or leptons, and hadrons are divided into baryons and mesons. Twentieth-century experiments found far more particles than expected, and the list became orderly once it was sorted by which interactions each particle feels. Hadrons are the particles subject to the strong interaction. Leptons are not subject 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 conserved quantum numbers

Baryon number, lepton number and strangeness are quantum numbers used to test whether an interaction is allowed. Each family carries its own 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 carries the condition. A neutron on its own lasts about a quarter of an hour, while a neutron bound inside a stable nucleus does not decay, because the binding energy of the nucleus leaves no lower-energy state to decay into. That is why stable nuclei can contain neutrons at all.

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, together with the neutrinos needed to conserve both lepton numbers separately.

Strange particles

Kaons posed a puzzle. 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 does not conserve strangeness: in a weak process S may change by 0, +1 or −1, which is how a single kaon can decay, and why the decay proceeds at weak-interaction rates rather than strong ones. Particle physics now relies on large collaborations of scientists and engineers, whose collective checking is how a new particle becomes accepted knowledge.