Classification of Particles: Hadrons & Leptons

The last lesson introduced the exchange particles that carry the four fundamental forces. This lesson sorts the particles those forces act on into two great families — hadrons, which feel the strong interaction, and leptons, which don't — and introduces the quantum numbers that govern which particle interactions are actually allowed to happen.

Hadrons: particles built from the strong interaction

A hadron is any particle that experiences the strong interaction — the force, carried by gluons, that holds nuclei together against electrostatic repulsion. Protons and neutrons are both hadrons, along with a family of less familiar, short-lived particles produced in accelerators and cosmic-ray showers.

Hadrons split into two classes, distinguished by how many quarks they're built from — you'll meet the quark structure properly in the next lesson, but the classification stands on its own for now.

Baryons and antibaryons

Baryons are one class of hadron. The proton and neutron are both baryons, and the only ones you'll meet regularly, though many more exist. Every baryon has a corresponding antibaryon — the antiproton and antineutron — matching it in mass but opposite in charge and every other additive quantum number.

Baryons carry a quantum number called baryon number, symbol $B$: $+1$ for every baryon, $-1$ for every antibaryon, $0$ for anything else. Baryon number is conserved: in any particle interaction, total baryon number before must equal total baryon number after.

Mesons

The second class of hadron is the meson. Mesons carry baryon number $B=0$ but, like baryons, feel the strong interaction. The two you need at this level are the pion ($\pi$) and the kaon (K).

The pion is the exchange particle of the strong nuclear force acting between nucleons — the short-range residual effect of the strong interaction met as an exchange particle in the previous lesson. The kaon is heavier and unstable, decaying into pions among other products.

Leptons: particles that don't feel the strong interaction

A lepton is a particle that does not experience the strong interaction. Leptons interact only through the weak interaction, and — if charged — the electromagnetic interaction too, alongside gravity. At this level you need two flavours of lepton, each with its own neutrino:

• the electron ($e^-$) and the electron neutrino ($\nu_e$) — the electron-type leptons;
• the muon ($\mu^-$) and the muon neutrino ($\nu_\mu$) — the muon-type leptons.

The muon behaves like a heavier, unstable version of the electron — same charge and interactions, but roughly 200 times the mass, decaying with a lifetime of a couple of microseconds into an electron plus two neutrinos. Every lepton has a corresponding antiparticle: the positron ($e^+$), the antimuon ($\mu^+$), and the electron and muon antineutrinos.

Leptons carry lepton number, kept as two separate quantum numbers, since electron-type and muon-type lepton number are each conserved on their own.

• Electron lepton number, $L_e$: $+1$ for $e^-$ and $\nu_e$, $-1$ for $e^+$ and $\bar\nu_e$, $0$ for everything else.
• Muon lepton number, $L_\mu$: $+1$ for $\mu^-$ and $\nu_\mu$, $-1$ for $\mu^+$ and $\bar\nu_\mu$, $0$ for everything else.

Both $L_e$ and $L_\mu$ must be separately conserved in any interaction — a process that conserves total lepton number but mixes up electron-type and muon-type totals incorrectly is still forbidden.

Strange particles

Certain hadrons — including the kaon — show a curious pattern: they're always produced via the strong interaction, in relatively copious amounts, yet always decay much more slowly, via the weak interaction. Particles with this behaviour were named strange particles, and the property responsible was given the quantum number strangeness, symbol $s$.

Strangeness is conserved in any interaction proceeding via the strong interaction. This is why strange particles are always created in pairs: a strong interaction starting from ordinary, non-strange particles (total $s=0$) must produce strange particles whose strangeness sums back to zero, typically one with $s=+1$ alongside another with $s=-1$.

Strangeness is not conserved in weak interactions, which is why strange particles decay via the weak interaction — there's no strong-interaction decay route that conserves strangeness. In a weak interaction, strangeness can change by $0$, $+1$ or $-1$, no more than that in a single step.

A genuinely collaborative science

Almost none of this picture — hadrons, leptons, strangeness, the whole catalogue of particles that turned up through the mid-twentieth century — came from a single scientist working alone. Cataloguing the growing "particle zoo" took large international collaborations before the underlying quark structure could even be proposed. Particle physics remains one of the most thoroughly collaborative fields in science today, with experiments run by teams of thousands of physicists from dozens of countries.

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