Physics › Particles › Classification of particles
Classification of particles
The particle zoo sorts into a short family tree. Hadrons feel the strong interaction and split into baryons and mesons; leptons are fundamental and do not. Two quantum numbers and one strange rule keep the whole zoo in order.
Builds on Particle interactions and exchange particles.
IN THIS TOPIC
- Classify particles as hadrons (baryons or mesons) or leptons, with the specified examples.
- Use baryon number and the two lepton numbers as conserved quantum numbers.
- Describe strange particles: produced by the strong interaction in pairs, decaying by the weak.
WHAT YOU PROBABLY THINK
Matter is just protons, neutrons and electrons.
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 not: the electron, the muon, and their neutrinos (electron-type and muon-type are the only flavours AQA tests), plus the antiparticles of all of them. Leptons are, as far as every experiment can tell, genuinely fundamental.
Hadrons split again. Baryons, with the proton and neutron as the specified examples, and their antibaryons, the antiproton and antineutron. Mesons: the pion and the kaon. The pion earns a special job title as the exchange particle of the strong nuclear force between nucleons, and the kaon is chiefly famous for what it does next: it decays into pions.
The bookkeeping numbers
Two families, two ledgers. Baryon number B: +1 for a baryon, −1 for an antibaryon, 0 for mesons and leptons. B is conserved in every interaction, which is why the proton, as the lightest baryon, is the only stable one: every other baryon eventually decays, in steps, down to a proton, because the baryon number has to end up somewhere and there is nowhere lighter to put it.
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, yet they decay by the weak interaction, slowly. The resolution is a third quantum number: strangeness, symbol S.
Strange particles are always created in pairs, one carrying S = +1 and one S = −1, because the strong interaction conserves strangeness: it can make strangeness only in cancelling pairs. The weak interaction is looser: in a weak process, strangeness can change by 0, +1 or −1, which is exactly why a lone kaon can decay at all, and why it must wait for the weak interaction to do it. One cultural note the spec asks for: the discoveries in this zoo came from large collaborations of scientists and engineers, whose collective checking is how new particles become accepted knowledge.
THE EXAM BIT
- Classify by interaction first: hadrons feel the strong interaction, leptons do not. That single sentence is the mark scheme's opening line.
- Baryons and the specified examples: p and n (with the antiproton and antineutron as antibaryons); mesons: π and K. B = +1, −1 and 0 respectively.
- The proton is the only stable baryon; other baryons eventually decay to it. Quote it as stated: it is a spec sentence.
- Lepton number is conserved per family: check electron-type and muon-type separately, or the audit fails.
- Strange particles: produced in pairs by the strong interaction, decay by the weak. Strangeness is conserved in strong interactions and can change by 0 or ±1 in weak ones.
CHECK YOURSELF
A kaon decays into pions. Identify the interaction responsible and give two pieces of evidence for your answer.
Show a hint
What happens to strangeness, and how quickly do these decays happen?
Show the answer
The decay is a weak interaction.
Evidence one: strangeness changes. The kaon carries S = ±1 and pions carry S = 0, and only the weak interaction permits ΔS = ±1; the strong interaction conserves strangeness exactly.
Evidence two: the decay is slow. Strong processes are effectively instantaneous, and the kaon's comparatively long life is the signature of a particle waiting for the weak interaction.
Hadrons feel the strong force; leptons are fundamental.
Strangeness is born in pairs and broken only weakly.
No animated video for this topic yet; these notes stand alone. InkPhysics on YouTube.