Summary — What's In This Chapter
- All matter is built from quarks and leptons — the fundamental building blocks of the Standard Model.
- Hadrons (particles made of quarks) split into baryons (3 quarks) and mesons (quark + antiquark).
- Leptons are fundamental — not made of anything smaller — and don't feel the strong force.
- Every matter particle has an antimatter twin with identical mass but opposite charge.
- Three quantities are always conserved in any interaction: charge (Q), baryon number (B), and lepton number (L).
- You can use conservation laws to work out whether a proposed reaction is even allowed, or to identify a mystery particle in an equation.
1. The Standard Model — Quarks & Leptons
Let's start from the very bottom. If you kept splitting matter into smaller and smaller pieces, you'd eventually hit a wall — particles that aren't made of anything else. These are called fundamental particles, and the Standard Model sorts every one of them into two families: quarks and leptons.
Think of it like LEGO
Quarks are like tiny LEGO bricks that only ever get used in combination — you never find a single quark sitting on its own. Leptons, on the other hand, are more like finished mini-figures — complete, independent, fundamental units that don't need to combine with anything else to exist.
The Three Generations
Both quarks and leptons come in three generations, with mass increasing as you go down the generations. Everyday matter (you, this screen, the air around you) is built entirely from Generation I particles — the up quark, down quark, electron, and electron neutrino. Generations II and III are heavier, unstable versions that only show up in high-energy events like cosmic rays or particle accelerators.
| Generation | Quarks | Charge | Leptons | Charge |
|---|
| I | up (u), down (d) | +2/3e, −1/3e | electron (e⁻), electron neutrino (νe) | −1e, 0 |
| II | charm (c), strange (s) | +2/3e, −1/3e | muon (μ⁻), muon neutrino (νμ) | −1e, 0 |
| III | top (t), bottom (b) | +2/3e, −1/3e | tau (τ⁻), tau neutrino (ντ) | −1e, 0 |
Neat historical fact
Only the up, down, strange, and charm quarks were found experimentally at first. The symmetry of the Standard Model — the neat pattern of three matching generations — predicted that a top and bottom quark must also exist before anyone had seen them. Physicists went looking specifically because the pattern demanded it, and found them exactly as predicted. That's the power of a good theory: it tells you what to look for.
Hadrons: Baryons & Mesons
Quarks never fly solo. They're always bound together in combinations, and any particle made of quarks is called a hadron. Hadrons feel the strong nuclear force (which is, unsurprisingly, the force that holds quarks together). There are two types:
Baryon = 3 quarks (e.g. proton, neutron)
Meson = quark + antiquark (e.g. pion, kaon)
2. Antimatter
identical in every way except charge
Think of it like a photo negative