Quarks & Antiquarks
Hadrons aren't fundamental — every baryon and meson you met last lesson is built from smaller particles called quarks. This lesson gives you the properties of the three quarks you need, shows how the proton and neutron are built from them, and uses that structure to explain what actually happens inside a neutron when it decays.
Quarks as the building blocks of hadrons
Every hadron — every baryon and meson — is made of smaller, more fundamental particles called quarks. Quarks feel the strong interaction directly, and the way they combine determines which hadron you end up with. Baryons are built from three quarks; mesons from one quark and one antiquark.
There are six known flavours of quark, but only three are required at this level: up (u), down (d) and strange (s). Each has a corresponding antiquark — $\bar u$, $\bar d$, $\bar s$ — with charge, baryon number and strangeness all reversed in sign.
Properties of the quarks
Every quark carries a fractional electric charge — a genuine surprise when first proposed, since no particle with a charge that isn't a whole multiple of $e$ had been observed directly. Each quark carries baryon number $\tfrac{1}{3}$, so three quarks combine to give baryon number $1$. Only the strange quark carries non-zero strangeness.
| Quark | Charge / e | Baryon number | Strangeness |
|---|---|---|---|
| Up (u) | +2/3 | +1/3 | 0 |
| Down (d) | −1/3 | +1/3 | 0 |
| Strange (s) | −1/3 | +1/3 | −1 |
| Up-antiquark (ū) | −2/3 | −1/3 | 0 |
| Down-antiquark (d̄) | +1/3 | −1/3 | 0 |
| Strange-antiquark (s̄) | +1/3 | −1/3 | +1 |
The strange quark itself is assigned strangeness $-1$, not $+1$ — a historical convention fixed before quarks were proposed. Best just to learn it: s has strangeness −1, s̄ has strangeness +1.
Building baryons: the proton and the neutron
The proton is built from two up quarks and one down quark, uud. The neutron is one up quark and two down quarks, udd — both non-strange, as expected for two everyday baryons.
The antiproton and antineutron are built the same way from antiquarks: antiproton = $\bar u\bar u\bar d$, antineutron = $\bar u\bar d\bar d$. Every charge and baryon-number contribution flips sign, giving the antiproton a charge of $-1e$ and the antineutron a charge of $0$, both with baryon number $-1$.
Building mesons: pion and kaon
Mesons are quark–antiquark pairs, so every meson has baryon number zero: one quark contributes $+\tfrac{1}{3}$, one antiquark contributes $-\tfrac{1}{3}$, and these cancel. A positive pion, $\pi^+ = u\bar d$, has charge $\tfrac{2}{3} + \tfrac{1}{3} = +1$, matching the pion's known charge. The kaon is built the same way but includes a strange quark or strange-antiquark — e.g. $K^+ = u\bar s$ — which is why the kaon carries non-zero strangeness while the pion doesn't.
Neutron decay at the quark level
A free neutron is unstable and undergoes beta-minus decay, emitting an electron and an antineutrino as it turns into a proton:
Baryon number stays at $+1$ throughout, and lepton number is conserved since $e^-$ and $\bar\nu_e$ carry $L_e = +1$ and $L_e=-1$, summing to zero.
The more revealing way to see this decay is at the level of the quarks. The neutron is udd; the proton is uud. Only one quark has changed: a down quark has turned into an up quark, while the other two are unaffected spectators.
Neutron decay isn't really "a neutron turning into a proton" as a single event — it's one down quark converting into an up quark, with the electron and antineutrino created directly out of that quark transition.
Neutron decay at the quark level: the full chain
Combining the quark transition with the exchange-particle picture of the weak interaction gives the complete, two-step chain behind every beta-minus decay. The down quark doesn't turn directly into an up quark plus an electron and antineutrino — it emits a W⁻ boson as it converts:
The W⁻ boson is the weak interaction's exchange particle: it carries away the charge lost by the quark (a change of $+1$, so the boson carries $-1$ to balance the vertex) and is extremely short-lived, decaying almost immediately:
producing the electron and antineutrino actually detected leaving the nucleus. Chaining the two steps and cancelling the intermediate W⁻ recovers $d \rightarrow u + e^- + \bar\nu_e$ — beta decay isn't a single event, but a quark emitting a virtual W boson, which then decays into the leptons observed.
Key takeaways
- Baryons are built from three quarks; mesons are built from one quark and one antiquark. Only up (u), down (d) and strange (s) quarks — and their antiquarks — are needed at this level.
- Quarks carry fractional charge (multiples of $\tfrac{1}{3}e$) and baryon number $+\tfrac{1}{3}$; only the strange quark carries non-zero strangeness ($-1$ for s, $+1$ for s̄).
- Proton = uud (charge $+1$, baryon number $+1$); neutron = udd (charge $0$, baryon number $+1$) — both check out by summing the quark contributions.
- Antiproton and antineutron are built from the corresponding antiquarks, with every charge and baryon-number contribution reversed in sign.
- Mesons (pion, kaon) always have baryon number zero, since a quark's $+\tfrac13$ and an antiquark's $-\tfrac13$ cancel; the kaon's strangeness comes from an s or s̄ in its make-up.
- Neutron decay, $n \rightarrow p + e^- + \bar\nu_e$, corresponds at the quark level to a single down quark converting into an up quark, with the other two quarks unaffected.
- The full chain behind that quark transition is two steps: $d \rightarrow u + W^{-}$, then $W^{-} \rightarrow e^- + \bar\nu_e$ — the electron and antineutrino are decay products of the short-lived, undetected W⁻ boson, not created directly by the quark itself.