Quarks and Antiquarks: Properties and Combinations Revision Notes | A Level AQA Physics

1. The Building Blocks Beneath the Proton

You already know that protons and neutrons sit inside the nucleus. But here is the deeper question: what are they made of? The answer is quarks — and once you understand their properties, you can build every hadron the AQA specification asks about from scratch.

At A Level, you only need three quarks and their three antiquarks: up (u), down (d), and strange (s). Every particle in this topic — proton, neutron, pion, kaon, and their antiparticles — is assembled from these six building blocks using a small set of rules.

2. Quark Properties: Charge, Baryon Number, and Strangeness

Each quark carries three key properties that you must know precisely: electric charge (in units of the elementary charge $e$), baryon number $B$, and strangeness $S$. These properties are additive — the total for any hadron is the sum of its constituent quarks' values.

Notice the pattern for antiquarks: every property is exactly negated compared to the corresponding quark. Charge flips sign, baryon number flips sign, and strangeness flips sign. This is the defining relationship between a particle and its antiparticle.

A common point of confusion: strangeness is not conserved in weak interactions. This is why strange particles (those containing an s or $\bar{s}$ quark) are produced in pairs via the strong force but decay individually via the weak force — a phenomenon historically called associated production.

3. Baryons — Three Quarks Combined

A baryon is always made of exactly three quarks. Because each quark carries baryon number $+\frac{1}{3}$, three quarks always give a total baryon number of $+1$. This is the defining feature of a baryon, and it is why protons and neutrons are stable constituents of nuclei.

The two baryons you need at A Level are the proton (uud) and the neutron (udd). Let us verify both from first principles so you can reproduce this in an exam without memorising the answer — you derive it instead.

Worked example: Verifying the Quark Content of the Proton (uud)

Show that the combination uud gives the correct charge, baryon number, and strangeness for a proton.

1. u: $+\frac{2}{3}e$, \quad u: $+\frac{2}{3}e$, \quad d: $-\frac{1}{3}e$ — Read directly from the quark properties table.
2. $+\frac{2}{3}e + \frac{2}{3}e - \frac{1}{3}e = \frac{4}{3}e - \frac{1}{3}e = +\frac{3}{3}e = +1e$ — The proton has charge $+1e$. This matches. [B1]
3. $+\frac{1}{3} + \frac{1}{3} + \frac{1}{3} = +1$ — Baryon number $B = +1$, confirming this is a baryon. [B1]
4. $0 + 0 + 0 = 0$ — Strangeness $S = 0$. The proton contains no strange quarks, so this is expected. [B1]

Answer: uud gives charge $+1e$, baryon number $+1$, strangeness $0$ — consistent with a proton.

Worked example: Verifying the Quark Content of the Neutron (udd)

Show that the combination udd gives the correct charge, baryon number, and strangeness for a neutron.

1. u: $+\frac{2}{3}e$, \quad d: $-\frac{1}{3}e$, \quad d: $-\frac{1}{3}e$ — Read directly from the quark properties table.
2. $+\frac{2}{3}e - \frac{1}{3}e - \frac{1}{3}e = \frac{2}{3}e - \frac{2}{3}e = 0$ — The neutron has charge $0$. This matches. [B1]
3. $+\frac{1}{3} + \frac{1}{3} + \frac{1}{3} = +1$ — Baryon number $B = +1$, confirming this is a baryon. [B1]
4. $0 + 0 + 0 = 0$ — Strangeness $S = 0$. No strange quarks present. [B1]

Answer: udd gives charge $0$, baryon number $+1$, strangeness $0$ — consistent with a neutron.

4. Antibaryons — Three Antiquarks Combined

An antibaryon is the antimatter counterpart of a baryon: it is composed of three antiquarks. Because each antiquark carries baryon number $-\frac{1}{3}$, three antiquarks always give a total baryon number of $-1$.

The two antibaryons required at A Level are the antiproton ($\bar{u}\bar{u}\bar{d}$) and the antineutron ($\bar{u}\bar{d}\bar{d}$). You can verify these by negating every property of the corresponding baryon. The antiproton has charge $-1e$, baryon number $-1$, and strangeness $0$. The antineutron has charge $0$, baryon number $-1$, and strangeness $0$.

5. Mesons — A Quark and an Antiquark

Mesons are structurally different from baryons. Each meson contains exactly one quark and one antiquark. Because a quark contributes baryon number $+\frac{1}{3}$ and an antiquark contributes $-\frac{1}{3}$, the total baryon number of any meson is always $0$.

The mesons required at A Level are pions ($\pi$) and kaons ($K$). Pions contain only u and d quarks (and their antiquarks); kaons always contain a strange quark or anti-strange quark, which is what gives them non-zero strangeness.

Let us verify the $\pi^+$ and the $K^+$ to cement the method. For $\pi^+$ ($u\bar{d}$): charge $= +\frac{2}{3}e + \frac{1}{3}e = +1e$; baryon number $= +\frac{1}{3} - \frac{1}{3} = 0$; strangeness $= 0 + 0 = 0$. For $K^+$ ($u\bar{s}$): charge $= +\frac{2}{3}e + \frac{1}{3}e = +1e$; baryon number $= +\frac{1}{3} - \frac{1}{3} = 0$; strangeness $= 0 + 1 = +1$.

Notice that $\pi^+$ and $K^+$ have identical charge and baryon number, but different strangeness. This is the key distinction between pions and kaons — strangeness is the differentiator, and it arises entirely from the presence of the strange or anti-strange quark.

6. Comparing Baryons and Mesons: A Structural Summary

Now that you have seen both families, it is worth comparing them directly. The structural difference — three quarks versus one quark plus one antiquark — drives every other difference in their properties.

One structural point worth emphasising: the $\pi^-$ ($\bar{u}d$) is the antiparticle of the $\pi^+$ ($u\bar{d}$). You can verify this because every quark in one is replaced by its antiquark in the other. The $\pi^0$, however, is its own antiparticle — replacing $u\bar{u}$ with $\bar{u}u$ gives the same particle.

7. Exam Technique and Common Errors

Questions on quark content tend to fall into two categories: either you are given a particle and asked to state its quark content, or you are given a quark combination and asked to identify or verify the particle. Both types reward the same systematic approach — always calculate charge, baryon number, and strangeness in turn, showing each addition explicitly.

Examiners frequently award marks for the working, not just the final answer. Writing $+\frac{2}{3}e - \frac{1}{3}e - \frac{1}{3}e = 0$ earns a method mark even if you then misidentify the particle. Never skip the arithmetic.

Across this topic, every particle you encounter can be decoded using the same three-step method: identify the quarks, sum the charges, sum the baryon numbers, sum the strangeness values. The AQA specification limits you to u, d, and s quarks and their antiquarks, so the combinations are finite and learnable — and deriving them is always faster and more reliable than memorising a list.