Structure of Matter: Leptons, Hadrons and Quarks

1Rutherford's experiment, one level deeper

In Senior 5 you met Rutherford's discovery: alpha particles bouncing back from gold foil revealed a tiny, hard nucleus inside the atom. In 1968, at the Stanford Linear Accelerator Center (SLAC) in California, physicists repeated the idea one level down. They fired electrons of up to about 17 GeV at protons. Far more electrons were scattered through large angles than a smooth, uniform proton would allow. Just as in 1911, the result meant that something small and hard was inside: the proton is not a fundamental particle but a bound group of smaller ones, now called quarks. In Unit 5 you treated the proton and neutron as building blocks. This unit asks what THEY are made of, and what holds everything together.

2How far down does matter go?

Figure 1.1: Each level of structure is found inside the one before.

A particle is called fundamental (or elementary) if no experiment has found any internal structure in it. Today the electron and the quarks are fundamental: collisions at the highest energies show no size for them down to about ${10}^{-18}10^\{-18\}$ m. The proton and neutron are not: they have a size of about 1 fm and contain quarks.

By the 1960s hundreds of short-lived particles had been found in cosmic rays and accelerators, a confusing "particle zoo". The quark model brought order to it, much as the periodic table brought order to the elements.

3Sorting the particles

Figure 1.2: A family tree of particles.

Particles are sorted in two ways. By spin (their intrinsic angular momentum, in units of $\frac{h}{2\pi }\backslash frac\{h\}\{2\backslash pi\}$): fermions have half-integer spin ($\frac{1}{2}\backslash frac\{1\}\{2\}$, $\frac{3}{2}\backslash frac\{3\}\{2\}$, ...) and obey Pauli's exclusion principle, so they make up matter; bosons have whole-number spin (0, 1, 2, ...) and include the particles that carry forces. And by which forces they feel: leptons do not feel the strong force; hadrons do.

Leptons

Leptons are fundamental fermions that do not feel the strong force. They come in three generations, each a charged lepton with its own neutrino:

Each charged lepton has charge $-e-e$; the neutrinos are neutral and have tiny masses (less than 0.45 eV, from tritium beta decay). The muon and tau behave like heavy electrons and decay into lighter particles.

Quarks

Quarks are fundamental fermions that DO feel the strong force. Their charges are fractions of $ee$:

All masses and lifetimes on this page are from the Particle Data Group's Review of Particle Physics, 2026 edition, rounded. The light quark masses are not measured directly, because no quark can be isolated.

Hadrons

Quarks are never found alone: they are always bound into hadrons. This is called quark confinement. There are two kinds:

• baryons, made of three quarks: proton (uud), neutron (udd), lambda ${\mathrm{\Lambda }}^0\backslash Lambda^0$ (uds), omega-minus ${\mathrm{\Omega }}^{-}\backslash Omega^-$ (sss);
• mesons, made of a quark and an antiquark: ${\pi }^{+}=\mathrm{u}\bar{\mathrm{d}}\backslash pi^+\; =\; \backslash mathrm\{u\}\backslash bar\{\backslash mathrm\{d\}\}$, ${\pi }^{-}=\mathrm{d}\bar{\mathrm{u}}\backslash pi^-\; =\; \backslash mathrm\{d\}\backslash bar\{\backslash mathrm\{u\}\}$, ${\mathrm{K}}^{+}=\mathrm{u}\bar{\mathrm{s}}\backslash mathrm\{K\}^+\; =\; \backslash mathrm\{u\}\backslash bar\{\backslash mathrm\{s\}\}$, ${\mathrm{K}}^{-}=\mathrm{s}\bar{\mathrm{u}}\backslash mathrm\{K\}^-\; =\; \backslash mathrm\{s\}\backslash bar\{\backslash mathrm\{u\}\}$.

An antiquark (written with a bar, $\bar{\mathrm{d}}\backslash bar\{\backslash mathrm\{d\}\}$) has the opposite charge to its quark: $\bar{\mathrm{d}}\backslash bar\{\backslash mathrm\{d\}\}$ has $+\frac{1}{3}e+\backslash frac\{1\}\{3\}e$. Antiparticles are the subject of Lesson 2. The only stable hadron is the proton; a free neutron decays with a mean life of 878 s.