Learn9702Paper 2 · AS Level Structured QuestionsParticle physicsFundamental particles

Particle physics · Subtopic 2 of 2

Fundamental Particles

Protons and neutrons are made of quarks. There are six flavours: up ( $+\tfrac23e$ ), down ( $-\tfrac13e$ ), strange ( $-\tfrac13e$ ), charm ( $+\tfrac23e$ ), top ( $+\tfrac23e$ ) and bottom ( $-\tfrac13e$ ), each with an antiquark of opposite charge. Hadrons are baryons (three quarks) or mesons (quark + antiquark). Leptons, such as electrons and neutrinos, are fundamental. In β⁻ decay a down quark becomes an up quark; in β⁺, up becomes down.

Cambridge 9702 Paper 1Cambridge 9702 Paper 2Edexcel YPH11 WPH14AQA 7408 Paper 1≈ 35 minRevision 5 · updated 2026-10-27

Section 1 of 14

Learning objectives

Recall the six quark flavours and their charges.
Find the charge of a hadron from its quark composition.
Distinguish baryons, mesons and leptons, and particles from antiparticles.
Describe the quark change in β⁻ and β⁺ decay.

Pause and think

A neutron has no charge, but it's made of charged quarks. How can three charged particles add up to zero?

Section 2 of 14

Core idea

Start hereInside the proton

Protons were once thought fundamental. Fire electrons hard enough at them and they scatter as if bouncing off three tiny points. Protons and neutrons are made of quarks, with charges in thirds of $e$ .

The big idea

Proton $uud$ , neutron $udd$ ; $u=+\frac23e$ , $d=-\frac13e$ . β⁻: $d\to u$ (weak interaction). Leptons are fundamental.

1. Quark content

Quark content of the proton and neutron

\beta^-

decay a d quark changes into a u quark (neutron → proton), via the weak interaction.

Worked exampleChecking charges

Proton, neutron, $u\bar d$ meson.

Goal: Charges.
Strategy: Add quark charges; antiquarks reverse sign.

$uud:\ \tfrac23+\tfrac23-\tfrac13=+1$
Proton.
$udd:\ \tfrac23-\tfrac13-\tfrac13=0$
Neutron.
$u\bar d:\ \tfrac23+\tfrac13=+1$
$\bar d$ has $+\frac13$ : the π⁺.

Answer

+1

0

+1

✓ CheckWhole-number charges for all hadrons ✓.

2. Particle families

Classifying particles

Drawing diagram…

Every particle has an antiparticle: same mass, opposite charge.

Worked exampleβ decay at quark level

β⁻ and β⁺.

Goal: Quark equations.
Strategy: One quark changes flavour.

$d\to u+e^-+\bar\nu_e$
β⁻.
$u\to d+e^++\nu_e$
β⁺.
$-\tfrac13=\tfrac23-1+0$
Charge check (β⁻).

AnswerAs shown.

✓ CheckLepton number:

0=+1-1

✓.

Common misconceptionElectrons from quarks Electrons are leptons: not made of quarks.

Summary

uud, udd.
Hadrons vs leptons.
β decay: flavour change.

Hadron: Particle made of quarks.
Lepton: Fundamental, not made of quarks.

HistoryThree quarks for Muster Mark. In

1964

Murray Gell-Mann proposed quarks to bring order to the "particle zoo" of hadrons discovered in accelerators. He took the name from a line in James Joyce's Finnegans Wake. Electron-scattering experiments at Stanford in

1968

found point-like particles inside the proton.

Fun factThe top quark is heavy. The top quark, found at Fermilab in

1995

, has a mass about

185

times that of a proton, roughly as heavy as a whole gold atom. It's so short-lived (about

5\times10^{-25}

s) that it decays before it can join with any other quark.

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