LearnYPH11WPH14 · Unit 4: Further Mechanics, Fields and ParticlesNuclear and particle physicsParticle interactions

Nuclear and particle physics · Subtopic 3 of 4

Particle Interactions

Every particle has an antiparticle with the same mass and opposite charge. They can annihilate into photons, and a photon with enough energy can create a particle–antiparticle pair: $E_{\min}=2mc^2$ . Forces act by exchange particles (photon, W bosons, gluons). A reaction can happen only if charge, baryon number and lepton number are all conserved; strangeness is conserved in strong interactions.

Edexcel YPH11 WPH14AQA 7408 Paper 1≈ 45 minRevision 1 · updated 2026-10-27

Section 1 of 14

Learning objectives

Describe annihilation and pair production and calculate photon energies.
Name the four interactions and their exchange particles.
Draw simple interaction diagrams for β decay and electron capture.
Use conservation laws to decide whether a reaction can occur.

Pause and think

A PET scanner detects two gamma photons flying apart in opposite directions. Why opposite?

Section 2 of 14

Core idea

Start hereMatter from light

A gamma-ray photon passes close to a nucleus and vanishes. In its place appear an electron and a positron. Energy has turned into matter, and antimatter, exactly as $E=mc^2$ allows.

The big idea

Particles interact by exchanging other particles. Every reaction must conserve charge, baryon number and lepton number (and strangeness, for strong interactions).

1. Antimatter

Threshold for pair production

$E_\gamma\ge m c^2+mc^2$
Both particles need their rest energy.
$E_{\min}=2mc^2$
Any extra becomes kinetic energy.
$e^-e^+:\ 2\times0.511=1.022\text{ MeV}$
Electron rest energy.

A nucleus nearby takes up momentum, which is why pair production happens near nuclei.

Worked exampleAnnihilation photons

An electron and positron at rest annihilate.

Goal: Photon frequency.
Strategy: Each photon gets $mc^2$ .

$E=0.511\text{ MeV}=8.19\times10^{-14}\text{ J}$
Rest energy each.
$f=\frac Eh=\frac{8.19\times10^{-14}}{6.63\times10^{-34}}$
$E=hf$ .
$f=1.24\times10^{20}\text{ Hz}$
Gamma rays.

AnswerTwo

1.24\times10^{20}

Hz photons, back to back.

✓ CheckZero total momentum before and after ✓.

2. Forces and conservation

Interactions and exchange particles

Drawing diagram…

Each interaction has its own carrier.

Worked exampleTesting a reaction

Is $p+p\to p+\pi^+$ possible?

Goal: Yes or no.
Strategy: Tabulate $Q$ , $B$ , $L$ .

$Q:\ 2\to2\ \checkmark$
Charge.
$B:\ 2\to1\ \times$
Pions are mesons, $B=0$ .

AnswerForbidden: baryon number not conserved.

✓ Check

p+p\to p+n+\pi^+

would conserve

B

✓.

Common misconceptionNeutrino or antineutrino? β⁻ makes an electron (

L=+1

), so it must also make an antineutrino (

L=-1

Key takeawayStrangeness Strange particles are made in pairs by the strong interaction but decay one at a time by the weak interaction.

Pause and think

Why is electron capture ( $p+e^-\to n+\nu_e$ ) a weak interaction?

Summary

$E_{\min}=2mc^2$ .
Exchange particles carry forces.
Conserve $Q$ , $B$ , $L$ .
$S$ conserved only in strong.

Antiparticle: Same mass, opposite charge and quantum numbers.
Annihilation: Particle and antiparticle become photons.
Exchange particle: Virtual particle that carries an interaction.

HistoryDirac and the positron. Paul Dirac's 1928 equation predicted a positive electron. Carl Anderson photographed one in a cloud chamber in 1932, and antimatter became real.

Fun factBananas emit antimatter. Potassium-40 in a banana occasionally decays by β

^+

, emitting a positron roughly once every 75 minutes.

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