3.2.1.4 - Particle Interactions

3.2.1.4 - Particle Interactions

When particles attract, repel, or change identity, physicists describe the process as an interaction. This lesson focuses on the four fundamental interactions, the idea of exchange particles, and the simple electromagnetic and weak-interaction diagrams you need to recognise for A-Level Physics.

Part 1 - Four Fundamental Interactions

There are four fundamental interactions in nature: gravity, electromagnetic, strong nuclear, and weak nuclear. Every other force you meet in everyday physics can be traced back to one of these.

Fundamental interaction

A fundamental interaction is one of the four basic forces of nature: gravity, electromagnetic, strong nuclear, or weak nuclear.

The key features are summarised below.

InteractionWhat it doesTypical rangeExchange particle you need here
GravityActs between particles with mass and is always attractiveInfiniteNot required
ElectromagneticActs between charged particles and can attract or repelInfiniteVirtual photon
Strong nuclearActs between hadrons and binds nucleons in nucleiAbout 3×10−15 m3 \times 10^{-15}\,\text{m}Not required
Weak nuclearChanges one particle into another in beta processesAbout 10−18 m10^{-18}\,\text{m}W+W^+ and W−W^- bosons

The names gluon, Z0Z^0, and graviton are not required knowledge for this specification point. The learner-facing names you do need are the virtual photon for the electromagnetic interaction and the W+W^+ and W−W^- bosons for the weak interaction.

Part 2 - Exchange Particles and the Electromagnetic Interaction

Forces between particles are explained using exchange particles. One particle emits an exchange particle and another absorbs it, so energy and momentum are transferred between them.

Exchange particle

An exchange particle is a particle passed between interacting particles to transfer energy and momentum and so mediate a force.

For the electromagnetic interaction, the exchange particle is a virtual photon. A virtual photon is not a real detected photon travelling to a detector. It is the exchange particle used in the interaction model.

In the three diagrams below, notice that the particles keep the same identity before and after the interaction while the virtual photon is the exchanged particle in every case.

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Diagram
In an electromagnetic interaction diagram, the particles entering and leaving are the same particles. A proton remains a proton, an electron remains an electron, and only their momenta change. The direction of the lines shows incoming and outgoing particles, while the wavy line shows the virtual photon being exchanged.

Electromagnetic interaction diagrams show the same charged particles before and after the interaction. The exchanged particle is always a virtual photon.

Part 3 - The Weak Interaction and Beta Decay

The weak interaction is different because it can change one particle into another. This is why it is responsible for beta decay. The exchange particles are the charged WW bosons, so the weak interaction is much shorter range than the electromagnetic interaction.

The two beta-decay processes you need are:

Beta-minus decay

n→p+e−+νˉen \rightarrow p + e^- + \bar{\nu}_e

In beta-minus decay, a neutron becomes a proton and emits a W−W^- boson at the first vertex. The W−W^- then produces the electron and the electron antineutrino at the second vertex.

In the beta-minus diagram below, notice the two vertices: the neutron changes into a proton first, and the emitted W−W^- then creates the electron and electron antineutrino.

[DIAGRAM: asset_name: 2.1.4 - Particle Interactions - Diagram 2; asset_slug: 2.1.4 - Particle Interactions - Diagram 2; recommended_method: retained_png; description: A simple beta-minus interaction diagram with a neutron line entering, a proton line leaving from the first vertex, a wavy line labelled W- heading to a second vertex, and electron plus electron antineutrino lines leaving that second vertex]
Diagram

Beta-plus decay

p→n+e++νep \rightarrow n + e^+ + \nu_e

In beta-plus decay, a proton becomes a neutron and emits a W+W^+ boson. The W+W^+ then produces the positron and the electron neutrino.

In the beta-plus diagram below, notice the same two-vertex structure, but this time the proton changes into a neutron and the exchanged particle is a W+W^+ instead of a W−W^-.

[DIAGRAM: asset_name: 2.1.4 - Particle Interactions - Diagram 3; asset_slug: 2.1.4 - Particle Interactions - Diagram 3; recommended_method: retained_png; description: A simple beta-plus interaction diagram with a proton line entering, a neutron line leaving from the first vertex, a wavy line labelled W+ heading to a second vertex, and positron plus electron neutrino lines leaving that second vertex]
Diagram
Because the WW bosons are charged and have non-zero rest mass, they behave very differently from virtual photons.

Reading Beta-Minus Interaction Diagrams

Those differences are exactly why a weak-interaction diagram is usually easy to spot. The next question checks whether you can read the particle changes in the beta-minus diagram itself.

Part 4 - Electron Capture and Electron-Proton Collisions

The weak interaction is also responsible for electron capture and electron-proton collisions. In both cases the overall equation is the same:

Electron capture and electron-proton collision

p+e−→n+νep + e^- \rightarrow n + \nu_e

However, the diagrams are not the same because the exchange particle is different.

In electron capture, the proton emits a W+W^+ boson and becomes a neutron. The electron absorbs the W+W^+ and becomes an electron neutrino.

In an electron-proton collision, the electron emits a W−W^- boson and becomes an electron neutrino. The proton absorbs the W−W^- and becomes a neutron.

Compare the two diagrams below and notice that the overall reaction is the same in both cases, but the WW boson has opposite charge and is emitted by a different particle.

[DIAGRAM: asset_name: 2.1.4 - Particle Interactions - Diagram 4; asset_slug: 2.1.4 - Particle Interactions - Diagram 4; recommended_method: retained_png; description: Two simple diagrams side by side. Left: electron capture with a proton line changing to a neutron and sending W+ to an electron line that changes to a neutrino. Right: electron-proton collision with an electron line changing to a neutrino and sending W- to a proton line that changes to a neutron]
Diagram

PET scanning depends on the weak interaction. A proton-rich tracer undergoes beta-plus decay inside the body, using a W+W^+ boson to change a proton into a neutron and emit a positron. That positron later annihilates with an electron, producing the photons detected by the scanner.

The important exam point is that the overall reaction can look identical while the exchange particle and its direction are different.

Part 5 - Reading Interaction Diagrams

To read a simple interaction diagram well, always ask three questions. Which particles go in? Which particles come out? Which exchange particle is shown between the vertices? For this topic, that is enough to identify the process cleanly.

The weak-interaction processes you need are summarised here:

ProcessOverall equationExchange particle
Beta-minus decayn→p+e−+νˉen \rightarrow p + e^- + \bar{\nu}_eW−W^-
Beta-plus decayp→n+e++νep \rightarrow n + e^+ + \nu_eW+W^+
Electron capturep+e−→n+νep + e^- \rightarrow n + \nu_eW+W^+
Electron-proton collisionp+e−→n+νep + e^- \rightarrow n + \nu_eW−W^-

Electromagnetic diagrams are easier because the particles do not change identity and the exchange particle is always the virtual photon. Weak-interaction diagrams need more care because the same overall reaction can be drawn in different ways depending on which particle emits the exchange particle.