Particle interactions

AQA·Physics·3.2.1.4

Fundamental interactions

A fundamental interaction is one of the basic ways in which particles or objects affect one another. There are four fundamental interactions:

  • Gravitational: acts between masses and attracts them towards one another.
  • Electromagnetic: acts between electrically charged particles. It can attract or repel them, depending on their charges.
  • Weak nuclear: causes processes including beta decay, in which a particle in an atomic nucleus changes and a beta particle is emitted.
  • Strong nuclear: binds the constituents of atomic nuclei together. It is also called the strong interaction.

The strong nuclear interaction holds the nucleus together despite the electromagnetic repulsion between its positively charged protons.

Exchange particles

When elementary particles interact, their energy and momentum can change. One way to represent the interaction is to describe the particles as exchanging another particle. This exchange particle transfers energy and momentum between the interacting particles.

An exchange particle is virtual: it is an internal part of the description of the interaction, not an incoming or outgoing particle detected independently. It should not be pictured as a particle that can be observed travelling freely between the interacting particles.

In an electromagnetic interaction, the exchange particle is a virtual photon. For example, the electromagnetic repulsion between two electrons can be represented by the exchange of a virtual photon. The transfer of momentum changes the electrons’ motion, providing a model for the force between them.

Beta decay

In beta decay, a neutron changes into a proton or a proton changes into a neutron. Both changes occur through the weak interaction and involve an intermediate $$W$$ particle.

A neutron has zero charge, while a proton has charge $$+1$$ in units of the elementary charge. The $$W$$ particle carries the charge needed to conserve charge when the nucleon changes type. It then produces two other particles.

Beta-minus decay

In beta-minus decay, a neutron changes into a proton and emits a $$W^-$$ particle. The $$W^-$$ produces an electron and an electron antineutrino:

n→p+W−W−→e−+νˉe\begin{aligned} n &\rightarrow p + W^- \\ W^- &\rightarrow e^- + \bar{\nu}_e \end{aligned}

Charge is conserved at each stage: the initial charge $$0$$ becomes $$+1 + (-1)$$, and the $$W^-$$ charge becomes $$-1 + 0$$. The electron antineutrino, $$\bar{\nu}_e$$, has zero charge.

Beta-plus decay

In beta-plus decay, a proton changes into a neutron and emits a $$W^+$$ particle. The $$W^+$$ produces a positron and an electron neutrino:

p→n+W+W+→e++νe\begin{aligned} p &\rightarrow n + W^+ \\ W^+ &\rightarrow e^+ + \nu_e \end{aligned}

Here the initial charge $$+1$$ becomes $$0 + 1$$, and the $$W^+$$ charge becomes $$+1 + 0$$. The positron has charge $$+1$$, while the electron neutrino, $$\nu_e$$, has zero charge.

The $$W^-$$ and $$W^+$$ are virtual exchange particles, not ordinary freely travelling particles. They represent the intermediate action of the weak interaction in these decays.

Electron capture and electron–proton collisions

In electron capture, a proton in a nucleus interacts with an electron captured from one of the atom’s electron shells. The proton changes into a neutron, and an electron neutrino is produced.

An electron–proton collision can produce the same particle transformation, provided sufficient energy is available:

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

Here, $$e^-$$ is an electron, $$p$$ is a proton, $$n$$ is a neutron and $$\nu_e$$ is an electron neutrino. In electron capture, the proton is part of a nucleus; in the collision, the same particle changes are represented by the equation above.

Weak-interaction representations

The transformation occurs through the weak interaction. It can be represented in either of two equivalent ways:

  • The electron emits a virtual $$W^-$$ boson and becomes an electron neutrino: $$e^- \rightarrow \nu_e + W^-$$. The proton absorbs the $$W^-$$ and becomes a neutron: $$p + W^- \rightarrow n$$.
  • The proton emits a virtual $$W^+$$ boson and becomes a neutron: $$p \rightarrow n + W^+$$. The electron absorbs the $$W^+$$ and becomes an electron neutrino: $$e^- + W^+ \rightarrow \nu_e$$.

These are alternative representations of the same interaction, not two separate transformations that both have to occur. In each representation, charge is conserved at both interaction points.

Particle interaction diagrams

A particle interaction diagram shows how particles enter an interaction, connect with one another and leave. Each particle is represented by a labelled line. A line entering the diagram represents an incoming particle; a line leaving it represents an outgoing particle.

Lines meet at vertices. A vertex marks a point in the interaction where a particle is emitted or absorbed, or where particles change into other particles. A chosen direction through the diagram represents the progression of the interaction. For example, if progression is from left to right, incoming lines are on the left and outgoing lines are on the right.

An exchanged virtual particle is represented by a line joining two vertices. It is emitted at one vertex and absorbed at the other, so it does not appear as an independently incoming or outgoing particle.

Reading a particle interaction diagram

The following sketch represents two electrons interacting by exchanging a virtual photon. The chosen direction of progression is from left to right.

incoming electron ───●──────── outgoing electron
                      ╲
                       ╲ virtual photon
                        ╲
incoming electron ───────●──── outgoing electron

At the first vertex, an electron emits the virtual photon and continues as an electron. At the second vertex, another electron absorbs the photon and continues as an electron. The virtual-photon line connects the two vertices; it is not an additional particle entering or leaving the whole interaction.

Particle labels and charge at vertices

The labels and connections must describe the particle changes consistently. In particular, electric charge is conserved at each vertex: the total charge entering a vertex must equal the total charge leaving it.

In the electron example, each electron has charge $$-e$$, where $$e$$ is the magnitude of the elementary charge, and the photon has zero charge. At the emission vertex, charge $$-e$$ enters and charge $$-e + 0$$ leaves. At the absorption vertex, charge $$-e + 0$$ enters and charge $$-e$$ leaves. Charge is therefore conserved at both vertices.

The lines show the structure of the interaction: which particles meet and how the vertices are connected. They are not literal particle tracks drawn to scale.