Fundamental interactions and exchange particlesAQA A-Level Physics: Revision notes

All forces in nature arise from four fundamental interactions:

• gravity: acts between all particles with mass
• electromagnetic: acts between charged particles
• weak nuclear: acts on all types of particle and is responsible for beta decay
• strong nuclear (the strong interaction): acts between hadrons, such as the protons and neutrons in a nucleus

Only the electromagnetic and weak interactions are needed in detail here.

Forces between particles are explained by the exchange of exchange particles. One particle emits an exchange particle and the other absorbs it. The exchange transfers momentum, and this produces the force.

Exchange particles are virtual: they exist only briefly and cannot be observed directly.

The electromagnetic force between charged particles is due to the exchange of virtual photons. The photon has no charge, so the particles that exchange it keep their identity.

The weak interaction is mediated by the W⁺ and W⁻ bosons, which carry charge, so a particle that emits one changes type and its charge changes.

You need four weak processes:

• beta-minus decay: n → p + e⁻ + $\bar{\nu}_e$ (W⁻)
• beta-plus decay: p → n + e⁺ + $\nu_e$ (W⁺)
• electron capture: p + e⁻ → n + $\nu_e$ (W⁺)
• electron–proton collision: e⁻ + p → n + $\nu_e$ (W⁺), the same reaction as electron capture

Charge is conserved in each. The gluon, Z⁰ and graviton are not tested.

Simple diagrams show incoming particles, outgoing particles and the exchange particle. Time runs in one direction (usually upwards).

For each weak process, draw the incoming particles at the bottom, the outgoing particles at the top and a wavy line for the W boson joining the two particle lines.

• beta-minus: the neutron line becomes a proton line; the W⁻ becomes e⁻ and $\bar{\nu}_e$
• electron capture: the proton line becomes a neutron line; a W⁺ goes to the electron line, which becomes $\nu_e$

Check that the charge is the same at the top and bottom of each diagram.