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Particle interactions and exchange particles

Modern physics has four forces and one mechanism: particles exert forces by exchanging other particles. The photon carries electromagnetism, the W bosons carry the weak interaction, and simple diagrams keep the bookkeeping honest.

Year 12AQA 3.2.1.4

Builds on Stable and unstable nuclei and Antimatter and photons.

IN THIS TOPIC

Name the four fundamental interactions and the exchange-particle concept.
Identify the virtual photon and the W bosons as the exchange particles of the electromagnetic and weak interactions.
Draw and interpret simple diagrams for β⁻ and β⁺ decay, electron capture and electron-proton collisions.

WHAT YOU PROBABLY THINK

When two things repel, nothing passes between them.

Four forces, one mechanism

Every interaction in nature belongs to one of four fundamental interactions: gravity, the electromagnetic interaction, the weak nuclear interaction and the strong nuclear interaction. The modern picture gives them all the same mechanism: two particles exert a force by exchanging an exchange particle, a short-lived messenger that carries energy and momentum between them, a little like two skaters pushed apart by throwing a heavy ball back and forth.

AQA's scope fence, worth knowing exactly: the gluon (strong), the Z⁰ and the graviton will not be tested. The two exchanges you must know are the photon and the W bosons.

Electromagnetism: the virtual photon

Charged particles interact by exchanging virtual photons: photons that exist only for the duration of the exchange and are never observed directly.

FIG. 1Two electrons repel by exchanging a virtual photon. The wavy line is the exchange; the kinks are the force being felt.

The diagram convention, which AQA calls a simple interaction diagram, reads with time running upward: solid lines for the incoming and outgoing particles, a wavy line for the exchange, and every junction balancing charge.

The weak interaction and the W bosons

The weak interaction is the only force that can change a particle's type, turning a neutron into a proton or the reverse, and its exchange particles are the charged W⁺ and W⁻ bosons. AQA limits it to four processes. Beta-minus decay: a neutron becomes a proton, the W⁻ carrying away the charge before becoming an electron and an electron antineutrino.

FIG. 2The simple diagram for beta-minus decay: n becomes p at the vertex, and the W minus becomes the electron and the electron antineutrino.

Beta-plus decay mirrors it: a proton becomes a neutron via a W⁺, which becomes a positron and an electron neutrino. Electron capture: a proton absorbs one of the atom's own inner electrons, becoming a neutron and emitting an electron neutrino, with the W⁺ as the exchange. And an electron-proton collision achieves the same conversion when a free electron strikes a proton: out come a neutron and a neutrino, exchanged through a W boson. In every diagram the giveaway that the weak interaction is at work is a particle changing type.

THE EXAM BIT

Name all four interactions when asked: gravity, electromagnetic, weak nuclear, strong nuclear. Leaving gravity out because it feels non-particle is the common slip.
Match exchange to force without hesitation: virtual photon for electromagnetic, W⁺/W⁻ for weak. The gluon, Z⁰ and graviton are explicitly untested.
In a simple diagram, check charge balance at every junction. The W's charge is exactly the difference it carries: W⁻ in β⁻, W⁺ in β⁺ and electron capture.
Neutrino flavours by process: β⁻ emits an electron antineutrino; β⁺, electron capture and electron-proton collisions emit an electron neutrino.
“Which interaction is responsible?”: if a quark-level identity changed, n to p or p to n, the answer is the weak interaction, and no other force can do it.

CHECK YOURSELF

In electron capture, a proton in a nucleus absorbs an atomic electron. Write the particle equation, name the exchange particle, and explain how you know the weak interaction is responsible.

Show a hint

What does the proton become, and what must leave to balance the leptons?

Show the answer

p + e⁻ → n + ν_e: the proton becomes a neutron and an electron neutrino leaves. Charge balances: +1 − 1 = 0.

The exchange particle is the W⁺, passed from the proton to the electron.

A proton changed into a neutron, and changing a particle's type is something only the weak interaction can do; neither the electromagnetic nor the strong interaction ever alters identity.

Forces are carried, not conjured.

Only the weak interaction changes what a particle is.

No animated video for this topic yet; these notes stand alone. InkPhysics on YouTube.