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Particle interactions and exchange particles
There are four fundamental interactions, and three of them work the same way: particles exert forces on one another by exchanging other particles. The photon carries the electromagnetic interaction and the W bosons carry the weak interaction, while gravity has no tested quantum description. Feynman diagrams track what is conserved at each vertex.
Pick your board and the few notes written for the other boards quietly fold away, here and in the practice players. Nothing is deleted: every folded piece reopens on a tap.
Builds on Stable and unstable nuclei and Antimatter and photons.
IN THIS TOPIC
- Name the four fundamental interactions, explain what an exchange particle does, and say why gravity sits outside the tested picture.
- 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.
- Choose the right W boson for a given process, and say why the weak interaction is the one at work.
COMMON MISCONCEPTION
When two things repel, nothing passes between them.
How forces are carried
Every interaction in nature belongs to one of four fundamental interactions, which are gravity, the electromagnetic interaction, the weak nuclear interaction and the strong nuclear interaction. For three of them the modern picture is settled and tested to great precision.
Two particles exert a force by exchanging an exchange particle, carrying energy and momentum between them, a little like two skaters shoving each other apart by throwing a heavy ball back and forth. The skaters are an analogy with limits: an exchanged particle is virtual, an internal step in the quantum calculation rather than an object that could be caught in flight, and the ball says nothing about how attraction works. Those three interactions make up the Standard Model.
Gravity is the odd one out, and the position is worth stating plainly. There is no accepted quantum theory of gravity, and the graviton is a proposed exchange particle that has never been detected and may not exist as described. General relativity, which describes gravity as curved spacetime rather than as an exchange, is the working theory instead. Treat the exchange account of gravity as conjecture and the other three as established physics.
Two of those messengers carry everything in this unit, the photon for the electromagnetic interaction and the W bosons for the weak. The gluon of the strong interaction and the Z0 are established particles, the Z0 observed directly in accelerator experiments and the gluon inferred from evidence such as three-jet events, but no question here turns on them, and the graviton stays the proposal it was above.
Electromagnetism: the virtual photon
Charged particles interact by exchanging virtual photons, photons that live only for the duration of the exchange and are never observed directly.
The diagram convention, the simple interaction diagram, reads with time running upward. Solid lines carry the incoming and outgoing particles, a wavy line carries the exchange, and charge balances at every junction.
The weak interaction and the W bosons
The weak interaction is the only force that can change a quark from one flavour to another, turning a neutron into a proton or the reverse, and its exchange particles are the charged W+ and W− bosons. Be precise about the monopoly. Strong and electromagnetic processes rearrange particles freely, and pair production converts a photon's energy into two new particles, but neither of them ever converts a down quark into an up. Four processes show the weak interaction at work. Take beta-minus decay first, where a neutron becomes a proton and the W− carries away the charge before turning into an electron and an electron antineutrino.
Beta-plus decay mirrors it, a proton becoming a neutron via a W+, which then becomes a positron and an electron neutrino. In electron capture a proton absorbs one of the atom's own inner electrons, becoming a neutron and emitting an electron neutrino, and the exchange particle is the W+. An electron-proton collision reaches the same end state when a free electron strikes a proton, giving a neutron and a neutrino, but here the exchange runs the other way and the boson is a W−. Those last two catch people out every year, so learn them as a pair. In every one of the four diagrams, the giveaway is a quark changing flavour.
WORKED EXAMPLE
Which interaction, and why?
A free neutron decays. Name the interaction responsible, the exchange particle, and justify both.
Ask the diagnostic question first. Does any particle change type? It does, since the neutron becomes a proton, which at quark level is one d becoming a u. Flavour change is the weak interaction's exclusive business, so the answer is weak.
The exchange particle carries the charge difference at the vertex. The quark line loses one negative third and gains two thirds, so one unit of negative charge has to leave, and the boson is the W−.
That W− becomes the electron and the electron antineutrino, giving n → p + e− + e. Check both books. Charge runs 0 → +1 − 1 + 0, and lepton number runs 0 → 0 + 1 − 1. Both balance, so the decay is allowed.
GUIDED PRACTICE
Build the beta-plus equation
In beta-plus decay a proton inside a nucleus becomes a neutron. Decide which quark changes flavour, which W boson is exchanged and why, then write the full particle equation and check its charge and lepton number.
Show the working
At quark level a u becomes a d, so the quark line sheds positive charge and the boson carrying it away is the W+.
The W+ becomes the positron and the electron neutrino, giving p → n + e+ + νe.
Audit both books. Charge runs +1 → 0 + 1 + 0. Lepton number runs 0 → 0 − 1 + 1, the positron counting as an antilepton. Both balance.
INDEPENDENT PRACTICE
Not everything is weak
Two protons drift towards each other and are pushed apart without touching. Name the exchange particle responsible for the repulsion, and give two reasons this interaction cannot be the weak one.
Show the working
The repulsion is electromagnetic, carried by the virtual photon.
First, no particle changes type anywhere in the event, and flavour change is the weak interaction's signature job. Second, the electromagnetic force has infinite range, which matches its massless exchange particle, while the large mass of the W bosons confines the weak interaction to a range smaller than a nucleus.
ASSESSMENT FOCUS
- Name all four interactions when asked. Gravity, electromagnetic, weak nuclear, strong nuclear. Leaving gravity out because it feels unlike the others is the common slip.
- Match exchange to force without hesitation. Virtual photon for electromagnetic, W+ or W− for weak. The gluon and the Z0 are established particles, but nothing at this level turns on them, so do not volunteer them. The graviton is the hypothetical one: if it has to be named, name it as proposed, since none has ever been detected.
- In a simple interaction diagram, which is the name AQA uses for it, check charge balance at every junction. The W carries exactly the difference, so it is W− in β− and W+ in β+.
- Learn the awkward pair separately. Electron capture uses the W+, emitted by the proton. An electron-proton collision uses the W−, emitted by the electron. Same products, different boson, and a mark rides on it.
- Get the neutrino flavours right by process. β− emits an electron antineutrino. β+, electron capture and electron-proton collisions all emit an electron neutrino.
- Asked which interaction is responsible, look for a change of quark flavour. If n became p, or p became 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 + νe, so the proton becomes a neutron and an electron neutrino leaves. Charge balances, +1 − 1 = 0 on the left and 0 on the right.
The exchange particle is the W+, passed from the proton to the electron. Note the contrast with an electron-proton collision, where the same end state comes from a W−.
A proton changed into a neutron, so a u quark became a d, and only the weak interaction changes quark flavour. Neither the electromagnetic nor the strong interaction can do it.
Forces are carried, not conjured, in all three Standard Model interactions.
Gravity has no tested exchange particle, and the graviton stays a proposal.
Only the weak interaction changes quark flavour.
WORKBOOK
Printable practice for this topic: original exam-style questions with room to work, and a fully worked answer book. Free to use; please do not redistribute or sell.
Or read them with their mark schemes on the particle interactions and exchange particles questions page.
CHECK YOUR PROGRESS
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- Name the four fundamental interactions, explain what an exchange particle does, and say why gravity sits outside the tested picture.
- 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.
- Choose the right W boson for a given process, and say why the weak interaction is the one at work.
Open the full revision checklist to track your progress across the whole unit.