Particle interactions — AQA A-Level Physics

Particle interactions are the processes by which fundamental particles exert forces on one another through the exchange of gauge bosons. This topic covers the four fundamental forces (strong nuclear, weak nuclear, electromagnetic, and gravitational), their relative strengths and ranges, and the particles that mediate them. You will also learn how to represent these interactions using Feynman diagrams, which are essential tools for visualising and analysing particle physics processes.

Understanding particle interactions is crucial for explaining phenomena such as radioactive decay, particle annihilation, and scattering experiments. It builds on your knowledge of particle classification (quarks, leptons, and hadrons) and conservation laws (charge, baryon number, lepton number, strangeness). This topic frequently appears in AQA A-Level exams, often requiring you to draw or interpret Feynman diagrams, identify exchange particles, and apply conservation rules to determine whether a proposed interaction is possible.

• →The four fundamental forces are the strong nuclear force (mediated by gluons), weak nuclear force (mediated by W+, W-, and Z0 bosons), electromagnetic force (mediated by photons), and gravitational force (mediated by hypothetical gravitons).
• →Exchange particles (gauge bosons) are virtual particles that transfer energy and momentum between interacting particles, and their mass determines the range of the force: heavier exchange particles mean a shorter range.
• →Feynman diagrams represent particle interactions with time flowing from left to right; fermions are drawn as straight lines with arrows, and bosons as wavy or dashed lines.
• →Conservation laws must be satisfied in all particle interactions: conservation of charge, baryon number, lepton number, and strangeness (strangeness is conserved in strong and electromagnetic interactions but not in weak interactions).
• →The weak interaction is unique in that it can change quark flavour, enabling processes such as beta decay and neutrino absorption.

• State that gravity acts between all masses and is the weakest of the four interactions, with infinite range.
• State that the electromagnetic interaction acts between charged particles and has infinite range.
• State that the weak nuclear interaction is responsible for beta decay and has a very short range.
• State that the strong nuclear interaction binds quarks and holds nucleons together, with range about 10⁻¹⁵ m.
• Explain that forces between particles are transmitted by the exchange of virtual particles, such as virtual photons for the electromagnetic force.
• Relate the range of an interaction to the mass of its exchange particle, with heavier exchange particles giving shorter range.
• State that the electromagnetic force acts between charged particles.
• Identify the virtual photon as the exchange particle for the electromagnetic force.
• Explain that a charged particle emits a virtual photon which is absorbed by another charged particle, transferring momentum.
• State that virtual photons are not directly observable and exist only during the interaction.
• Relate the infinite range of the electromagnetic force to the zero rest mass of the photon.
• Incoming particles are drawn at the bottom and outgoing particles at the top, with time increasing upwards.
• The exchange particle is drawn as a wavy line connecting two vertices, not as a solid particle line.
• Each vertex conserves charge, baryon number and lepton number; the diagram must be consistent with these conservation rules.
• The correct exchange particle is chosen: virtual photon for electromagnetic, W⁺ or W⁻ for weak charged-current interactions.
• For beta-plus decay, a proton changes into a neutron, emitting a W⁺ that decays into a positron and an electron neutrino.
• For electron-proton collisions, a proton and an electron interact via a W⁻ boson to produce a neutron and an electron neutrino.

• 💡Learn a table of the four interactions with their relative strengths, ranges and exchange particles.
• 💡When explaining exchange particles, always state that a virtual particle is emitted by one particle and absorbed by another, transferring momentum.
• 💡Use the phrase 'strong nuclear force may also be called the strong interaction' to show awareness of alternative terminology.
• 💡Link the range of an interaction to the mass of its exchange particle: heavier exchange particles give shorter range.
• 💡Remember that the exchange particle for the electromagnetic force is the virtual photon.
• 💡When explaining the force, describe the emission and absorption of a virtual photon between two charged particles.
• 💡Link the zero rest mass of the photon to the infinite range of the electromagnetic force.
• 💡Use the term 'virtual photon' rather than just 'photon' when discussing exchange particles.
• 💡Sketch the diagram roughly first, then check charge and baryon number at every vertex before finalising it.
• 💡Label each particle with its symbol and ensure the W boson has the correct charge sign (W⁺ or W⁻) to conserve charge at the first vertex.
• 💡When drawing Feynman diagrams, always label the exchange particle and ensure that charge, baryon number, and lepton number are conserved at each vertex; examiners award marks for correct arrows and labels.
• 💡For questions asking you to determine whether an interaction is possible, systematically check each conservation law (charge, baryon number, lepton number, strangeness) and state clearly which law is violated if the interaction is forbidden.
• 💡Learn the relative strengths and ranges of the four fundamental forces: strong (strength 1, range 10^-15 m), electromagnetic (strength 10^-2, infinite range), weak (strength 10^-6, range 10^-18 m), and gravitational (strength 10^-39, infinite range).

• Confusing the strong nuclear force with the weak nuclear force: the strong force binds quarks and nucleons, while the weak force causes beta decay.
• Thinking gravity is the strongest force because it dominates on large scales; in fact it is the weakest but acts over infinite range and on all masses.
• Believing exchange particles are real detectable particles; they are virtual and exist only during the interaction.
• Forgetting that the electromagnetic force acts only between charged particles, not between all particles.
• Thinking the electromagnetic force acts between all particles; it acts only between charged particles.
• Believing virtual photons are real photons that can be detected; they are virtual and only mediate the force.
• Confusing the exchange particle of the electromagnetic force with that of the weak interaction (W and Z bosons).
• Forgetting that the photon has zero rest mass, which allows the electromagnetic force to have infinite range.
• Drawing the exchange particle as a solid line like a fermion: the correction is to use a wavy line for bosons so it is visually distinct.
• Confusing electron capture and electron-proton collisions: the correction is to note that electron capture involves an atomic electron and a W⁺ boson from the proton, whereas a collision involves a free electron and a W⁻ boson from the electron.
• Using a photon as the exchange particle for beta decay: the correction is to use a W⁻ or W⁺ boson because beta decay is a weak interaction.
• Students often think that the strong nuclear force acts between all nucleons equally, but it actually acts between quarks via gluons and only between nucleons as a residual effect; it is also charge-independent and has a very short range.
• Many students believe that exchange particles are real particles that can be detected, but they are virtual particles that exist only during the interaction and cannot be observed directly.
• Students frequently assume that strangeness is always conserved, but it is not conserved in weak interactions, which is why strange particles decay via the weak force.

1. 1Week 1, Day 1-2: Review the four fundamental forces and their exchange particles. Create a table summarising the force, exchange particle, relative strength, and range.
2. 2Week 1, Day 3-4: Learn the rules for drawing Feynman diagrams. Practice drawing diagrams for beta decay, electron capture, and annihilation, ensuring correct arrows and labels.
3. 3Week 1, Day 5-7: Apply conservation laws to a variety of particle interactions. For each example, check charge, baryon number, lepton number, and strangeness, and determine if the interaction is allowed.
4. 4Week 2, Day 1-3: Work through past paper questions on particle interactions, focusing on Feynman diagrams and conservation law checks. Mark your answers using the official mark scheme.
5. 5Week 2, Day 4-5: Create flashcards for key definitions and exchange particles. Use active recall to test yourself on the strengths and ranges of forces and the conditions for allowed interactions.

• 📋Feynman diagram drawing: You may be asked to draw a Feynman diagram for a given interaction, such as beta decay or electron-positron annihilation. Ensure time flows left to right, label exchange particles, and include arrows for particle direction.
• 📋Conservation law checks: You may be given a proposed interaction and asked to determine whether it is possible by checking conservation of charge, baryon number, lepton number, and strangeness. State clearly which law is violated if it is forbidden.
• 📋Exchange particle identification: You may be asked to identify the exchange particle for a specific force or interaction, or to explain how the exchange particle model accounts for the range and strength of a force.
• 📋Calculation of energy or momentum in particle interactions: You may need to use E = mc^2 or conservation of momentum to calculate the energy or momentum of products in a decay or annihilation.