Physics • Year 12 • Module 8 • Lesson 16
Particles and Antiparticles
Lock in the core vocabulary of antimatter, annihilation, pair production, and the lepton–hadron classification before tackling harder questions.
1. Term–definition match
The definitions below are shuffled. In the right-hand column write the matching term from this list: antiparticle, annihilation, pair production, lepton, hadron, baryon, meson, positron, baryon number, lepton number. 10 marks (1 each)
| # | Definition | Matching term |
|---|---|---|
| 1.1 | A particle with the same rest mass as its partner but opposite charge and opposite quantum numbers (e.g. baryon number, lepton number). | |
| 1.2 | The process in which a particle and its antiparticle collide and convert their total mass-energy into photons or other particle pairs. | |
| 1.3 | The process in which a high-energy photon near a nucleus converts into a particle–antiparticle pair. | |
| 1.4 | A fundamental fermion that does not experience the strong nuclear force; includes the electron, muon, tau, and their associated neutrinos. | |
| 1.5 | A composite particle made of quarks that does experience the strong nuclear force; includes protons, neutrons, and pions. | |
| 1.6 | A hadron composed of three quarks (or three antiquarks); has baryon number ±1. Examples: proton, neutron. | |
| 1.7 | A hadron composed of a quark–antiquark pair; has baryon number 0. Examples: pion, kaon. | |
| 1.8 | The antiparticle of the electron: same mass (0.511 MeV/c²), but charge +1 e. Discovered by Carl Anderson in 1932. | |
| 1.9 | A conserved quantum number assigned +1 to baryons, −1 to antibaryons, and 0 to all other particles. | |
| 1.10 | A conserved quantum number that counts leptons minus antileptons in an interaction; there is a separate value for each lepton family. |
2. True or false — with correction
Circle T or F for each statement. If the statement is false, write the corrected version on the line below it. 12 marks (1 T/F + 1 correction each)
2.1 An antiparticle always has a different rest mass from its corresponding particle. T / F
2.2 When an electron and a positron at rest annihilate, exactly one photon is produced to conserve energy. T / F
2.3 The minimum photon energy required for electron–positron pair production is approximately 1.022 MeV. T / F
2.4 A proton is classified as a meson because it is composed of quarks. T / F
2.5 Lepton number is conserved in all known particle interactions. T / F
2.6 A pion (π+) is a lepton because it does not carry baryon number. T / F
3. Fill-in-the-blank paragraph
Use the word bank to complete the passage. Each word is used once. 8 marks (1 per blank)
Word bank:
annihilate · baryon · conservation · hadrons · lepton · momentum · nucleus · photons
Paul Dirac’s 1928 relativistic quantum theory predicted that for every particle there exists a corresponding antiparticle. When a particle meets its antiparticle, they ___________, converting their rest-mass energy into ___________. Electron–positron annihilation at rest produces two photons to satisfy the ___________ of ___________; a single photon could not recoil to balance the zero total momentum of the initial system. Pair production — the creation of a particle–antiparticle pair from a high-energy photon — requires the presence of a ___________ to absorb the recoil. All known matter particles are divided into ___________ (quark composites, subject to the strong force) and leptons. A ___________ such as the electron carries ___________ number and does not experience the strong nuclear force.
4. Function recall
Answer each question in 1–2 sentences using precise terms from the lesson. 8 marks (2 each)
4.1 What is the role of baryon number as a conservation law in particle physics?
4.2 Why must a nucleus be present for pair production to occur?
4.3 What distinguishes a lepton from a hadron?
4.4 State the quark composition of the proton and use it to explain why the proton has charge +1.
5. Build a concept map
Draw labelled arrows between the six terms below to show how they are related. Each arrow must carry a linking phrase (e.g. “produces”, “is the antiparticle of”, “conserves”, “requires”). Aim for at least 6 labelled arrows. 6 marks (1 per valid labelled arrow)
Supplied terms: electron · positron · annihilation · pair production · photon · conservation of momentum.
Q1 — Term–definition match
1.1 antiparticle • 1.2 annihilation • 1.3 pair production • 1.4 lepton • 1.5 hadron • 1.6 baryon • 1.7 meson • 1.8 positron • 1.9 baryon number • 1.10 lepton number.
Q2 — True / false with correction
2.1 False. An antiparticle has the same rest mass as its partner; it is the charge (and other quantum numbers such as baryon number and lepton number) that are opposite.
2.2 False. Two photons are produced, not one. A single photon cannot simultaneously conserve both energy and momentum in the centre-of-mass frame where the total initial momentum is zero.
2.3 True. The minimum photon energy equals the rest-mass energy of both particles: Emin = 2mec² = 2 × 0.511 MeV = 1.022 MeV.
2.4 False. A proton is a baryon (three quarks: uud), not a meson. Mesons are quark–antiquark pairs with baryon number 0.
2.5 True. Lepton number (separately for each lepton family) is conserved in all known interactions.
2.6 False. A pion is a meson (quark–antiquark pair), which is a type of hadron. Leptons are fundamental fermions that do not experience the strong force; pions do.
Q3 — Cloze paragraph
In order: annihilate / photons / conservation / momentum / nucleus / hadrons / lepton / lepton.
Q4.1 — Role of baryon number conservation
Baryon number is a quantum number assigned +1 to baryons, −1 to antibaryons, and 0 to all other particles. Its conservation means the net baryon number of a system cannot change in any interaction, which is why free protons do not spontaneously decay and why baryon–antibaryon pairs must be created together.
Q4.2 — Why a nucleus is needed for pair production
A nucleus (or heavy charged particle) must be present to absorb the recoil momentum. A photon alone carries energy but zero net momentum cannot be converted into a particle–antiparticle pair moving symmetrically unless a third body (the nucleus) absorbs the surplus momentum, satisfying both energy and momentum conservation.
Q4.3 — Lepton vs hadron
Leptons are fundamental, point-like fermions that do not experience the strong nuclear force and have no known substructure. Hadrons are composite particles made of quarks and do experience the strong force. Leptons carry lepton number; hadrons carry baryon number (baryons) or zero baryon number (mesons).
Q4.4 — Quark composition of the proton and its charge
The proton is made of two up quarks and one down quark (uud). Up quarks carry charge +⅔ e each; the down quark carries −⅓ e. Total charge: +⅔ + ⅔ − ⅓ = +1e, giving the proton its +1 elementary charge.
Q5 — Sample concept map
Correct maps should include arrows such as:
- electron — is the antiparticle of → positron
- electron + positron — undergo → annihilation
- annihilation — produces → photon (two photons)
- photon — undergoes → pair production
- pair production — requires → conservation of momentum
- conservation of momentum — explains why two photons form in → annihilation
Award 1 mark per valid labelled arrow (minimum 6, maximum 6 marked).