AQA A-level Physics 7408 · Paper 1 · Spec 3.2.1
Particles and Antiparticles
Build nuclei, make matter from light, draw weak interactions and test reactions against the conservation laws. Free simulations, exam questions and a printable booklet.
On this page: ● particle · ● antiparticle (bar over the symbol, or the opposite charge sign such as e+) · ∿ photon or W boson (wavy line) · in interaction diagrams, arrows point forward in time and an antiparticle’s arrow points backwards
What is inside an atom?
Choose a nucleus on the stability chart and watch the atom change: Its nuclide notation, charge and specific charge. Switch between the Bohr and quantum models of the electrons. Unstable nuclei can be made to decay. The second tab shows the strong nuclear force that holds the nucleus together.
Click the chart to choose a nucleus, or use the ▼ and ▲ buttons (hold to go faster).
Black dots are the stable nuclei: Together they make the line of stability, which bends away from the N = Z line for heavier nuclei. Above it nuclei have too many neutrons (β− decay); below it too many protons (β+ decay or electron capture); very heavy nuclei decay by alpha emission.
A nucleus, stable or unstable, can exist only in the shaded band. Outside it, the protons and neutrons cannot hold together: A nucleon would leave at once, so no nucleus can form there.
Stable nuclei and the decays of well-known radioactive nuclei are real data. Other nuclei follow the region they are in, a model prediction. Positron capture (n + e+ → p + νe) is allowed by the conservation laws but needs a place full of positrons, such as the early Universe. Radioactive decay and this chart come again in Nuclear physics (3.8.1).
Mass uses 1.67 × 10−27 kg per nucleon plus 9.11 × 10−31 kg per electron. e = 1.60 × 10−19 C.
Drag particle B towards particle A, then let go. Keyboard: Arrow keys on the picture.
Positive force = repulsive (pushes B away), negative = attractive. Electrostatic force from F = Q1Q24πε0r2 (exact). The strong force has no simple equation: Its values here are a model, right to an order of magnitude. The real motion lasts about 10−22 s; here it is slowed down enormously, and sped up further where the forces are tiny (shown on the picture).
This is a classical model. In reality only a proton and a neutron stay together as a pair: The deuteron, the nucleus of hydrogen-2. When they join, the extra energy leaves as a gamma photon (so the motion here is damped to let B settle). Two protons or two neutrons on their own do not stay together.
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The Particle Family
Every particle in this topic has a place on this chart. Click any box to see what it is, its values from the AQA data booklet and how it comes up in the exam. Switch to the wider Standard Model to see what lies beyond A-level.
QuarksLeptonsExchange particlesHadronsClick any box for details
The pion is a meson (made of quarks), but in the A-level model it is also the exchange particle of the strong force between nucleons, so it appears in both places. Rest energies are from the AQA Data and Formulae Booklet where it lists them.
Hadron Builder and Conservation Laws
Drag quarks into the circle and press Check to see whether they make a hadron. Then test reactions: A reaction can happen only if charge, baryon number and both lepton numbers balance. Strangeness tells you which interaction it uses.
Rules for a hadron
- Baryon: Three quarks (B = +1). Antibaryon: Three antiquarks (B = −1).
- Meson: One quark and one antiquark (B = 0).
- Each quark has B = +⅓ and each antiquark B = −⅓, so B must come out as a whole number. The charge Q must be a whole number too.
- Strangeness: s has S = −1, s has S = +1.
- Particle
- —
- Class
- —
- Charge Q
- —
- Baryon number B
- —
- Strangeness S
- —
For each quantity, is it conserved? Then press Check.
| Quantity | Conserved? | Before | After |
|---|
Quark composition (AQA data booklet)
| Particle | Quarks | Q | B | S |
|---|
Quarks
| Quark | Q | B | S |
|---|---|---|---|
| u | +⅔ | +⅓ | 0 |
| d | −⅓ | +⅓ | 0 |
| s | −⅓ | +⅓ | −1 |
| Antiquarks | Opposite signs | ||
Lepton numbers
| Particles | Electron lepton number | Muon lepton number |
|---|---|---|
| e−, νe | +1 | 0 |
| e+, νe | −1 | 0 |
| μ−, νμ | 0 | +1 |
| μ+, νμ | 0 | −1 |
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Particle properties from the AQA Data and Formulae Booklet. Energy and momentum are always conserved too: A single particle at rest can decay only into particles with less total rest energy.
Antimatter: Annihilation and Pair Production
A particle and its antiparticle have the same mass and rest energy, but opposite charge. When they meet they annihilate into two photons. A photon with enough energy can do the reverse and make a particle–antiparticle pair.
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Photons are drawn as wave packets: A short burst of wave inside an envelope. Gamma rays cannot be seen, so the purple is only a code; a more energetic photon has a shorter wavelength (drawn wavelengths are not to scale). Particle speeds use special relativity, E0 + Ek = E0 / √(1 − v2/c2) (not needed for this topic); on screen, photons move at c and particles at their real fraction of c. Rest energies from the AQA Data and Formulae Booklet. h = 6.63 × 10−34 J s, c = 3.00 × 108 m s−1, 1 MeV = 1.60 × 10−13 J.
Drawing Interaction Diagrams
Particles interact by swapping exchange particles. Press Play to watch each interaction happen: A “now” line moves up the time axis, the particles move along their lines and the exchange particle travels as a wave packet from one vertex to the other, drawing its wavy line behind it. Check that charge balances at every vertex.
On the AQA specification
Beyond A-level
- Equation
- Exchange particle
- Interaction
House style: Time runs up the page; each particle is a straight line with its symbol at the end and an arrow in the direction of time (an antiparticle’s arrow points backwards); the line bends where the particle changes; the exchange particle is a wavy line between two vertices, sloping upwards because it moves forwards in time; a W has an arrow showing which way it travels (from the vertex that emits it to the vertex that absorbs it). W bosons are drawn in blue, photons in purple. At quark level the quark that changes is drawn in red.
Key equations for particles and antiparticles
Quick catch
Three rounds of five multiple-choice questions, each round harder than the last. Pick A, B, C or D and the reasons appear straight away. Get 4 of 5 right to unlock the next round.
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Where it comes from
Three ideas behind this topic, built up step by step. The neutrino hypothesis is on the AQA specification; the two-photon argument uses momentum, which you meet again in the mechanics topics; the third tab is for interest.
| Interaction | Exchange particle | Rest energy | Range |
|---|---|---|---|
| electromagnetic | virtual photon | 0 | infinite |
| strong (between nucleons) | pion | about 140 MeV | about 1–3 × 10−15 m |
| weak | W+, W− | about 80 000 MeV | about 10−18 m |
Test yourself
35 exam-style questions in three levels: Core, Exam-style and Stretch for A*. Each level shows a few at a time. Press Try another question for a different one. Type numbers without units. For the explain parts, write your answer, then mark it against the mark scheme.
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Download the booklet
Notes, worked examples, equation cards, 21 quick checks and 30 exam-style questions on particles and antiparticles, to print and write on. The answers are at the back.
Particles and Antiparticles: Notes and practice booklet
Print it, write in the spaces, then check your answers at the back.
It opens in a new tab. Use the download or print button in the PDF viewer to keep a copy. Print it double-sided and write in the spaces. In the PDF, each question links to its answer and back again.
Teachers: Free for classroom use. Please share this page link rather than the file, so students always get the latest version.
Particles and antiparticles: Common questions
Why does beta-minus decay give an antineutrino, not a neutrino?
Lepton number must be conserved. Before the decay there are no leptons, so the electron-lepton number is 0. The electron that is made has Le = +1.
To bring the total back to zero, the third particle must have Le = −1: An electron antineutrino. In β+ decay the positron has Le = −1, so a neutrino is emitted instead.
Exam trap. Writing νe in a β− equation loses the mark, even if everything else is right.
Why do annihilation photons go in opposite directions?
When a particle and its antiparticle annihilate at rest, their total momentum is zero. The photons must carry zero total momentum too.
A single photon always has momentum, so at least two photons are needed. Two photons with equal energies travelling in exactly opposite directions have momenta that cancel. Each photon gets the rest energy of one particle: 0.511 MeV for an electron and a positron.
Exam trap. Each photon has E0, not 2E0. The total energy is 2E0, shared between two photons.
Why are strange particles made in pairs?
Strange particles are made by the strong interaction, which conserves strangeness. In a collision between ordinary hadrons, such as protons and pions, the total strangeness is 0.
So for every particle with S = −1 made, there must be one with S = +1. The strange particles then decay by the weak interaction, which lets strangeness change by 1. That is also why they live so long.
Exam trap. A change in strangeness does not make a reaction impossible: It means it can only be a weak interaction. A change of 2 or more is impossible.
Is the W a meson?
No. The W+ and W− are W bosons, the exchange particles of the weak interaction. They are not made of quarks.
Mesons, such as pions and kaons, are hadrons made of a quark and an antiquark. The pion is the exchange particle of the strong nuclear force between nucleons, which may be where the mix-up comes from.
Exam trap. “W meson” is marked wrong. Write W boson, and give its charge: W− in β− decay, W+ in β+ decay and electron capture.