AS Physics: Particle Physics — Revision Notes

Condensed for the final weeks. For the full explanation, use the Particle Physics study guide.

Syllabus note: Cambridge 9702 (2025–2027) Topic 11 requires quarks, hadrons, leptons and the quark-level description of β decay. It does not name exchange bosons (gluon, photon, W/Z, graviton) or require baryon/ lepton/strangeness conservation checks as explicit AS outcomes. The “Four fundamental forces” and “Conservation rules” sections below are useful, standard extension material, not a guaranteed AS exam requirement in their own right.

The nuclear atom and radioactive decay

The alpha-particle scattering experiment showed that an atom’s mass and positive charge are concentrated in a tiny, dense nucleus, with electrons occupying the much larger surrounding space.

Nucleon number A is protons plus neutrons; proton number Z is protons only. A nuclide is written ᴬZX. Both nucleon number and charge are conserved in nuclear processes.

Radiation	Composition	Mass (u)	Charge
α	2p + 2n (helium nucleus)	4	+2
β⁻	electron	~0	−1
β⁺	positron	~0	+1
γ	EM radiation	0	0

Worked example — alpha decay: ²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂α. Nucleon number balances (238 = 234 + 4) and proton number balances (92 = 90 + 2).

β particles have a continuous range of energies, unlike α particles, which are discrete — because a neutrino or antineutrino shares the released energy with the β particle in a variable proportion, whereas an α decay shares a fixed energy release between just two bodies.

The standard model

Fundamental particles cannot be broken down further. Quarks and leptons are the fundamental matter particles — but this doesn’t exclude other fundamental particles outside this matter classification, such as the photon, gluon and W/Z bosons (see exchange particles, below). Two families of fundamental matter particles:

Quarks — up, down, strange, charm, top, bottom.

Quark	Charge	Baryon number
up (u)	+2/3	+1/3
down (d)	−1/3	+1/3
strange (s)	−1/3	+1/3

Leptons — electron, muon, tau, and their neutrinos. Leptons are fundamental; quarks are too. Protons and neutrons are not — they are composite.

proton  = uud   ->  2(+2/3) + (-1/3) = +1
neutron = udd   ->  (+2/3) + 2(-1/3) = 0

Being able to derive the charge from the quark composition is the standard question.

Hadrons

Particles made of quarks, and they feel the strong force.

• Baryons — three quarks (proton, neutron). Baryon number ±1.
• Mesons — one quark and one antiquark (pion, kaon). Baryon number 0.

Leptons are not hadrons and do not feel the strong force. Classifying an electron as a hadron is a common error.

The four fundamental forces

Force	Acts on	Range	Exchange particle
Strong	Quarks and hadrons	~10⁻¹⁵ m	Gluon
Electromagnetic	Charged particles	Infinite	Photon
Weak	All particles	~10⁻¹⁸ m	W⁺, W⁻, Z⁰ bosons
Gravitational	All mass	Infinite	Graviton (hypothetical)

The weak force is the only one that changes quark flavour, which is why it is responsible for beta decay. That link is the key to the whole topic.

Exchange (force-carrying) particles are also fundamental particles — they are just not matter particles like the quarks and leptons above.

Beta decay

beta-minus:  n -> p + e- + antineutrino     at quark level  d -> u
beta-plus:   p -> n + e+ + neutrino         at quark level  u -> d

The neutrino was postulated because energy and momentum did not appear to be conserved in beta decay — the emitted electron had a range of energies rather than a fixed value. Rather than abandon conservation, physicists predicted an undetected particle carrying the balance. That is the standard “why was the neutrino proposed” answer.

Conservation rules

In every interaction, these must all balance:

• Charge
• Baryon number
• Lepton number (separately for each lepton family)
• Energy and momentum

Strangeness is conserved in strong and electromagnetic interactions but may change by ±1 in weak interactions.

Method for “is this interaction possible?” questions: check each conserved quantity in turn on both sides. If any fails, the interaction cannot occur — and you must say which rule is violated.

Antiparticles

Same mass, opposite charge and opposite quantum numbers. When a particle and its antiparticle annihilate from rest (negligible initial kinetic energy), the two-photon channel produces two photons travelling in opposite directions, as required by conservation of momentum — the initial total momentum is zero, so the photons’ momenta must cancel.

Exam traps

• Calling a proton or neutron fundamental.
• Classifying leptons as hadrons.
• Forgetting that lepton number is conserved per family.
• Saying only one photon is produced in annihilation.
• Not stating which conservation rule is violated when rejecting an interaction.
• Confusing β⁻ (d → u) with β⁺ (u → d).

Self-test

1. Which particles are fundamental, and which of proton, neutron, electron are not?
2. Derive the charge of a neutron from its quark composition.
3. What distinguishes a baryon from a meson?
4. Why was the neutrino postulated?
5. Which force changes quark flavour, and what process does that explain?
6. Complete the alpha-decay equation ²³⁸₉₂U → ²³⁴₉₀Th + ? and check it balances.
7. Why do β particles have a continuous range of energies while α particles have discrete energies?

Answers: 1. Quarks and leptons are fundamental; the electron is fundamental, but the proton and neutron are composite. 2. udd gives (+2/3) + (−1/3) + (−1/3) = 0. 3. A baryon consists of three quarks; a meson consists of one quark and one antiquark. 4. Beta decay appeared to violate conservation of energy and momentum because the emitted electron had a range of energies; a neutrino was proposed to carry the missing energy and momentum. 5. The weak force; it explains beta decay, in which a down quark changes to an up quark or vice versa. 6. ⁴₂α (an alpha particle); nucleon number balances as 238 = 234 + 4, and proton number balances as 92 = 90 + 2. 7. A neutrino or antineutrino shares the released energy with the β particle in a variable proportion, giving a continuous spread; an α decay shares a fixed energy release between just two bodies, giving a single discrete value.