Particle physics

Q: - PPQ C+ - How do you test whether a proposed particle reaction is possible?

Compare total charge, baryon number, electron lepton number and muon lepton number before/after; also check strangeness for the interaction involved, plus energy and momentum. State the specific quantity that fails if the reaction is impossible.

Q: - PPQ C+ - A reaction conserves charge and baryon number but changes strangeness by ±1. What does this imply?

It cannot be a strong interaction, because strong interactions conserve strangeness. The change is consistent with a weak interaction if all other conservation laws hold.

Q: - PPQ C+ - How do you identify an unknown particle in a reaction?

Use conservation laws to calculate the missing charge, baryon number, electron lepton number, muon lepton number and (if relevant) strangeness; match the result to a possible particle or quark composition.

Q: - PPQ C+ - What is the correct mark-scheme structure for explaining why a particle reaction is forbidden?

Name the violated conservation law and show the unequal totals before and after; do not merely state that the reaction is impossible.

Q: - PPQ C+ - How do you check electron and muon lepton number?

Check them separately. The corresponding lepton has +1, its antilepton −1, and hadrons have 0. Each family total must be conserved.

Q: - PPQ C+ - What happens at quark level in β⁻ and β⁺ decay, and which W boson is involved?

β⁻: d → u + W⁻, then W⁻ → e⁻ + ν̄ₑ. β⁺: u → d + W⁺, then W⁺ → e⁺ + νₑ. Spectator quarks remain unchanged; charge must balance at each vertex.

Q: - PPQ C+ - Why does β⁻ decay produce an electron antineutrino?

Electron lepton number is conserved: e⁻ has Lₑ = +1 and ν̄ₑ has Lₑ = −1, giving zero total as initially in the neutron.

Q: - PPQ C+ - A K⁺ (S = +1) decays to pions (S = 0). Which interaction and why?

Weak interaction: strangeness changes by −1, whereas the strong interaction conserves strangeness. The fact that the products are mesons does not make the decay strong.

Q: - PPQ C+ - Why can strange particles be produced strongly but decay weakly?

Strong production conserves total strangeness, so strange particles are produced in combinations with balanced strangeness. Weak decay can change strangeness by 0 or ±1.

Q: - PPQ C+ - How do you choose W⁺ or W⁻ in a quark-level diagram?

Follow the quark flavour change and conserve charge at each vertex: d → u requires W⁻ emission; u → d requires W⁺ emission.

Q: - PPQ C+ - How do you solve a missing-quark or missing-particle question efficiently?

Write initial and final particle/quark identities, retain unchanged spectator quarks, then balance charge, baryon number, lepton numbers and strangeness as applicable.

Q: - PPQ C+ - What are the minimum energy and photon energies for electron–positron pair production/annihilation?

Electron and positron rest energies are 0.511 MeV each, so pair creation needs at least 1.022 MeV. For slow e⁻e⁺ annihilation, two opposite photons each have about 0.511 MeV.

Q: - PPQ C+ - How do you calculate a photon’s frequency or wavelength from its energy?

Use E = hf = hc/λ. Convert MeV to joules before using h in J s; for slow electron–positron annihilation each photon has 0.511 MeV.

Q: - PPQ C+ - How do you calculate specific charge?

Specific charge = q/m in C kg⁻¹. For a nucleus q = +Ze; use mass in kg. For an ion, use its net charge and total mass.

Knowing the specific charge of a proton, you can multiple by the ratio of charge (of ion or nucleus) to mass in the nucleus (including mass of electrons)

Q: - PPQ C+ - How do you identify the daughter nuclide after α, β⁻ or β⁺ decay?

α: A decreases by 4 and Z by 2. β⁻: A unchanged, Z increases by 1. β⁺: A unchanged, Z decreases by 1. Balance A and Z in the nuclear equation.

What determines the length of a streak a particle leaves in a cloud chamber?

The mass (and momentum). A muon leaves a longer streak then a pion since it is less massive.

What are the 3 ways to test for alpha particles

cloud chamber (leaves a short trail)

spark counter (ionises the air between the gap allowing sparks)

Geiger counter (beeps close and doesn’t beep when far)

How did beta decay predict the existence of the neutrino

Beta particles were observed to have a CONTINUOUS RANGE of energies (this couldn’t be due to any photons released as the energy of these photons were measured as em radiation and didn’t account for the range of energies)

suggesting energy and momentum were shared with an additional particle. The neutrino was proposed to account for the missing energy, momentum and angular momentum.

Also it preserved conservation laws.

How do neutron decay (HINT: they decay about every 800s when on their own!)

neutron —> proton + electron + anti-electron neutrino

what are all the interactions to memorise?

Beta-minus decay: n → p + e⁻ + ν̅ₑ

Beta-plus decay: p → n + e⁺ + νₑ

Electron capture: p + e⁻ → n + νₑ

Beta-minus quark interaction: d → u + W⁻

Beta-plus quark interaction: u → d + W⁺

W⁻ decay: W⁻ → e⁻ + ν̅ₑ

W⁺ decay: W⁺ → e⁺ + νₑ

Electron–positron annihilation: e⁻ + e⁺ → 2γ

Pair production: γ → e⁻ + e⁺

Strangeness-changing decay: Λ⁰ → p + π⁻

do you need to use E=mc²

NO NO NO!

How are cosmic ray showers produced and detected, and what evidence do they provide?

High-energy cosmic rays collide with particles in the atmosphere, producing showers of secondary particles, including pions and kaons. Stray tracks in a cloud chamber provide evidence of these particles.

The pions and kaons themselves leave tracks in the cloud chamber as they travel through it, because they ionise gas molecules along their paths.

Two Geiger counters can detect a cosmic ray shower by registering particles arriving within a short time interval (coincidence counting).

This provides evidence that multiple particles from the same cosmic ray shower are reaching the detectors

What is the only stable baryon that all other baryons decay into?

The proton

What do Kaons decay into? What do pions decay into?

Kaons decay into pions.

(pions decay into muon neutrinos and muons) NOT NEEDED

What interaction creates and causes the decay of strange particles? How many strange particles are produced at once?

Strong nuclear interaction creates kaons

Weak nuclear interaction causes the decay of kaons

Strange particles are ALWAYS produced in pairs. Hence why strangeness is -1, to highlight how another particle with strangeness +1 was produced as well to conserve strangeness (since strange particles are produced by the snf, which conserves strangeness)

What are the charge and approximate masses of proton, neutron and electron?

Proton: +1.60 × 10⁻¹⁹ C, 1.67 × 10⁻²⁷ kg; neutron: 0 C, 1.675 × 10⁻²⁷ kg; electron: −1.60 × 10⁻¹⁹ C, 9.11 × 10⁻³¹ kg. Relative masses: proton ≈ 1, neutron ≈ 1, electron ≈ 1/1836.

What do A and Z mean in nuclide notation, and how do you find neutron number?

A is nucleon number (protons + neutrons); Z is proton number. Neutrons = A − Z. Nuclide notation is ᴬZX.

What are isotopes, and how do you calculate relative atomic mass from isotopic data?

Isotopes have the same Z but different neutron numbers. Relative atomic mass = Σ(isotopic mass × abundance) / Σ(abundance); use consistent percentage or fractional abundances.

What is specific charge, and what are the approximate values for a proton and electron?

Specific charge = q/m, unit C kg⁻¹. Proton: +9.58 × 10⁷ C kg⁻¹; electron: −1.76 × 10¹¹ C kg⁻¹.

What is the strong nuclear force’s role and range?

It attracts nucleons and helps overcome proton–proton electrostatic repulsion at nuclear distances. It is attractive up to about 3 fm and strongly repulsive below about 0.5 fm; 1 fm = 10⁻¹⁵ m.

What are α decay and β decay equations, and how do A and Z change?

α: ᴬZX → ᴬ⁻⁴Z⁻²Y + ⁴₂He. β⁻: ᴬZX → ᴬZ₊₁Y + e⁻ + ν̄ₑ. β⁺: ᴬZX → ᴬZ₋₁Y + e⁺ + νₑ. A changes only in α decay.

Why was the neutrino/antineutrino proposed in beta decay?

Beta electrons/positrons have a continuous energy spectrum; the neutrino/antineutrino carries away the remaining energy and momentum so conservation laws hold.

What is an antiparticle?

It has the same mass as its corresponding particle but opposite electric charge and opposite additive quantum numbers such as baryon number, lepton number and strangeness. Neutral particles can still have distinct antiparticles.

What are the required antiparticles of the electron, proton, neutron and neutrino?

Electron → positron; proton → antiproton; neutron → antineutron; neutrino → antineutrino. Each has the same mass as its partner and opposite relevant quantum numbers.

What are annihilation and pair production?

Annihilation converts a particle and antiparticle into other particles, commonly photons. Pair production converts photon energy into a particle–antiparticle pair. Conserve energy, momentum, charge and relevant quantum numbers.

How does PET use annihilation?

A positron from a radioactive tracer annihilates with an electron, producing two gamma photons detected in coincidence; the data are used to reconstruct tracer distribution.

What is the photon model and what equation relates photon energy, frequency and wavelength?

Electromagnetic radiation consists of photons with E = hf = hc/λ; h = 6.63 × 10⁻³⁴ J s and c is the speed of light.

What are the four fundamental interactions, and the exchange particles required by AQA for this topic?

Gravitational, electromagnetic, strong nuclear and weak nuclear. Required exchange particles: virtual photon for electromagnetic; W⁺ and W⁻ for weak; pion for the strong nuclear force between nucleons. Detailed gluon, Z⁰ and graviton knowledge is not required here.

What is an exchange particle in the exchange-particle model?

It transfers momentum between interacting particles; one particle emits it and another absorbs it. Diagrams should show incoming/outgoing particles, the exchange particle and charge balance at each vertex.

Which weak-interaction processes are required, and what exchange particles do they involve?

β⁻ decay, β⁺ decay, electron capture and electron–proton collisions; the specified weak interaction is mediated by W⁺ or W⁻ bosons.

What are hadrons, baryons, antibaryons and mesons?

Hadrons experience the strong interaction. Baryons contain three quarks; antibaryons contain three antiquarks; mesons contain one quark and one antiquark.

What are the baryon numbers of baryons, antibaryons, mesons and leptons, and what is conserved?

Baryon: +1; antibaryon: −1; meson and lepton: 0. Total baryon number is conserved in particle interactions.

Which baryon is stable in the AQA model, and what is the pion’s role?

The proton is the stable baryon; other baryons ultimately decay into a proton and other particles. Pions are the exchange particles for the strong nuclear force between nucleons.

What are leptons, and which are required by AQA?

Electron, muon, electron neutrino and muon neutrino, plus their antiparticles. Leptons do not experience the strong interaction.

What are electron and muon lepton numbers for the leptons and antileptons?

e⁻: (Lₑ,Lμ)=(+1,0); e⁺: (−1,0); νₑ: (+1,0); ν̄ₑ: (−1,0); μ⁻: (0,+1); μ⁺: (0,−1); νμ: (0,+1); ν̄μ: (0,−1). Each family number is conserved separately.

What is the negative muon decay equation, and how do you check it?

μ⁻ → e⁻ + ν̄ₑ + νμ. Charge: −1 on both sides; Lₑ = 0 on both sides (+1 − 1); Lμ = +1 on both sides.

What are strange particles, and how is strangeness assigned?

Strange particles have non-zero strangeness, often from an s or anti-s quark. An s quark has S = −1; an anti-s quark has S = +1. Strong interactions conserve total S; weak interactions can have ΔS = 0 or ±1.

What are the charge, baryon number and strangeness of u, d and s quarks?

u: charge +⅔e, B=+⅓, S=0. d: charge −⅓e, B=+⅓, S=0. s: charge −⅓e, B=+⅓, S=−1. Antiquarks have the opposite values.

What are the quark compositions of proton, neutron, antiproton and antineutron?

Proton uud; neutron udd; antiproton ūū d̄; antineutron ū d̄ d̄.

What are the quark compositions of charged pions and kaons?

π⁺ = u d̄; π⁻ = ū d. K⁺ = u s̄; K⁻ = ū s.

What are the quark compositions of neutral pions and neutral kaons at AQA level?

π⁰ is a neutral quark–antiquark combination (commonly represented as a mixture of uū and d d̄). K⁰ = d s̄; K̄⁰ = d̄ s.

How do you calculate a hadron’s charge, baryon number and strangeness from its quarks?

Add the contributions of every quark and antiquark. A quark has B=+⅓, an antiquark −⅓; add electric charges and strangeness too, then check against the particle identity.

How do you classify a quark combination?

Three quarks = baryon; three antiquarks = antibaryon; one quark plus one antiquark = meson. Verify by summing charge, baryon number and strangeness.

What is neutron decay at quark level?

n (udd) → p (uud) + e⁻ + ν̄ₑ. One down quark changes to an up quark via the weak interaction; the other two quarks are spectators.

Why are strange particles often produced in pairs in strong interactions?

Strong interactions conserve total strangeness. If initial total S is zero, products must also total zero, so strange and anti-strange content balances.

How do you write a precise explanation of a weak decay involving strangeness?

State the numerical change in strangeness (e.g. +1 to 0, so ΔS=−1), state that strong interactions conserve strangeness, then conclude the decay is weak.

What is the general conservation checklist for AQA particle questions?

Charge, baryon number, electron lepton number and muon lepton number; strangeness where relevant to strong/weak interactions; and total energy and momentum.

What should you say about scientific progress in particle physics?

Particle physics requires collaboration among large teams of scientists and engineers; new knowledge is established through evidence, scrutiny and validation by the scientific community.