Particles, antiparticles, quarks and conservation laws
8 min read · about 1 h 50 min with practice3 quick checks≈3% of the testCore: Core: tested on most papers
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Particle physics is one of the most reliable sources of marks at A-Level. The content is finite and the questions follow a small number of patterns. Examiners test it with quick multiple-choice classification items, “is this interaction possible?” conservation checks, pair production and annihilation calculations in MeV, and short explanations of beta decay at quark level. On AQA it sits in Paper 1 and regularly supplies a full structured question.
By the end you’ll be able to
Use proton number, nucleon number and specific charge for nuclei, ions and particles
Describe annihilation and pair production and calculate minimum photon energies using rest energies in MeV
Classify particles as hadrons (baryons/mesons) or leptons and write the quark composition of protons, neutrons, pions and kaons
Apply conservation of charge, baryon number, lepton number (electron and muon) and strangeness to decide whether an interaction can occur
Draw and interpret Feynman diagrams for beta decay, electron capture and electron–proton collisions, identifying the exchange particle (W±, Z, photon, gluon)
What the exam asks
Constituents of the atom: charge, mass and specific charge (charge ÷ mass, in C kg⁻¹) of particles, ions and nuclei.
Antiparticles: annihilation into two photons and pair production from one photon, with minimum energies worked out from rest energies.
Classification: hadrons (baryons and mesons) and leptons, with quark compositions.
Conservation laws: charge, baryon number, lepton number and (AQA) strangeness, used to decide whether an interaction can happen and which interaction it is.
Beta decay at quark level, the neutrino hypothesis and (AQA) Feynman diagrams with exchange particles.
Core ideas
Specific charge
vii.Check your understanding
3 questions on particles, antiparticles, quarks and conservation laws. Every option is explained once you answer.
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PromptCard 1 of 3
Define specific charge and give its unit.
specific charge=mQ(C kg−1)
Particle
Charge / C
Mass / kg
Specific charge / C kg⁻¹
electron
−1.60×10−19
9.11×10−31
1.76×1011
proton
+1.60×10−19
1.673×10−27
neutron
0
1.675×10−27
0
For an ion, the charge is (protons − electrons) × e, but the mass is essentially all in the nucleus (nucleon number × about 1.67×10−27 kg). For a nucleus, the charge is Ze.
Antiparticles, annihilation and pair production
Every particle has an antiparticle with the same rest energy (mass) and opposite charge, baryon number, lepton number and strangeness. Examples are the positron e⁺, the antiproton p̄, the antineutron n̄ and the electron antineutrino ν̄ₑ.
Annihilation: a particle and its antiparticle meet and are converted into two photons, which travel in opposite directions so that momentum is conserved. If both are at rest, each photon has energy E0, the rest energy of one particle.
Pair production: a photon becomes a particle–antiparticle pair. This happens near a nucleus, which recoils so that momentum is conserved. The minimum photon energy is
hfmin=2E0
Any extra photon energy becomes kinetic energy of the pair. To convert: 1MeV=1.60×10−13J.
Antiquarks reverse every sign, so s̄ has strangeness +1. The other three quarks (Edexcel, Cambridge, OCR B) are c (+32), t (+32) and b (−31).
The proton is the only stable baryon, and all other baryons eventually decay into one. A free neutron decays by the weak interaction with a half-life of about 10 minutes. Kaons decay into pions, and the muon decays into an electron.
The four interactions
Interaction
Acts on
Exchange particle
Strong
hadrons (quarks)
gluon; AQA: the pion between nucleons
Electromagnetic
charged particles
virtual photon
Weak
all particles
W⁺, W⁻ (and Z⁰)
Gravity
all masses
(graviton, not detected)
The strong nuclear force between nucleons is repulsive below about 0.5 fm, attractive up to about 3 fm, and negligible beyond that.
Conservation laws
Every interaction conserves energy, momentum, charge Q, baryon number B and lepton number L. Electron-family and muon-family lepton numbers are conserved separately: Le=+1 for e⁻ and νₑ, and Lμ=+1 for μ⁻ and ν_μ. Their antiparticles have −1.
Strangeness is conserved in strong interactions but can change by 0 or ±1 in weak interactions. That is why strange particles are produced in pairs by the strong interaction and decay one at a time by the weak interaction.
Beta decay at quark level
β⁻: d→u+e−+νˉe (overall, n→p+e−+νˉe), with a W⁻ exchanged.
β⁺: u→d+e++νe (overall, ), with a W⁺ exchanged.
Electron capture: p+e−→n+νe.
The beta particles have a continuous range of kinetic energies up to a maximum, even though each decay releases the same total energy. Energy (and momentum) would not be conserved unless another particle, the (anti)neutrino, carried away the rest. This is how the neutrino was predicted.
Feynman diagrams (AQA): show time running up the page. Particles enter at the bottom and leave at the top. The exchange particle is a wavy line between two vertices, and charge is conserved at each vertex.
Worked examples
Exam technique
Use a conservation table every time. Write columns for Q, B, Le, Lμ and S with one row per particle, and tick or cross each total. This takes 30 seconds and prevents most errors.
Which interaction? Neutrinos involved, a quark changes flavour, or strangeness changes: weak. Only hadrons, with everything conserved: strong. Photons and charged particles only: electromagnetic.
“Explain why the decay is weak”: name the quantity that changes (strangeness, or quark flavour such as d → u). “Because it is slow” earns nothing.
Energies: keep calculations in MeV until the final step, then multiply by 1.60×10−13 to get joules.
Feynman diagrams: label every line, put arrows on particle lines, and check that the W charge balances at both vertices. Electron-family leptons attach to one vertex and hadrons to the other.
Common mistakes
Quick recap
Specific charge = Q/m in C kg⁻¹. For ions the mass is essentially all in the nucleus.
Antiparticles have the same rest energy and opposite quantum numbers.
Pair production: hfmin=2E0. Annihilation at rest: two photons, each of energy E0.
Baryons are qqq (B=1), mesons are qq̄ (B=0), and leptons are fundamental.
Q, B, Le and Lμ are always conserved. is conserved only in strong interactions.
β⁻ is d → u (W⁻) and β⁺ is u → d (W⁺). The continuous beta spectrum led to the neutrino hypothesis.