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What are the fundamental particles, and how does the standard model classify them?

Quarks, leptons and the standard model, hadrons as baryons and mesons, antiparticles and annihilation, and the conservation laws governing particle interactions.

A CCEA A-Level Physics answer on quarks and leptons and the standard model, the classification of hadrons into baryons and mesons, antiparticles and annihilation, and the conservation laws that govern particle interactions.

Generated by Claude Opus 4.811 min answer

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  1. What this dot point is asking
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What this dot point is asking

CCEA wants you to describe quarks and leptons and the standard model, classify hadrons as baryons and mesons in terms of their quark content, explain antiparticles and annihilation, and apply the conservation laws to particle interactions. Conservation-law checks on a written interaction are a recurring question type.

The answer

Quarks and leptons

A proton is made of two up quarks and one down quark (uud), giving charge 23+23−13=+1\tfrac{2}{3} + \tfrac{2}{3} - \tfrac{1}{3} = +1; a neutron is two down and one up (udd), giving −13−13+23=0-\tfrac{1}{3} - \tfrac{1}{3} + \tfrac{2}{3} = 0.

Hadrons: baryons and mesons

Antiparticles and conservation laws

Every particle has an antiparticle with the same mass but opposite charge (and opposite quantum numbers). When a particle meets its antiparticle they annihilate, converting their entire mass into energy as photons; an electron and a positron at rest annihilate to give two gamma photons of total energy 2mc22mc^2, emitted back to back to conserve momentum.

Worked example: testing a proposed reaction

Examples in context

Example 1. A PET scanner. Positron emission tomography uses a radioactive tracer that emits positrons. Each positron annihilates with a nearby electron, producing two 511 keV511\ \text{keV} gamma photons travelling in opposite directions. Detectors in a ring register the simultaneous pair and trace the line between them back to the annihilation point, building a three-dimensional map of metabolic activity.

Example 2. The Large Hadron Collider. Protons (baryons of quark content uud) are smashed together at high energy. Because energy can become mass via E=mc2E = mc^2, the collisions create showers of new particles, every one of which must respect charge, baryon and lepton number conservation, which is exactly how the Higgs boson was confirmed in 2012.

Try this

Q1. State the quark composition of a proton and a neutron. [2 marks]

  • Cue. Proton is up, up, down (uud); neutron is up, down, down (udd).

Q2. Explain what happens when an electron meets a positron. [2 marks]

  • Cue. They annihilate, converting their mass into energy as two gamma photons emitted in opposite directions.

Q3. State the three quantities (besides energy and momentum) that must be conserved in a particle interaction. [3 marks]

  • Cue. Charge, baryon number and lepton number.

Exam-style practice questions

Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

CCEA 20206 marksBeta-minus decay can be written as a neutron changing into a proton, an electron and an electron antineutrino. Write the equation in terms of the particles, and show that charge, baryon number and lepton number are each conserved. State which fundamental force is responsible.
Show worked answer →

The decay is

n→p+e−+νˉen \rightarrow p + e^{-} + \bar{\nu}_e.

Charge: left side 00; right side (+1)+(−1)+0=0(+1) + (-1) + 0 = 0. Conserved.

Baryon number: the neutron and proton are baryons (+1+1 each); the electron and antineutrino are leptons (00). Left +1+1, right +1+0+0=+1+1 + 0 + 0 = +1. Conserved.

Lepton number: left side 00; right side the electron is a lepton (+1+1) and the electron antineutrino is an antilepton (−1-1), giving +1−1=0+1 - 1 = 0. Conserved.

The interaction is governed by the weak nuclear force, the only force that can change a down quark into an up quark (turning a neutron into a proton).

Markers reward the correct equation, all three conservation checks shown, and naming the weak force.

CCEA 20184 marksDescribe the difference between a baryon and a meson in terms of their quark content, giving one example of each. Explain what is meant by annihilation and calculate the minimum energy of each photon produced when an electron meets a positron, both effectively at rest. Take the electron rest mass as 9.11 times 10 to the minus 31 kg and c as 3.0 times 10 to the 8.
Show worked answer →

A baryon is made of three quarks, for example the proton (up, up, down). A meson is made of a quark and an antiquark, for example a pion.

Annihilation is when a particle meets its antiparticle and their entire mass is converted into energy, released as photons.

Total rest energy released is

E=2mc2=2×9.11×10−31×(3.0×108)2=2×8.2×10−14=1.64×10−13E = 2mc^2 = 2 \times 9.11 \times 10^{-31} \times (3.0 \times 10^{8})^2 = 2 \times 8.2 \times 10^{-14} = 1.64 \times 10^{-13} J.

To conserve momentum two photons are produced in opposite directions, sharing the energy equally, so each photon has a minimum energy of

1.64×10−132=8.2×10−14\frac{1.64 \times 10^{-13}}{2} = 8.2 \times 10^{-14} J.

Markers reward the quark-content distinction with examples, the mass-to-energy statement, and dividing the total 2mc22mc^2 between two photons.

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