Fundamental particles

AS Level Physics · Particle physics. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

Test this topic freeAll AS Level Physics topics

The idea

A fundamental particle is not made of anything smaller. Protons and neutrons are not fundamental: each is made of three quarks. There are six flavours of quark: up, down, strange, charm, top and bottom. Each has a charge that is a fraction of e, and each antiquark has the opposite charge.

Particles made of quarks are called hadrons. A baryon has three quarks. A meson has one quark and one antiquark. Add the quark charges to get the charge of the hadron.

Electrons and neutrinos are leptons. They are fundamental and contain no quarks. Beta decay changes one quark in a nucleon into another flavour.

Rules to remember

Common mistake

Students mix up the two nucleons. The proton has two up quarks (uud) and the neutron has two down quarks (udd); check by adding the charges.

Worked example

A particle is made of one up quark, one down quark and one strange quark (uds). What is its charge, and is it a baryon, a meson or a lepton?

  1. Charges: up +⅔e, down −⅓e, strange −⅓e.
  2. Total charge = ⅔ − ⅓ − ⅓ = 0.
  3. It contains three quarks, so it is a baryon (and therefore a hadron). Leptons contain no quarks.

Answer: The particle has zero charge and is a baryon.

Practice questions

Try each one, then open the answer.

1. In β− decay at the quark level, what happens?

  1. A
    an up quark changes into a down quark
  2. B
    a down quark changes into an up quark, emitting an electron and an antineutrino
  3. C
    a proton splits into three quarks
  4. D
    a quark is created from a photon
Show answer

Answer: B. A neutron (udd) becomes a proton (uud): one down quark becomes an up quark via the weak interaction, releasing e− and ν̄e.

2. Which list contains only fundamental particles?

  1. A
    proton, neutron, electron
  2. B
    pion, electron, neutrino
  3. C
    electron, electron neutrino, charm quark
  4. D
    neutron, up quark, positron
Show answer

Answer: C. Leptons (electron, neutrino) and quarks are fundamental. Protons, neutrons and pions are hadrons made of quarks.

3. A meson consists of

  1. A
    three quarks
  2. B
    a quark and an antiquark
  3. C
    two quarks
  4. D
    a lepton and a quark
Show answer

Answer: B. Baryons (proton, neutron) are three quarks; mesons are quark–antiquark pairs. Both are hadrons.

More questions on this topic

Read the full notes

Keep going

← Atoms, nuclei and radiationAll rules on one pageStuck? Ask a question