CIE AS Physics (9702) 2019-2021

Revision Notes

13.2.1 Fundamental Particles

Fundamental Particles: Quarks

  • Quarks are fundamental particles that make up other subatomic particles such as protons and neutrons
  • Protons and neutrons are in a category of particles called hadrons
    • Hadrons are defined as any particle made up of quarks
  • Fundamental means that quarks are not made up of any other particles. Another example is electrons
  • Quarks have never been observed on their own, they’re either in pairs or groups of three
  • There are six flavours (types) of quarks that exist:

Quarks, downloadable AS & A Level Physics revision notes

The six flavours of quarks

The Weak Interaction

  • The weak interaction is one of the four fundamental forces of nature
    • It is responsible for the radioactive decay of atoms
  • β decay occurs because of the weak interaction between quarks
  • β minus and plus decay are examples of the weak interaction in action
    • In beta minus decay, a neutron turns into a proton emitting an electron and an electron antineutrino
    • In beta plus decay, a proton turns into a neutron emitting a positron and an electron neutrino
  • At the quark level, a down quark turns into an up quark (or vice versa)

Beta decay, downloadable AS & A Level Physics revision notes

The weak interaction turns a down quark turns into an up quark in β decay 

  • The carrier particles that mediate the weak interaction are the: W+, W or Z0 bosons
    • Bosons are a group of particles that mediate the fundamental forces
  • The W boson ‘carries away’ the negative charge of the down quark and transforms it into an up quark


  • Note: you will only need to specify that the weak interaction that gives rise to beta decay, you will not need to remember the details about bosons

Fundamental Particles: Leptons

  • Leptons are a group of fundamental (elementary) particles
  • This means they are not made up of any other particles (no quarks)
  • There are six leptons altogether:


Leptons, downloadable AS & A Level Physics revision notes

The six leptons are all fundamental particles

  • The muon and tau particle are very similar to the electron but with slightly larger mass
  • Electrons, muon and tau particles all have a charge of -1e and a mass of 0.0005u


  • There are three flavours (types) of neutrinos (electron, muon, tau)
  • Neutrinos are the most abundant leptons in the universe
    • They have no charge and negligible mass (almost 0)


  • Leptons interact with the weak interaction, electromagnetic and gravitational forces
  • However, they do not interact with the strong force
  • Although quarks are fundamental particles too, they are not classed as leptons
  • Leptons do not interact with the strong force, whilst quarks do

Worked example

Worked example - leptons, downloadable AS & A Level Physics revision notes

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