Hadrons & Leptons (OCR A Level Physics)

Revision Note

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Katie M


Katie M




  • Hadrons are particles made up of quarks
    • This means they are affected by the strong nuclear force
  • There are two classes of hadrons:
    • Baryons (3 quarks)
    • Mesons (quark and anti–quark pair)
  • The most common baryons are protons and neutrons
  • The most common mesons are pions and kaons
  • If charged they experience the electromagnetic force
  • They decay via the weak nuclear force
  • Quarks have never been discovered on their own, always in pairs or groups of three
  • The large hadron collider at CERN (LHC) is used to look inside fundamental particles


  • Leptons are fundamental particles
    • This means they have no internal structure and cannot be divided or split into smaller particles
  • Unlike hadrons, they are not composed of quarks
    • This means they are not affected by the strong nuclear force 
  • There are 6 leptons in total and 3 different flavours (types) of lepton:
    • Electron, e
    • Muon, μ
    • Tau, τ


Leptons, downloadable AS & A Level Physics revision notes

The six leptons are all fundamental particles

  • Electrons, muon and tau particles all have a charge of −1e
    • This means they experience the electromagnetic force
  • Muon and tau particles are very similar to electrons but with slightly larger masses
    • The mass of an electron is about 0.0005u, whereas the mass of a muon is about 0.1u and the mass of a tau is about 2u
    • Where u is the unified atomic mass unit, equal to 1.661 × 10−27 kg
  • There are also three flavours of neutrino
    • The electron neutrino, νe
    • The muon neutrino, νμ
    • The tau neutrino, ντ
  • Neutrinos are the most abundant leptons in the universe
    • They have no charge and negligible mass (almost 0)
  • Neutrinos are produced in particle interactions which also involve the other leptons
    • For example, if an electron is produced in a particle interaction, an electron neutrino will also be produced
  • Leptons interact through the weak interaction, electromagnetic force 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

Circle all the anti-leptons in the following decay equation.

WE - Leptons question image, downloadable AS & A Level Physics revision notes

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

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Katie M

Author: Katie M

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.

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