Hadrons & Leptons (OCR A Level Physics): Revision Note

Hadrons

• 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

• 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

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, ν_e

  • The muon neutrino, ν_μ

  • The tau neutrino, ν_τ

• 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