Fundamental Particles (Cambridge International A-Level Physics)

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 the particles are not made up of any other particles

• Quarks have never been observed on their own, they are either in pairs or groups of three

• There are six flavours (types) of quarks that exist:

  • up
  • down
  • charm
  • strange
  • top
  • bottom

The six flavours of quarks

The six flavours of quarks are up, down, charm, strange, top and bottom

Fundamental Particles: Leptons

• Leptons are also a group of fundamental (elementary) particles

• This means they are not made up of any other particles (no quarks)

• There are six leptons altogether:

  • electron
  • electron neutrino
  • muon
  • muon neutrino
  • tau
  • tau neutrino

The six leptons

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

• 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 State all the anti-leptons in the following decay equation.

$π^{+}\rightarrowμ^{+}+v_{μ}$

Answer:

• The pion (π⁺) is a meson (a type of hadron) and is made up of quarks

  • Therefore it is not a fundamental particle and hence not a lepton

• The anti-muon (μ⁺) is the anti-particle of the muon

  • Therefore it is an anti-lepton

• The muon neutrino is a lepton, not an anti-lepton

  • The anti-lepton would be the anti-muon neutrino

• Therefore, the only anti-lepton is an anti-muon (μ⁺)