Standard Model of Matter: Quarks and Leptons

This topic is part of the HSC Physics course of the section Deep Inside the Atom.

HSC Physics Syllabus

  • investigate the Standard Model of matter, including:

– quarks, and the quark composition hadrons
– leptons
– fundamental forces (ACSPH141, ACSPH142)
  • analyse the evidence that suggests:
    – that protons and neutrons are not fundamental particles
    – the existence of subatomic particles other than protons, neutrons and electrons 

    Quarks and Leptons


    What is the Standard Model of Matter?

    The discovery of new fundamental particles led scientists to propose the Standard Model of Matter. The Standard Model states that all matter is composed of small fundamental particles that exist on their own or in groups to:
    1. Form other subatomic particles
    2. Mediate forces between matter particles
    In the Standard Model of Matter, there are three types of fundamental particles:
    • Quarks (matter particles)
    • Leptons (matter particles)
    • Bosons (force-mediating particles)

      *Gravitons are force-mediating particles for gravitational force which are yet to be discovered and are typically not included in the Standard Model of Matter.


      Quarks combine together to make other particles, while Leptons do not – both are classified as fermions due to their half integer spin values. Bosons are not classified as fermions because they have whole integer spin values).




      Leptons also have generations but unlike quarks, they do not make up larger particles. 

      • First generation: electron and electron neutrino
      • Second generation: muon and muon neutrino
      • Third generation: tau and tau neutrino

      How was the electron discovered?

      The electron was discovered by Thomson in his experiment on determining the nature of cathode rays. The electron was the first fundamental and subatomic particle to be discovered.

      How was the muon discovered?

      The muon was discovered by Anderson and Neddermeyer when they were studying comic radiation.

      • Cosmic rays (radiation) originate as primary cosmic rays, from outside the Solar System. They are formed as a result of various astronomical processes. Primary cosmic rays are composed of alpha particles and a small proportion of heavier nuclei (<1%)
      • Primary cosmic rays eventually decay into secondary cosmic rays. Secondary cosmic rays consist of various sub-atomic particles including photons, leptons, hadrons (protons and neutrons), electrons, positrons, muons and pions.

      Anderson and Neddermeyer applied a magnetic field to the cloud chamber which contained cosmic radiation. Upon interacting with molecules inside the chamber, sub-atomic particles were produced from cosmic rays. This released a particle whose deflection trail was similar to that of an electron.

      The unknown particles were deflected to the same direction as electrons, but they were deflected to a lesser extent. This suggested that the particle has a much greater mass than an electron (assuming they have the same charge of -1). It was later confirmed that muons are approximately 200 times heavier than electrons.

      Muons were initially thought to be mesons (particles composed of two quarks) but were later corrected to be leptons because they do not interact with strong nuclear force (gluons).

      How was the electron neutrino discovered?

      A He-4 nucleus produced from alpha decay is monokinetic, meaning its kinetic energy is constant for a given alpha decay. This is because the kinetic energies of the alpha particle and daughter nucleus formed from the decaying process must be specific to upload the laws of conservation of energy and momentum. 

      Initially, beta particles from beta plus and beta minus decay were also thought to be monokinetic. It was thought that only two particles were emitted during beta-decay: an electron/positron and the daughter nucleus. 



        However, unlike alpha decay, beta decay was shown to not be monokinetic because the emitted electron varied in kinetic energy. In addition, the energy of the beta particle could never account for the total energy released from the decaying process. This observation violated the law of conservation of energy.

        Pauli, in response to this observation, proposed that the missing energy was given to a particle with very small mass that was produced in addition to the daughter nuclide and beta particle. This was termed the neutrino and is known as the electron neutrino. The existence of the neutrino was later confirmed in nuclear fission reactions.


          There are six different quarks, grouped into three generations. 

          • First generation quarks: up and down
          • Second generation quarks: charm and strange
          • Third generation quarks: top and bottom

          Quarks from different generations vary in mass and abundance. First generation quarks are the most abundant (and most important to know) while third generation quarks are the rarest.

          Quarks have fractional charges. Up, charm and top quarks all have `+2/3` of electric charge. Down, strange and bottom quarks all have `-1/3` of electric charge.

          Antiquarks are the antiparticles to quarks. Antiquarks have the same mass as their particle counterpart but the opposite charge. For example, an antiup quark has the same mass as an up quark but `-2/3` of an electric charge.

          How were quarks discovered?

          In an experiment called Deep Inelastic Scattering, high energy electrons were fired at protons in a particle accelerator. The inelastic collision between electrons and protons caused quarks to be removed from protons. This experiment showed that protons were not fundamental particles.


            Hadrons are particles made from a combination of quarks. There are two kinds of hadrons: baryons and mesons. 

              Baryons are hadrons which consist of three quark particles. For example, protons and neutrons are baryons.

              • A proton consists of two up quarks (uu) and one down quark (d), which combine to give a total charge of +1
              • A neutron consists of one up quark (u) and two down quarks (dd), which combine to give a total charge of 0

              Meson are hadrons which consist of one quark and one anti-quark. For example, pions are mesons.

              • A positive pion consists of one up quark and one antidown quark, which combine to give a total charge of +1
              • A negative pion consists of one antiup quark and one down quark, which combine to give a total charge of –1 

              All hadrons are affected by strong force (gluons) because gluons interact with quarks.


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              Next section: Standard Model of Matter: Bosons