M8-S12b: Evidence for the Standard Model of Matter

  • analyse the evidence that suggests:

    • that protons and neutrons are not fundamental particles

    • the existence of subatomic particles other than protons, neutrons and electrons 


  • Thomson discovered the first sub-atomic particle – the electron in 1897 in his experiment investigating the nature of cathode rays.
  • In the 20th century, Rutherford and Chadwick discovered particles constituting the nucleus (protons and neutrons respectively). Thus, they provided a deeper understanding of the atom.
  • Scientists have been able to investigate the size and nature of the atom by using these subatomic particles, namely the electron due to its much smaller size compared to neutrons and protons.


Using Electron Scattering to Probe Sub-atomic Structure

  • A beam of electron projected at an atom experiences scattering due to Coulomb electrostatic forces with sub-atomic particles.


  • An electron beam with a few hundred electron volts (eV) is scattered by atomic electrons surrounding the nucleus. By studying the pattern of scattering, the size and distribution of electrons can be determined.
    • An atom on average is 10-10 m in diameter – most of which is empty space, as outlined by the Rutherford model of the atom.


  • When the electron beam’s energy is increased to a few hundred mega electron volts (MeV), electrons become unaffected by atomic electrons and thus are able to penetrate deeper into the atom. As a result, electron beams with this magnitude of energy are scattered by nucleons in the nucleus. By studying the pattern of scattering, the size and composition of the nucleus can be determined.
    • A nucleus on average is 10-14 m in diameter. This is approximately 1/10000 the size of the whole atom.
    • A nucleon (proton or neutron) is 10-15 m in diameter. This is approximately 1/10 the size of the nucleus.


  • When the electron beam’s energy is further increased to a giga-electron volt (GeV), electrons are able to penetrate within protons and neutrons. Likewise, by studying the pattern of scattering, the structure of a proton and neutron can be determined.
    • Experiments in the early 1970s using particle accelerators (which can emit electron beams with energy up to 1 GeV) showed that neutrons and protons are composed of smaller particles. Therefore, it was shown that nucleons are not fundamental particles because they are divisible like an atom.


Discovery of Other Sub-atomic Particles

Muon (µ)

  • The muon was discovered by Carl Anderson and Seth Neddermeyer in 1936 when they were studying comic radiation. It was later confirmed (by other scientists) in 1937 when analysed in a cloud chamber.
  • 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 an external 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 (beta-particle).
    • The unknown particles were deflected to the same direction as electrons, but their curvature was less sharp than that of an electron. This indicates that muons have 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 originally 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.


Pion (pi-meson, p)

  • Pions are examples of mesons (particles consisting of two quarks). They can be charged or neutral.
  • Charged pions were discovered in 1947 whereas neutral pions were discovered in 1950.
  • Powell used photographic films containing silver halide salts dispersed within gelatin to study traces of pions. The film with gelatin-silver emulsion was referred to as photographic emulsion. The photographic films were set-up and exposed to cosmic radiation on top of mountain ranges - high altitudes.


  • Initially, the method was ineffective because the film had low sensitivity to ionising particles and traces of ionisation faded over time. Powell solved this problem by collaborating with industry-giants in photograph at the time (Kodak) to develop films that were more sensitive to ionisation. He also adopted state-of-the-art microscopes to study the photographic films at higher resolution.
  • Neutral pions (p0) could not be discovered with photographic emulsions nor in Wilson cloud chambers due to their neutral charge and lack of ionising ability. Instead, they were first indirectly inferred from observing its decay products in cosmic rays. This was confirmed by observing its decay into two photons.


Neutrino (electron neutrino, v)

  • Initially, our understanding of beta-minus decay was simple. It was thought that only two particles are emitted during beta-minus decay: an electron and the daughter nucleus. Beta-minus decay was also thought to be monokinetic where the emitted electron took away the energy difference between the parent and daughter nuclides.



  • However, unlike alpha decay, beta-minus decay was shown to not be monokinetic because the emitted electron varied in kinetic energy – which never added to up to the total energy before the decay. This was not consistent with the Law of conservation of energy.



  • The existence of the neutrino was first proposed by Wolfgang Pauli in 1931. In response to the missing energy observed in beta-minus decay, Pauli proposed that a very light particle is also emitted in addition to the electron, and the kinetic energy of this light particle would account for the missing energy.
    • This was later corrected to be an antineutrino. Beta-minus decay emits an electron and antineutrino while beta-plus decay emits a positron and neutrino.
    • This particle was termed neutrino because it was small and neutral in charge.
    • The mass of neutrinos is undetermined.


  • The existence of the neutrino was experimentally confirmed by Fredrick Reines and Clyde Cowan in 1956 when they were studying particles emitted from products resulted from nuclear fission. This was not possible during Pauli’s time because nuclear reactors were during development and strictly used for creating atomic bombs.


Previous section: The Standard Model of Matter

Next section: Particle Accelerators