Unit 4 Physics – Topic 3: Standard Model, Forces, Conservation & Feynman Diagrams
Historical Evolution of Particle Physics
19th-Century view: atoms considered indivisible “hard billiard balls,” representing the smallest fundamental units of matter.
Dalton’s atomic model (1808): proposed the first scientific model of atoms, describing them as uniform, solid spheres with distinct properties for each element. This laid the foundation for modern chemistry.
J.J. Thomson (1897): discovery of the electron using cathode ray tubes ➜ demonstrated that atoms possess an internal structure and are not fundamental, challenging Dalton's model and leading to the “plum-pudding” model.
Ernest Rutherford (1898–1911): conducted the seminal gold-foil experiment, which showed that most alpha particles passed through the foil, but a small fraction were deflected at large angles or even reflected. This led to the conclusion that atoms have a small, dense, positively charged nucleus surrounded by largely empty space occupied by electrons.
Particle accelerators (mid-20ᵗʰ C.): groundbreaking technologies like cyclotrons and synchrotrons enabled particles to be accelerated to extremely high energies, facilitating deep inelastic scattering experiments. These experiments led to the discovery of a “particle zoo” – a multitude of previously unknown particles, including anti-matter, various leptons (muons, taus), and numerous hadrons (e.g., baryons and mesons).
Need for unification: The proliferation of observed particles highlighted the need for a more coherent and predictive theoretical framework. This led to the development of the Standard Model (SM) – a quantum field theory that successfully consolidates our understanding of subatomic particles and their interactions.
• The SM classifies all known elementary particles based on their properties (mass, charge, spin) and interactions.
• It unifies 3 of the 4 fundamental forces of nature: the strong nuclear force, the electromagnetic force, and the weak nuclear force. Gravity remains outside the SM's scope.
Elementary Particles & Antiparticles
Elementary (fundamental) particle: a particle with no known internal substructure; it cannot be broken down into smaller, simpler constituents, representing the most basic building blocks of matter and energy.
• The current Standard Model regards four primary families of particles as elementary: leptons (e.g., electrons, neutrinos), quarks (building blocks of protons and neutrons), gauge bosons (force-carrying particles like photons), and the Higgs boson.
Antiparticle: Every elementary particle has a corresponding antiparticle, which shares identical mass and spin but possesses opposite additive quantum numbers (e.g., electric charge, baryon number, lepton number, and magnetic moment).
• Example: the electron ( with charge ) has a positron ( with charge ) as its antiparticle.
• When a particle encounters its antiparticle, they undergo annihilation. Their masses are converted entirely into energy, typically manifesting as two (or more) high-energy -ray photons, in accordance with Einstein's mass-energy equivalence . This process conserves energy and momentum.
Matter versus Antimatter in the Universe
Early Big-Bang conditions: In the extremely hot and dense early universe, thermal energy was sufficient to continually produce particle–antiparticle pairs through pair production ( in reverse). These pairs subsequently annihilated, creating a dynamic equilibrium.
Exact symmetry expectation: If an exact symmetry existed between matter and antimatter, all particles and antiparticles produced in the early universe would have completely annihilated, leaving behind only radiation today. However, the observation of our universe, which is overwhelmingly dominated by matter, implies a crucial matter–antimatter asymmetry must have existed at an early stage.
• Empirical estimate: Cosmological observations suggest that for every antiparticles created in the early universe, approximately particles were produced. This minuscule surplus of