Overview
This lecture covers the development, classification, and properties of elementary particles, the Standard Model, fundamental forces, symmetries, and open problems in particle physics and cosmology.
Historical Evolution of Elementary Particles
- Ancient Greeks considered atoms as indivisible, elementary particles.
- Discovery of the atomic nucleus showed atoms are mostly empty space.
- Atoms consist of nuclei (protons and neutrons) and electrons.
- Quantum mechanics explained atomic structure and particle stability.
- Discovery of the neutron led to the idea of protons, neutrons, and electrons as elementary particles.
- Neutron decay introduced the neutrino as a new elementary particle.
Particle Classification and Discovery
- New particles (mesons, muons, etc.) found in cosmic rays and accelerators increased the particle "zoo."
- Particles classified by properties like spin: bosons (integer spin) and fermions (half-integer spin).
- Particles further divided into hadrons (feel strong force) and leptons (do not).
- Hadrons later found to be composite, made of quarks.
Accelerators and Particle Detection
- Particle accelerators use electric/magnetic fields to reach high energies and discover new particles.
- Colliders allow two beams to collide, increasing chances of discovering heavy particles.
- Detectors record traces and properties of created particles.
Quark Model
- Hadrons are made of quarks; baryons have three quarks, mesons have two.
- Initially, three quarks (up, down, strange) explained observed particles.
- More quarks (charm, bottom, top) discovered, leading to six flavors grouped into three generations.
Leptons and Neutrino Physics
- Three charged leptons (electron, muon, tau) and their neutrinos exist, forming three generations.
- Neutrino oscillations (changing type) show neutrinos have mass, but absolute mass is unknown.
- Neutrinos interact weakly; large underground detectors are needed to observe them.
Symmetry and the Standard Model
- Particle classification and interactions rooted in symmetry (group theory).
- Types of symmetries include spatial reflection, charge conjugation, and phase (continuous) symmetry.
- Gauge symmetries dictate conservation laws and predict force carriers.
- Symmetry breaking (e.g., Higgs mechanism) gives particles mass.
Fundamental Interactions
- Four fundamental forces: electromagnetic (photon), strong (gluons), weak (W/Z bosons), gravitational (graviton - hypothetical).
- Higgs boson discovered in 2012, linked to particle mass generation.
Structure and Content of the Standard Model
- Standard Model: six quarks, six leptons, force carriers, one Higgs boson.
- All matter and interactions (except gravity) described by three gauge groups: SU(3), SU(2), U(1).
- Experimental evidence for three generations of particles.
Unsolved Problems and Open Questions
- Why are there three generations of quarks and leptons?
- What is dark matter (not explained by the Standard Model)?
- Why is there more matter than antimatter in the universe?
- What is the nature of dark energy?
- How to unify gravity with other forces?
- Possibility of new particles, interactions, or symmetries beyond the Standard Model.
Key Terms & Definitions
- Quark — fundamental particle making up hadrons, comes in six flavors.
- Lepton — light, weakly interacting particles (e.g., electron, neutrino).
- Hadrons — composite particles made of quarks, including baryons and mesons.
- Boson — particle with integer spin, force carrier (e.g., photon, gluon).
- Fermion — particle with half-integer spin, makes up matter (e.g., quarks, leptons).
- Symmetry — invariance under transformation (reflection, rotation, etc.); foundational in physics.
- Gauge symmetry — a symmetry that dictates conservation laws and interactions.
- Standard Model — theory describing fundamental particles and their interactions (except gravity).
- Higgs mechanism — process by which particles acquire mass via spontaneous symmetry breaking.
Action Items / Next Steps
- Review particle classification (leptons, hadrons, bosons, fermions).
- Study the role of symmetry and group theory in the Standard Model.
- Read about neutrino oscillations and their significance.
- Prepare for questions regarding the open problems (dark matter, dark energy, matter-antimatter asymmetry).