Particle Physics
Particle physics asks what everything is made of at the smallest scale, and what holds it together. The answer is the Standard Model: a short list of elementary particles and three of the four known forces, written in the language of relativistic quantum fields.
Matter is built from two families — quarks and leptons — each appearing in three generations of rising mass. The forces are carried by gauge bosons: the gluon, the photon, and the massive W and Z.
Every prediction runs through the same machinery. A process is a sum of Feynman diagrams, each vertex a factor and each internal line a virtual particle, and the calculus turns those pictures into rates and cross-sections you can measure.
The strong force is stranger than gravity or electromagnetism. Its charge, colour, grows more binding with distance, so quarks are permanently confined inside hadrons and never seen alone.
The weak force and electromagnetism are two faces of one electroweak interaction, split apart by the Higgs field. Its potential has no stable centre, so the field settles off-axis and the symmetry breaks.
That single act of symmetry breaking gives the W and Z their mass, hands every fermion a mass through its coupling, and leaves behind a physical Higgs boson — found at the LHC in 2012.
What makes the theory formidable is not just its scope but its precision: quantities like the electron's magnetic moment agree with experiment to more than ten digits, a level of confirmation no other physical theory matches.
The Standard Model is not the end. Neutrino masses, the nature of dark matter, the matter–antimatter asymmetry, and gravity itself all point past it — the open frontier the course closes on.