The Standard Model
- Twelve fermions in three generationsnot yet tested
- Three gauge forces and their bosonsnot yet tested
- The Higgs field giving particles massnot yet tested
- Gravity, dark matter, and what it omitsnot yet tested
Between 1968 and the 2012 discovery of the Higgs boson at CERN, particle physicists assembled what is now called the Standard Model — a quantum field theory describing all known elementary particles and their interactions except gravity. The Standard Model has been tested with extraordinary precision: the electron's anomalous magnetic moment matches the theory's prediction to twelve significant figures, the most precise agreement between theory and experiment in all of science. It is also frustratingly incomplete: it gives no account of dark matter (~25% of the universe), dark energy (~70%), neutrino masses, or gravity. The Standard Model is the most successful and most unsatisfying physical theory ever written.
Matter in the Standard Model is built from twelve fermions in three generations: the first contains the up and down quarks, the electron, and the electron neutrino; the second the charm and strange quarks, the muon, and the muon neutrino; the third the top and bottom quarks, the tau, and the tau neutrino. Quarks combine into baryons (three quarks — protons uud, neutrons udd) and mesons (quark-antiquark pairs); leptons interact only via electroweak forces. Three of the four known forces are gauge interactions mediated by twelve gauge bosons: the electromagnetic force (one photon, U(1)), the weak nuclear force (three bosons — W⁺, W⁻, Z, responsible for beta decay, SU(2)), and the strong nuclear force (eight gluons, hadrons held together, SU(3)). Standing apart from these force-carriers is a single scalar boson: the Higgs field (quantum: the Higgs boson, discovered 2012 at the LHC) gives mass via the Higgs mechanism. The electroweak unification (Glashow-Weinberg-Salam, Nobel 1979) shows electromagnetic and weak forces are one at high energies. Quantum chromodynamics exhibits confinement — quarks are never observed in isolation. The Standard Model has ~20 free parameters (masses, mixing angles, couplings), measured not predicted. What it does not include is conspicuous: gravity, dark matter, dark energy (the cosmological-constant problem: naive SM estimates give a value 10¹²⁰ times the observed), neutrino masses, and the matter-antimatter asymmetry.