Definition
The Standard Model is the current theory of elementary particles and three of the four fundamental forces. It is a quantum field theory built on the gauge group U(1)×SU(2)×SU(3), containing three generations of quarks and leptons, four force-carrying gauge bosons (photon, W, Z, gluon), and the Higgs boson that gives the other particles their masses.
It does not include gravity, dark matter, or dark energy, but within its scope it matches experiment to extraordinary precision.
Why it matters
How it works
The Standard Model's content is small enough to list. Matter: six quarks (up, down; charm, strange; top, bottom) and six leptons (electron, muon, tau and their neutrinos), arranged in three generations of identical structure with rising mass. Forces: the U(1) gauge boson is the photon, the three SU(2) bosons combine with U(1) after electroweak symmetry breaking to produce the photon, W⁺, W⁻, and Z, and SU(3) yields the eight gluons. Mass-giver: the Higgs scalar field, whose nonzero vacuum value spontaneously breaks SU(2)×U(1) down to the electromagnetic U(1) and gives mass to W, Z, and the charged fermions.
Each interaction follows from a gauge symmetry. Electromagnetism couples to electric charge; the weak force couples to a "weak isospin" carried by left-handed fermions; the strong force couples to color carried by quarks and gluons. The Higgs mechanism explains how the W and Z bosons can be massive without breaking the gauge structure, by hiding the symmetry in the vacuum rather than in the equations.
The model's predictive triumph came in stages. Glashow-Salam-Weinberg unified the electroweak sector in the 1960s; the W and Z bosons were found at CERN in 1983; the top quark at Fermilab in 1995; the tau neutrino at Fermilab in 2000; and the Higgs at the LHC in 2012. Each appeared roughly where predicted with roughly the predicted properties.
What it does not contain: gravity (general relativity is separate and not quantized), dark matter, dark energy, neutrino masses (they were originally assumed zero, then shown nonzero by oscillation experiments — a small extension), and an explanation of the matter-antimatter asymmetry. Each gap is an open invitation to physics beyond the Standard Model.