Standard Model of Particle Physics

The Standard Model of Particle Physics is the theoretical framework describing the elementary constituents of matter and three of the four fundamental forces — the electromagnetic, weak nuclear, and strong nuclear forces, mediated respectively by photons, W and Z bosons, and gluons. It classifies all known elementary particles: six quarks, six leptons, four gauge bosons, and the Higgs boson. It is the most experimentally confirmed theory in science, with some predictions (such as the anomalous magnetic moment of the electron) verified to twelve significant figures.

Its limitations are precisely known, which is rare in science. The Standard Model excludes gravity, offers no candidate for dark matter, provides no mechanism for the matter-antimatter asymmetry observed in the universe, and contains approximately 19 free parameters with no theoretical derivation. A theory with 19 adjustable constants is not obviously more than an extremely well-organized summary of experimental results. Whether those constants will eventually be derived from a deeper principle — some symmetry not yet discovered, or a connection to quantum gravity — or whether they are simply the universe's arbitrary choices, is the open question that defines the frontier of physics.