The Standard Model of particle physics

Through most of the twentieth century, particle physics discovered an ever-growing list of "elementary" particles — hundreds of them by the 1960s — until it became clear that most were not elementary at all but composites built from a much smaller set of truly fundamental constituents. The Standard Model is the theory that organizes those genuine fundamentals and the forces that act between them.

Matter particles fall into two families, both fermions (obeying the Pauli exclusion principle). Quarks come in six types ("flavours": up, down, charm, strange, top, bottom) and combine in groups of two or three, bound by the strong force, to form composite particles called hadrons — a proton is two up quarks and one down quark, a neutron is two down quarks and one up quark. Quarks are never observed in isolation; the strong force between them grows rather than shrinks with separation, so pulling a quark out of a hadron costs so much energy that it instead creates new quark-antiquark pairs from the vacuum, a phenomenon called confinement. Leptons are the second matter family and include the electron along with its heavier cousins the muon and tau, plus three corresponding neutrinos; unlike quarks, leptons such as the electron are observed directly and individually and are not subject to the strong force at all.

Forces between these matter particles are carried by a second class of particles, bosons, one type for each interaction the Standard Model describes: the photon carries the electromagnetic force, the W and Z bosons carry the weak force (responsible for processes like beta decay), and the gluon carries the strong force that binds quarks together inside hadrons and indirectly binds protons and neutrons together inside nuclei. A separate particle, the Higgs boson, is associated with the mechanism that gives many Standard Model particles their mass. Gravity is notably absent from this picture: no confirmed force-carrying particle for gravity has been found within the Standard Model's framework, and unifying gravity with the other three forces described by the Standard Model remains one of the major open problems in physics.