Standard Model is the established quantum field theory describing the known elementary particles and three of the four fundamental interactions, namely the electromagnetic interaction, the weak interaction and the strong interaction. It does not include gravity. The model is based on gauge symmetries and describes matter using quark and lepton fields while describing interactions through gauge fields. Its central gauge symmetry is commonly written as SU3 times SU2 times U1. The first factor corresponds to colour and Quantum Chromodynamics, while the latter two describe the electroweak sector. The model successfully predicts a vast range of experimental measurements and is one of the most extensively tested theories in science.
The Standard Model developed through the synthesis of several major theoretical and experimental advances. Quantum Electrodynamics provided a highly successful quantum description of electromagnetism. The electroweak theory developed by Sheldon Glashow, Abdus Salam and Steven Weinberg unified electromagnetic and weak interactions within a common theoretical framework. Quantum Chromodynamics supplied the theory of the strong interaction. The discovery of quarks, gluons and the W and Z bosons progressively confirmed important elements of the framework. The Higgs boson was discovered at the Large Hadron Collider in 2012, providing experimental evidence for the Higgs mechanism incorporated into the model.
The matter content of the Standard Model consists primarily of quarks and leptons. There are six known quark flavours, conventionally named up, down, charm, strange, top and bottom, and six known leptons, consisting of the electron, muon, tau and three associated neutrinos. Each matter particle has a corresponding antiparticle. Quarks participate in the strong, electromagnetic and weak interactions, subject to the relevant charges. Charged leptons participate in electromagnetic and weak interactions, while neutrinos participate in the weak interaction and are electrically neutral. The model organises these particles into three generations with similar interaction structures but different masses.
The force carrying particles in the Standard Model are bosons. Gluons mediate the strong interaction, photons mediate electromagnetism, and W and Z bosons mediate the weak interaction. The Higgs boson is different because it is not conventionally classified as a gauge force carrier in the same way as these particles. Instead, it represents an excitation of the Higgs field. The Higgs field has a nonzero vacuum expectation value, and interactions with this field contribute to the masses of the W and Z bosons and to the masses of quarks and charged leptons through Yukawa interactions. The Higgs mechanism is therefore central to the electroweak structure of the Standard Model.
Gauge symmetry determines much of the mathematical architecture of the Standard Model. Gauge theories describe interactions through local symmetries and associated gauge fields. The strong interaction is governed by SU3 colour symmetry, while the electroweak sector is based on SU2 times U1 before spontaneous symmetry breaking. The Higgs field breaks electroweak symmetry in a way that leaves the electromagnetic U1 symmetry unbroken. As a consequence, the photon remains massless while the W and Z bosons become massive. Electric charge emerges from the relationship between the underlying electroweak generators, and the observed interaction structure follows from the resulting symmetry breaking.
The Standard Model has achieved remarkable predictive success. It correctly describes phenomena ranging from electromagnetic scattering to weak decays and high energy strong interaction processes. Precision measurements of electroweak observables have repeatedly tested its predictions, while collider experiments have measured properties of many of its particles with increasing accuracy. Quantum corrections are essential for obtaining precise predictions, meaning that the model is more than a simple collection of classical force rules. Calculations often require sophisticated perturbative methods, numerical techniques and extensive experimental input. Agreement between theory and experiment is assessed by comparing measured quantities with predictions including statistical and systematic uncertainties.
Despite its success, the Standard Model has important limitations. It does not provide a quantum theory of gravity, and it does not identify the particle responsible for dark matter. It also does not explain why the fundamental parameters have their observed numerical values or why there are three generations of matter. Neutrino oscillations demonstrate that neutrinos have mass, requiring an extension or modification of the minimal Standard Model because its original neutrino sector treated neutrinos as massless. The model also does not by itself explain the observed cosmic matter antimatter asymmetry. These limitations motivate extensive research into physics beyond the Standard Model.
The Standard Model should not be interpreted as claiming that everything in nature is completely understood. It is a specific quantum field theory with a defined domain and particle content, not a general synonym for modern physics. Composite objects such as atoms and nuclei can be understood using theories that ultimately connect to Standard Model interactions, but their emergent properties can be extremely complicated. Likewise, gravity remains outside the model even though gravity is fundamental to nature. The Standard Model is therefore both highly successful and incomplete, providing a precise description of known elementary particle interactions while leaving several major questions open.
