Elementary particle

Article By:

Close, Frank E. Rudolf Peierls Centre for Theoretical Physics, Oxford University, Oxford, United Kingdom.

Last reviewed:August 2022

DOI:https://doi.org/10.1036/1097-8542.227700

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• Elementary particle, published February 2019:Download PDF Get Adobe Acrobat Reader
• Elementary particle, published January 2018:Download PDF Get Adobe Acrobat Reader
• Elementary particle, published January 2015:Download PDF Get Adobe Acrobat Reader

Fundamental, irreducible units that constitute the matter of the universe and express the forces of nature. The development of quantum field theories in the twentieth century have provided the natural language for describing the interactions and properties of elementary particles. These descriptions are assembled in the standard model of particle physics. The standard model, which emerged in the 1970s, classifies the 17 known elementary particles in terms of their properties such as mass, spin and charge (Fig. 1 and table). Elementary particles are accordingly grouped as either fermions or bosons. Fermions are defined by having the same half-integer spin, or intrinsic angular momentum, and conserved quantum numbers, while bosons are defined by having whole-integer spin. The two families of elementary fermions are quarks and leptons. Quarks and leptons are further divided into six particle species, each of which has a corresponding antimatter particle. As for bosons, four kinds—called gauge bosons—are recognized as elementary particles. These gauge bosons convey three of the four known fundamental interactions or forces of nature, namely electromagnetism, the strong nuclear interaction (or strong force), and the weak nuclear interaction (or weak force). A hypothetical elementary particle, called the graviton, is theorized to convey the fourth known force of nature, gravity. A fifth boson, the Higgs boson, is the seventeenth elementary particle predicted by the standard model and experimentally confirmed. See also: Angular momentum; Electric charge; Fundamental interaction; Lepton; Mass; Quantum field theory; Quantum numbers; Quark; Spin (quantum mechanics); Standard model

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