particle

A particle is a localized object that can be assigned properties such as mass, electric charge, and momentum. In classical physics, a particle is treated as a point-like body whose motion is described by equations of motion. In quantum mechanics, particles are described by wavefunctions and probabilities; phenomena such as wave-particle duality reflect that a particle can show both localized and spread-out behavior.

Particles are categorized as elementary or composite. Elementary particles are not known to be made of smaller components; the Standard Model lists quarks, leptons, gauge bosons, and the Higgs boson. Composite particles include protons, neutrons, and many mesons and nuclei, which are bound states of elementary particles by the strong interaction.

Interactions between particles are mediated by force carriers: photons for electromagnetism, gluons for the strong force, W and Z bosons for the weak force. Gravity would be mediated, in quantum theories, by gravitons (hypothetical). The Standard Model describes electromagnetic, weak, and strong forces; gravity is described separately by general relativity. Many properties are characterized by quantum numbers such as spin, charge, and color.

In practice, particle physics investigates the fundamental constituents by high-energy experiments using particle accelerators and detectors. Observations have confirmed the Standard Model to high precision, including the discovery of the Higgs boson, and continue to test for new physics beyond it.

Outside physics, the term particle also appears in everyday language to refer to a small piece of matter or a constituent particle such as a dust particle or aerosol.