Particle physics (or high energy physics) studies the nature of the particles that constitute matter (particles with mass) and radiation (massless particles). Although the word "particle" can refer to various types of very small objects (e.g., protons, gas particles, or even household dust), "particle physics" usually investigates the irreducibly smallest detectable particles and the irreducibly fundamental force fields necessary to explain them.
Thumbnail: In this Feynman diagram, an electron and apositron annihilate, producing a photon(represented by the blue sine wave) that becomes aquark–antiquark pair, after which the antiquark radiates a gluon (represented by the green helix). Image used with permission (CC-SA-BY 2.5; Joel Holdsworth).
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