= Elementary particle
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An <elementary particle> has no resolved constituent substructure within the physical theory and experiments under discussion. In the <Standard Model>, examples include <electrons>, <quarks> and <photons>. A <proton> is instead a composite <hadron>, even though it often acts as one particle in a detector or an atomic model.
The distinction is tested through interactions rather than by picturing a smaller material ball inside a larger one. Scattering at increasing <momentum> transfer probes shorter spatial scales and can reveal internal response. A fundamental <quantum field> describes the particle's excitations; being elementary does not mean that all measured electromagnetic properties reduce to those of a classical point charge.
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