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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Elementary particles are the fundamental constituents of matter and radiation in the universe. According to the current understanding in particle physics, especially as described by the Standard Model, elementary particles are not made up of smaller particles; they are the most basic building blocks of the universe. Elementary particles can be classified into two main categories: 1. **Fermions**: These are the particles that make up matter. They have half-integer spin (e.g., 1/2, 3/2).
The opposite of quasiparticle, see notaby: quasiparticles vs elementary particles.