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A subatomic particle is a constituent or excitation studied at the level below ordinary atomic structure. The category includes elementary particles, such as the electron, and composite hadrons, such as the proton and neutron.
Calling something a particle does not require a tiny classical solid sphere. Quantum mechanics describes localized detections and wave-like propagation, while high-energy scattering probes whether internal structure is resolved. A particle detector usually infers a particle from transferred energy, emitted light or a trajectory reconstructed from many interactions. The measured track width and the particle's intrinsic spatial structure are therefore different quantities.
