preon

A preon is a hypothetical fundamental constituent proposed to lie beneath the quarks and leptons of the Standard Model. In preon theories, these known fermions are composite particles formed from more elementary entities bound by a new, yet-unobserved interaction that would operate at very high energy scales.

History and models: The idea emerged in the 1970s as a possible explanation for patterns among quarks and leptons and for charge quantization. The best-known version is the Harari–Shupe preon model, which posits a small set of basic preons with fractional electric charges. In such schemes, all quarks and leptons would be bound states of these preons, with the different observed fermions arising from distinct combinations and configurations. Over the years, many variants have been proposed, varying in the number of preon types and in the assumed binding dynamics. Despite this variety, no model has achieved wide experimental support or unique predictive success.

Experimental status: High-energy experiments have sought signs of substructure in quarks and leptons, but no deviations from point-like behavior have been observed. Deep inelastic scattering and collider data place stringent limits on any possible compositeness, pushing the scale of new binding interactions to the multi-TeV range or higher, depending on the specific model and observable.

Assessment: Preon theories remain speculative and face substantial theoretical and experimental challenges. They are not part of the established framework of particle physics, but they have informed discussions about the possibility that quarks and leptons are not fundamental and prompted consideration of new layers of matter beyond the Standard Model.