Hypothetical particle physics models

Speculative theoretical frameworks that propose particles or interactions beyond those established in the Standard Model, often to explain unresolved observations or theoretical problems.

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Standard Model: The quantum field theory describing known elementary particles and three fundamental interactions, excluding gravity. It provides the baseline that proposed new particles and interactions must extend.
Dark matter: Matter inferred from gravitational effects that does not emit or absorb enough light to be seen directly. Many extensions propose new particles that could account for this unseen matter.
Supersymmetry: A proposed symmetry relating bosons and fermions, with a superpartner for each known particle. It predicts a broad spectrum of new particles and can address several theoretical puzzles.
Loop quantum gravity: A research program that attempts to quantize spacetime geometry without introducing a particle-physics extension. It addresses quantum gravity by a route distinct from adding new particle sectors.
Hierarchy problem: The puzzle of why the weak interaction scale is so small compared with the Planck scale. It motivates extensions that protect the Higgs mass from large quantum corrections.
Quantum field theory: A framework combining quantum mechanics and special relativity to describe fields and their particle excitations. Most particle extensions are formulated as quantum field theories.
Neutrino mass: The property that neutrinos have nonzero mass, established by neutrino flavor oscillations. The Standard Model's original formulation cannot explain observed neutrino masses.
String theory: A theoretical framework in which fundamental entities are one-dimensional strings rather than point particles. Its possible particle spectra and extra dimensions extend far beyond the Standard Model.
Phenomenological model: A model designed to capture observed or testable features without specifying a complete underlying theory. Some proposed particle frameworks prioritize testable signatures over fundamental explanation.
Strong CP problem: The question of why quantum chromodynamics appears to preserve charge-parity symmetry so precisely. Its possible resolution motivates the axion and related extensions.