Hypothetical particle

A particle proposed by a physical theory or to explain observations whose existence has not been experimentally established. Such proposals may later be confirmed, revised, or rejected.

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Axion: A hypothetical light particle proposed to solve the strong CP problem and considered a dark matter candidate. It was proposed to explain a symmetry puzzle and could account for dark matter.
Symmetry breaking: A change in which a system's state no longer displays a symmetry of its governing laws. Broken symmetries can require additional particles in a theory's spectrum.
Elementary particle: A particle not known to consist of smaller constituents, within the limits of current experiments. Hypothetical particles are proposed additions to the set of elementary particles.
Particle discovery: The establishment of a particle's existence through converging experimental evidence. A proposal becomes a discovery only when evidence meets demanding standards.
Magnetic monopole: A hypothetical particle carrying isolated magnetic charge, rather than paired north and south poles. Its possible existence follows from some theories that extend electromagnetism.
Quantum field theory: A framework combining quantum mechanics and special relativity by describing particles as excitations of fields. Most modern particle proposals are formulated as fields and their excitations.
Standard Model: The quantum field theory describing known elementary particles and three of their fundamental interactions. A proposed particle may fill a gap in, or extend, this established framework.
Anomaly (physics): A measurement or theoretical result that conflicts with an established physical framework or expectation. An anomaly can motivate a particle proposal but does not identify its cause.
Graviton: A hypothetical quantum of the gravitational field, often described as a massless spin-2 particle. It would be the particle description of gravity in a quantum theory.
Effective field theory: A theory describing physical phenomena at a chosen energy scale without specifying all higher-energy details. It can predict effects of unseen particles even when their masses exceed experimental reach.