Secondaryon

Secondaryon is a hypothetical elementary particle proposed in certain speculative extensions of the Standard Model. In these theories, a secondaryon can act as a mediator between the visible sector and a hidden or dark sector, or emerge as a remnant of a broken symmetry. The term reflects its suggested role as a secondary mediator beyond the known gauge bosons. The concept is used in discussions of physics beyond the Standard Model, particularly in the context of portal interactions between sectors.

Theoretical properties of a secondaryon vary by model. It may be a scalar (spin-0) or a vector (spin-1), and many realizations posit a neutral particle with only weak couplings to Standard Model fields. These couplings are typically suppressed by a small mixing parameter or portal coupling, making the secondaryon difficult to detect directly.

Interactions with Standard Model particles are usually described through portals, such as the Higgs portal or the vector portal. Through these mechanisms, a secondaryon could couple to SM fermions, photons, or other visible-sector states, potentially decaying into fermion pairs, photon pairs, or entering invisible channels. Depending on its mass and couplings, a secondaryon could be short-lived or long-lived, potentially producing displaced vertices in detectors.

Phenomenology of the secondaryon spans collider, fixed-target, and cosmological contexts. In cosmology, it might influence early-universe dynamics, contribute to dark matter production, or modify the effective number of neutrino species. In experiments, searches focus on resonant production, missing energy signals, or displaced decays, with constraints arising from collider data, flavor physics, fixed-target experiments, and astrophysical observations. To date, there is no confirmed observation of a secondaryon.

Status: Secondaryons remain speculative and are discussed within broader dark-sector and portal frameworks. They are not part of the confirmed particle inventory.

See also: dark photon, Higgs portal, vector portal, hidden sector, beyond Standard Model.