kappaon

Kappaon is a hypothetical elementary particle proposed in several extensions of the Standard Model of particle physics. In most models, a kappaon is a neutral, spin-1/2 fermion that interacts with Standard Model fields primarily through mixing with neutrinos or via portal interactions to new sectors. The exact properties vary between theories, but common assumptions include electric charge zero, color neutrality, and a mass that can span from the MeV to the TeV scale. Depending on the model, the particle may be a Dirac or Majorana fermion and may be referred to as a heavy neutral lepton or sterile-neutrino-like state.

Potential roles and motivations include generating neutrino masses via seesaw mechanisms, acting as a dark matter candidate in certain parameter regions, or participating in baryogenesis through CP-violating decays. Its interactions are typically mediated indirectly with the weak sector, allowing decays and productions that depend on the kappaon’s mass and mixing with active neutrinos.

Interactions and decays: kappaons typically couple to the weak sector through mixing with active neutrinos, enabling decays such as kappaon → W l, Z ν, or h ν when kappaon mass lies above the corresponding thresholds. For lighter kappaons, decays may proceed via suppressed channels such as to three neutrinos or to a neutrino plus a photon in loop processes. Production mechanisms in experiments include decays of W and Z bosons, meson decays, and high-energy collisions.

Experimental status: no confirmed observation. Searches in collider experiments, fixed-target setups, and meson-decay experiments have placed limits on mixing parameters and lifetimes, leaving broad regions of parameter space unconstrained.

Etymology: the name combines the Greek letter kappa with the common particle-name suffix -on.