neutrinieller

Neutrinieller is a hypothetical neutral fermion invoked in certain extensions of the Standard Model of particle physics. The term is used in theoretical discussions to describe a neutral lepton related to the known neutrinos but heavier or more weakly coupled. In many models, a neutrinieller corresponds to a heavy neutral lepton (HNL) or sterile neutrino that mixes with active neutrinos, allowing indirect interactions with the weak sector.

Properties and variants: A neutrinieller is typically assumed to be a spin-1/2 fermion with no electric charge. Its mass scale is model dependent, ranging from sub-eV to well above the electroweak scale in various proposals. Depending on the model, the neutrinieller may be a Dirac or Majorana particle and may couple to Standard Model particles only through mixing with active neutrinos or through new interactions. These properties govern its production and decay channels in experiments.

Role in theory: The introduction of a neutrinieller is motivated by attempts to explain active neutrino masses via seesaw mechanisms, contribute to leptogenesis in the early universe, or serve as a dark matter candidate if sufficiently stable and light. It also provides a framework to test new symmetries or mechanisms beyond the Standard Model.

Experimental status: There is no confirmed observation of a neutrinieller. Experimental searches focus on meson decays, collider signatures, and fixed-target experiments that could reveal heavy neutral leptons, as well as cosmological and astrophysical data that constrain their properties. No parameter region has been decisively established.

See also: sterile neutrino, heavy neutral lepton, beyond the Standard Model.