Fermionidel

Fermionidel is a hypothetical fermionic particle proposed within certain extensions of the Standard Model of particle physics. In these theoretical frameworks, fermionidels are spin-1/2 particles that can interact with ordinary matter through new portal couplings or through a hidden sector. Depending on the model, a fermionidel may be neutral under the Standard Model gauge group and carry charges under an additional symmetry, or it may mix with Standard Model fermions.

Commonly envisioned properties include a mass that can span from a few GeV to multi-TeV scales, and a Dirac or Majorana nature. The stability of a fermionidel is model dependent; it may be stabilized by a discrete symmetry or decay into Standard Model particles with missing energy carried away by other hidden-sector states. If stable on cosmological timescales, a fermionidel could contribute to the dark matter content of the universe; if unstable, its decays would produce Standard Model fermions and potentially missing energy signals.

Theoretical context for fermionidels arises in models with hidden sectors and portal mechanisms that connect them to the visible sector. They may appear in neutrino portals, vector or Higgs portals, or in composite theories that include fermionic constituents. Their role is typically to address questions such as the nature of dark matter, neutrino mass generation, or anomalies in cosmology and flavor physics.

Experimental status is currently unsettled; no confirmed observations of fermionidels exist. Collider searches look for events with missing energy, displaced vertices, or resonant production, while astrophysical and cosmological data constrain relic density and interaction strength with ordinary matter. Ongoing and future experiments aim to explore broader regions of mass and coupling space.

See also: Dark matter, sterile neutrino, heavy neutral lepton, hidden sector, portal models.