achion

An achion is a hypothetical elementary particle that is a fermionic counterpart to the axion, a proposed particle associated with solving the strong CP problem in quantum chromodynamics. While axions are predicted to be bosons, achions would be fermions. The concept of achions arises from certain extensions of the Standard Model of particle physics, particularly those that introduce a new U(1) symmetry.

Like axions, achions are expected to be very light and interact weakly with ordinary matter. Their existence has not been experimentally confirmed, and searches for them are ongoing. Theoretical models suggest that achions could have a small mass and potentially contribute to dark matter. Their fermionic nature means they would obey the Pauli exclusion principle, which differs from the bosonic behavior of axions.

The search for both axions and achions is a significant area of research in particle physics and cosmology. Experiments are designed to detect their potential interactions with photons or their contribution to the mass density of the universe. The precise properties and existence of achions remain speculative, pending further theoretical development and experimental evidence.