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
The search for both axions and achions is a significant area of research in particle physics and