Self-interacting Elko dark matter with an axis of locality

Here we report that Elko (for Eigenspinoren des Ladungskonjugationsoperators) breaks Lorentz symmetry in a rather subtle and unexpected way by containing a ``hidden'' preferred direction. Along this preferred direction, a quantum field based on Elko enjoys locality. In the form reported here, Elko offers a mass dimension one fermionic dark matter with a quartic self-interaction and a preferred axis of locality. The locality result crucially depends on a judicious choice of phases.