We study dark matter (DM) models in which there are two dark sector particles, ${\chi }_1$ and ${\chi }_2$, of near mass. In such models, co-annihilation of ${\chi }_1$ and ${\chi }_2$ may be the dominant process controlling the DM relic density during freeze-out in the early Universe. In this scenario, there is no significant contribution to direct and indirect detection signals, unless there exists an extreme degeneracy in the masses of the lightest dark sector particles. Therefore, relic density constraints and collider searches provide the most relevant information about these models. We consider Dirac fermion dark matter which couples to standard model (SM) particles via an effective operator. For the collider phenomenology, where an effective field theory may not be valid, we adopt a simple Z’ model to provide an appropriate UV completion. We explore the interesting LHC signals that arise from the dark matter production process $pp\to \overline{{\chi }_1}+{\chi }_2+\text{jet}$, followed by the decay ${\chi }_2\to {\chi }_1+\mathrm{SM}$.

• Received 2 December 2013

DOI:https://doi.org/10.1103/PhysRevD.89.115001