Co-annihilating Dark Matter: Effective Operator Analysis and Collider
  Phenomenology
  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 freezeout 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 $pp \rightarrow \overline{\chi_1} + \chi_2 + \textrm{
jet}$, followed by the decay $\chi_2 \rightarrow \chi_1 + SM$.
