Simplified models of dark matter with a long-lived co-annihilation
  partner
  We introduce a new set of simplified models to address the effects of 3-point
interactions between the dark matter particle, its dark co-annihilation
partner, and the Standard Model degree of freedom, which we take to be the tau
lepton. The contributions from dark matter co-annihilation channels are highly
relevant for a determination of the correct relic abundance. We investigate
these effects as well as the discovery potential for dark matter
co-annihilation partners at the LHC. A small mass splitting between the dark
matter and its partner is preferred by the co-annihilation mechanism and
suggests that the co-annihilation partners may be long-lived (stable or
meta-stable) at collider scales. It is argued that such long-lived electrically
charged particles can be looked for at the LHC in searches of anomalous charged
tracks. This approach and the underlying models provide an
alternative/complementarity to the mono-jet and multi-jet based dark matter
searches widely used in the context of simplified models with the s-channel
mediators. We consider four types of simplified models with different particle
spins and coupling structures. Some of these models are manifestly gauge
invariant and renormalizable, others would ultimately require a UV completion.
These can be realised in terms of supersymmetric models in the neutralino-stau
co-annihilation regime, as well as models with extra dimensions or composite
models.
