Dark matter with $t$-channel mediator: a simple step beyond contact interaction

Effective contact operators provide the simplest parameterization of dark matter searches at colliders. However, light mediator can significantly change the sensitivity and search strategies. Considering simple models of mediators is an important next-step for collider searches. In this paper, we consider the case of a $t$-channel mediator. Its presence opens up new contributions to the monojet$+\displaystyle{\not{E}}_T$ searches and can change the reach significantly. We also study the complementarity between searches for processes of monojet$+\displaystyle{\not{E}}_T$ and direct pair production of the mediators. Mediator pair production also gives important contribution to a CMS-like monojet$+\displaystyle{\not{E}}_T$ search where a second hard jet is allowed.There is a large region of parameter space in which the monojet$+\displaystyle{\not{E}}_T$ search provides the stronger limit. Assuming the relic abundance of the dark matter is thermally produced within the framework of this model, we find that in the Dirac fermion dark matter case, there is no region in the parameter space that satisfies the combined constraint of monojet$+\displaystyle{\not{E}}_T$ search and direct detection; whereas in the Majorana fermion dark matter case, the mass of dark matter must be larger than about 100 GeV. If the relic abundance requirement is not assumed, the discovery of the $t$-channel mediator predicts additional new physics.