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We analyse the constraints on models of WIMP Dark Matter that can be derivedfrom upper bounds on the "monojet" cross section at the LHC. These constraintswere originally interpreted in the context of an effective field theory (EFT)where the Standard Model is extended by a dimension--6 operator whosecoefficient is $1/\Lambda^2$. We show that combining the 8 TeV data of theATLAS and CMS collaborations improves the bounds only slightly. We then analyzethis final state in the context of simplified models with $s-$channel mediator.We show that if the decay width of the mediator is small, these simplifiedmodels can be accurately modeled by the effective field theory only if themediator mass is above 5 TeV. Finally, we point out that even if the EFTaccurately describes the ${\cal O}(\Lambda^{-2})$ contributions to the matrixelement, for values of $\Lambda$ near the current bound it receives significantcontributions of order $\Lambda^{-4}$, in the context of simplified models,these correspond to diagrams where two mediators are exchanged. Thisobservation challenges the internal consistency of the EFT description sincedimension$-8$ operators, which would also contribute to ${\calO}(\Lambda^{-4})$ to the matrix element, are not included.

Analysis of the Bounds on Dark Matter Models from Monojet Searches at the LHC