An Investigation of Constraints on Dark Matter Models from Mono-jet Searches at LHC

Abstract

In this work we investigate the Weakly Interacting Massive Particle (WIMP) dark matter searches in colliders. We focus on the mono-jet searches and derive the upper bounds on the WIMP production cross-sections at the LHC. Using the derived bounds we further constrain the models describing WIMP dark matter. We start by using Effective Field Theory (EFT) for our analysis. In our context, EFT for WIMP dark matter is an extension of the Standard Model by a dimension--6 operator which has a coefficient $1/Lambda^2$, where $Lambda$ is the regulating scale upto which the effective theory analysis is safely applicable. We do the analysis for both the 8 $tev$ and 13 $tev$ searches at the ATLAS and CMS experiments of LHC. We also combine the 8 $tev$ data of ATLAS and CMS collaborations and observe that this approach improves the individual bounds only very little. <br /> The EFT is able to accurately describe the contributions from the ${cal O}(Lambda^{-2})$ to the tree-level matrix element. However, we show that the tree level matrix element receives significant contributions of order $Lambda^{-4}$ for the values of $Lambda$ near the current bound. When these higher order contributions of $Lambda$ are considered they strengthen the current upper bounds on the WIMP cross-section significantly. In case of a simplified model, such contributions further correspond to the processes where two mediators are exchanged. EFT interpretations face difficulties when the higher order operators, like dimension--8 operators, are not taken into consideration. The $Lambda^{-4}$ contributions to the matrix element from a double mediator exchange are similar to the ${cal O}(Lambda^{-4})$ contributions coming from the dimension--8 operators. Based on our observations, we challenge the internal consistency of the EFT description. <br /> We further translate the bounds obtained from the effective theory to the simplified models. We study the mono-jet signals for the $s-$channel mediator in the simplified model where the mediator's width can be accurately calculated. We then show that if the mediator's width is small then the simplified models can be accurately derived from the effective field theory only when the mediator has a mass greater than 5 $tev$. This requires that the upper bounds on the cross-sections to be 16 times stronger than the current bounds at LHC. This is very unlikely for the considered parameters and hence, such a model cannot be described accurately by EFT. This application is even more redundant if the bounds at 13 $tev$ are to be considered. In the end we show that the inconsistency of the EFT description further makes it difficult to apply its model independent results to a well-defined model, in any sensible way.