Recommendations of the LHC Dark Matter Working Group: Comparing LHC searches for dark matter mediators in visible and invisible decay channels and calculations of the thermal relic density

Abstract

Weakly-coupled TeV-scale particles may mediate the interactions between normal matter and dark matter. If so, the LHC would produce dark matter through these mediators, leading to the familiar "mono-X" search signatures, but the mediators would also produce signals without missing momentum via the same vertices involved in their production. This document from the LHC Dark Matter Working Group suggests how to compare searches for these two types of signals in case of vector and axial-vector mediators, based on a workshop that took place on September 19/20, 2016 and subsequent discussions. These suggestions include how to extend the spin-1 mediated simplified models already in widespread use to include lepton couplings. This document also provides analytic calculations of the relic density in the simplified models and reports an issue that arose when ATLAS and CMS first began to use preliminary numerical calculations of the dark matter relic density in these models.

Direct detection and LHC constraints on a t-channel simplified model of Majorana dark matter at one loop

“…Much of previous work has considered "s-channel models" in which the mediator is a dark force carrier, a neutral boson which has interactions both with a pair of dark matter particles and with a pair of Standard Model particles [2][3][4][5]. While interesting parameter space remains to be explored, such constructions are generically constrained by searches for visible decays of the mediator [6,7]. A different, and equally compelling class of models contains colored mediator particles, which can either interact directly with a quark and a dark matter particle [8][9][10][11][12][13][14][15], or act as a bridge at loop level between a pair of dark matter particles and a pair of gluons [16][17][18].…”

Interference effects in dilepton resonance searches for Z′ bosons and dark matter mediators

“…In this model the Z width can easily be large compared to the detector resolution and therefore large enough for interference effects to be relevant. As a specific benchmark we considered g χ = 1, g q = 0.1, and g = 0.01 (a choice recommended by the LHC Dark Matter Working Group and used by both ATLAS and CMS to present exclusion limits [42]), as well as an additional example with g = 0.02. Deriving bounds on this model as a function of DM mass and mediator mass, we demonstrated that interference effects lead to substantially stronger constraints on the parameter space of this model (see figure 5).…”

“…Such large widths typically cannot be obtained from decays into SM particles (as the required couplings would violate experimental constraints), but they are a generic prediction in models with additional contributions to the Z width arising from decays into new invisible light degrees of freedom. As a specific example of such a model, we consider a spin-one simplified DM model [23], which has been employed by the LHC collaborations [41,42] to create benchmark points in theory space that allow for different LHC DM searches to be compared to each other and to non-collider experiments.…”

“…In section 3 we describe our implementation of an ATLAS search for dilepton resonances with 139 fb −1 of data [5], including the modeling of detector effects, the statistical method, and a validation via comparison to published bounds. In section 4 we then present our results on the importance of interference effects for various Z models, with a special focus on a DM simplified model, with benchmark couplings recently proposed by the LHC DM working group [42]. Finally, our conclusions are presented in section 5.…”

“…On a more phenomenological level, they have also received substantial attention as the mediator of spin-one simplified models of Dark Matter (DM) [22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40]. These simplified models have been advertised by the LHC DM working group [41,42] in order to explore the complementarity between different LHC analyses and across different DM experiments, which include direct and indirect detection, as well as observations of the DM relic density.…”