Cornering pseudoscalar-mediated dark matter with the LHC and cosmology

Shankha Banerjee,Benjamin Fuks,Geneviève Bélanger,Bryan Zaldivar,Daniele Barducci,Andreas Goudelis

doi:10.1007/jhep07(2017)080

Abstract

Models in which dark matter particles communicate with the visible sector through a pseudoscalar mediator are well-motivated both from a theoretical and from a phenomenological standpoint. With direct detection bounds being typically subleading in such scenarios, the main constraints stem either from collider searches for dark matter, or from indirect detection experiments. However, LHC searches for the mediator particles themselves can not only compete with — or even supersede — the reach of direct collider dark matter probes, but they can also test scenarios in which traditional monojet searches become irrelevant, especially when the mediator cannot decay on-shell into dark matter particles or its decay is suppressed. In this work we perform a detailed analysis of a pseudoscalar-mediated dark matter simplified model, taking into account a large set of collider constraints and concentrating on the parameter space regions favoured by cos-mological and astrophysical data. We find that mediator masses above 100-200 GeV are essentially excluded by LHC searches in the case of large couplings to the top quark, while forthcoming collider and astrophysical measurements will further constrain the available parameter space.

Highlights

At the price of introducing only a handful of additional free parameters
Barring somehow singular kinematic configurations that can occur in freeze-out scenarios [22], the two most notable situations in which direct detection constraints fall short concern models in which the dark matter particles can be lighter than a few GeV, where direct detection searches suffer from recoil energy threshold limitations [23], or models in which the spin-independent dark matter-nucleon scattering cross section is suppressed due to the Lorentz structure of the underlying theory
This suppression can occur in models of axial-vector mediated fermionic dark matter or, which is the topic of this work, in scenarios featuring a pseudoscalar-mediated fermionic dark matter candidate in which the dark matter-nucleon scattering cross section is suppressed by the momentum transfer involved in the reaction

Summary

Introduction

At the price of introducing only a handful of additional free parameters. simplified models capture, with a minimal set of assumptions, some important features of more ultraviolet-complete (UV) theories and, perhaps more importantly, they can provide a (semi-)consistent framework in order to analyse the experimental results [10]. Amongst all mono-X searches, the monojet one has garnered the most attention given the relative magnitude of the strong coupling with respect to the electroweak one This channel was shown to provide powerful constraints on dark matter models, especially in cases where direct detection experiments become inefficient [20, 21]. In this work we investigate the constraints on the parameter space of a simplified pseudoscalar-mediated fermionic dark matter model stemming from both monojet and multijet plus E/ T searches at the LHC. As we investigate scenarios with a large coupling of the mediator to top quarks, we assess the sensitivity of the direct measurement of the top quark pair production cross section to the new physics parameter space This observable turns out to play a key role even when a top-antitop pair is produced via an off-shell pseudoscalar exchange. Two appendices follow, discussing some technical aspects of our analysis

Results

Discussion

Conclusion