Cornering pseudoscalar-mediated dark matter with the LHC and cosmology

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 cosmological 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.