Constraints on spin-0 dark matter mediators and invisible Higgs decays using ATLAS 13 TeV $pp$ collision data with two top quarks and missing energy in the final state

In this presentation, the statistical combination of searches targeting final states with two top quarks and invisible particles and characterised by the presence of zero, one or two leptons, at least one jet originating from a b-quark and missing transverse momentum is presented. The analyses are performed with the ATLAS detector using 139 fb$^{−1}$ of pp collisions at a centre-of-mass energy of 13 TeV at the LHC and search for beyond the standard model phenomena consistent with the production of dark matter particles at colliders. The results are interpreted in terms of simplified dark matter models with a spin-0 scalar or pseudoscalar mediator particle. In addition, the results are interpreted in terms of upper limits on the Higgs boson invisible branching ratio, where the Higgs is produced according to the Standard Model in association with a pair of top quarks.