Combination of searches for invisible Higgs boson decays with the ATLAS experiment

Many extensions of the Standard Model predict the production of dark matter particles at the LHC. Sufficiently light dark matter particles may be produced in decays of the Higgs boson $H$ that would appear invisible to the detector. This document presents a statistical combination of searches for $H\to\text{invisible}$ decays where the $H$ is produced according to the Standard Model via vector boson fusion or in association with a pair of top quarks in final states with zero or two leptons. These searches are performed with the ATLAS detector using 139~fb$^{-1}$ of $pp$ collisions at a centre-of-mass energy of $\sqrt s = 13$ TeV at the LHC. In combination with the results at $\sqrt s = 7$ and $8$ TeV, an upper limit on the $H\to\text{invisible}$ branching ratio of $0.11~(0.11^{+0.04}_{-0.03})$ at 95\% confidence level is observed (expected). These results are also interpreted in the context of models where the 125 GeV Higgs boson acts as a portal to dark matter, and limits are set on the scattering cross-section of weakly interacting massive particles and nucleons.