Probing the dark sector with the Higgs boson and missing energy at the LHC

Abstract: The Standard Model (SM) of particle physics describes a vast range of particle physics experiments across many orders of magnitude. Despite the enormous experimental success of the SM, it does not explain dark matter (DM). I focus on two open questions in particular: (a) What is the nature of DM? (b) How can we use the Higgs boson as a tool to discover DM or other physics beyond the SM?

This thesis presents two searches emphasizing missing energy in the vector boson fusion channel: one for invisible decays of the Higgs boson and one for semi-visible decays. These analyses use 139 fb$^{-1}$ of proton-proton collision data at center-of-mass energy $\sqrt{s} = 13$ TeV with the ATLAS detector at the Large Hadron Collider.  Assuming a SM Higgs, an upper limit of 10\%(14\%) expected (observed) at 95\% C.L. is derived for the branching fraction of Higgs to invisible. Results are interpreted in the context of models in which the Higgs acts as a portal to the dark sector, translating to limits on the scattering cross-section of weakly interacting massive particles and nucleons. Future results of the semi-visible Higgs decay will be interpreted in the context of supersymmetry and Higgs portal models.