Abstract

As the only non-composite scalar in the Standard Model, associated with an all-permeating, ever-present field thought to generate the mass of all other particles, the study of the Higgs boson represents a historic opportunity to probe reality on the most fundamental level. At the current level of precision, all of the measured properties of the Higgs boson are found to be consistent with their SM predictions, and no additional Higgs boson has been observed to date. However, extended Higgs sectors are well-motivated and provide a rich phenomenology of additional scalars. While these new scalars could exist at higher energy scales, they could also exist at or below the electroweak scale undiscovered by previous experiments, if their most significant coupling is to the Higgs boson. Given the small natural decay width of the Higgs boson, even small additional couplings to these resonances would lead to final states with substantial, and thus possibly detectable, branching fractions. This talk will introduce two popular extensions to the SM Higgs sector, the Two Higgs Doublet Model and Two Higgs Doublet Model with an additional singlet, before focusing on the searches for decays of the Higgs boson to final states including such light scalars at the ATLAS detector. Specific emphasis will be placed on the recently published first search for Higgs boson decays to a Z boson and a hadronically decaying resonance. This novel final state probes previously inaccessible parts of the 2HDM(+S) parameter space, and is enabled by track-base substructure techniques and a dual-stage neural network.