Symmetries in Two Higgs Doublet Model effective field theories and searching for heavy Higgs bosons with graph neural networks at the ATLAS detector

A theoretical study of accidentally symmetric potentials in the framework of Two Higgs Doublet Model effective field theory (2HDMEFT) including higher-order operators of dimension-6 and dimension-8 is presented. A method is derived which can be used to identify operators of any higher dimension by utilising the generators of each symmetry in the bi-adjoint representation. The full classification of the 17 accidental symmetries in the 2HDMEFT including dimension-6 and dimension-8 operators is presented. The relations that govern the theoretical parameters of the corresponding effective potentials are also derived. An analysis searching for the resonant production of heavy neutral Higgs bosons arising from the 2HDM in the process ttH/A→tttt in the single-lepton and opposite-sign dilepton final state in proton-proton collisions at 13 TeV with the ATLAS detector is presented. This analysis utilises several novel ML techniques such as a NN-based kinematic reweighting function and a mass-parameterised GNN signal discriminant which will be discussed. The median expected limits for this analysis range between 28 fb and 9 fb for neutral heavy Higgs masses between 400 and 1000 GeV respectively. The neutral heavy Higgs bosons in this analysis are interpreted in the context of a CP-conserving type-II 2HDM.