Probing the nature of the Higgs Boson: A study of the Higgs spin and parity through the $ZZ^* \to 4l$ final state at the ATLAS Experiment

A series of studies are presented of the Higgs-like resonance that was discovered in July 2012. The studies focus on measurements of the resonance spin and parity using the full dataset collected by the ATLAS experiment during the LHC Run-1. Alternative spin models are compared to the Standard Model Higgs. Spin-2 models are described as graviton-like particles in an e↵ective field theory approach. Several assumptions on the boson coupling to the QCD sector are tested, as is the analysis dependency on transverse momenta cuto↵s. The Standard Model is favoured over all alternative models and except for the negative parity spin-2 and a spin-2 boson that does not couple to the quark sector, each are excluded with 95% confidence level. Notably, the spin-2 universal couplings model is assigned a CLS value of 0.0009. Spin-1 models are tested in the $H \to ZZ^* \to 4l$ channel and excluded in favour of the Standard Model. The resonance decay to also disfavours the possibility of spin-1. The spin-0 boson coupling to the heavy vector bosons is described in an e↵ective field theory approach with three components, the Standard Model and a CP-even BSM and CP-odd sector. Assuming only one coupling to be present at a time, the Standard Model $(J^P = 0^+)$ is tested against the CP-even BSM $(J^P = 0^+_h)$ and CP-odd $(J^P = 0^-$) model. The Standard Model is favoured in data and the two alternate models are assigned CLS values of 0.10 (CP-even) and 0.054 (CP-odd). The analysis is extended by allowing a mixture of the three spin-0 components, first assuming only one BSM coupling to be present at a time. Discriminative power between models is obtained in a Matrix-Element Observable approach. The $H \to ZZ^* \to 4l$ channel is combined with the $H \to WW^* \to \epsilon\nu\mu\nu$ analysis. The e↵ective BSM cross section contributions to the resonance is found to be limited to the ranges: fg2 · cos g2 2 [0.16, 0.12] fg4 · cos g4 2 [0.41, 0.090] at 95% confidence level. The data is thus found to be in agreement with the Standard Model. In a final extension of the analysis the possibility of the simultaneous presence of both BSM sectors is studied.