Measurement of the Higgs boson tensor coupling in $H \rightarrow ZZ^{*} \rightarrow 4\ell$ decays with the ATLAS detector - How odd is the Higgs boson?

The tensor structure of the Higgs boson couplings to gluons and heavy weak gauge bosons has been probed for small admixtures of non-Standard Model CP-odd and, only for heavy vector bosons, CP-even couplings to the CP-even Standard Model coupling. The Higgs boson candidates are reconstructed in the $H \to ZZ^{*} \to 4\ell$ $(\ell\equiv e,\mu)$ decay channel using proton-proton collision data recorded by the ATLAS detector at the Large Hadron Collider (LHC) in 2011 and 2012 at centre-of-mass energies of $\sqrt{s}=7$ and $8$ TeV corresponding to an integrated luminosity of $24.8\,\mathrm{fb}^{-1}$ and in 2015 and 2016 at $\sqrt{s} = 13$ TeV corresponding to $36.1\,\mathrm{fb}^{-1}$. The non-Standard Model coupling parameters are defined within an effective field theory, the so-called Higgs characterisation framework. The relative contributions of the CP-even and CP-odd terms are described by the CP mixing angle $\alpha$. The parameter $\kappa_{\mathrm{Agg}}$ denotes the CP-odd non-Standard Model coupling at the Higgs to gluon interaction vertex and $\kappa_{\mathrm{HVV}}$ and $\kappa_{\mathrm{AVV}}$ the CP-even and CP-odd non-Standard Model couplings of the Higgs boson to weak gauge bosons, respectively. The observable coupling parameters are the product of the coupling strengths $\kappa$ and the sine or cosine of the CP mixing angle $\alpha$, $c_{\alpha} \equiv \cos\alpha$ and $s_{\alpha} \equiv \sin\alpha$. Two complementary methods have been used for the measurement. One is solely based on kinematic variables of the four-lepton final state which are sensitive to the presence of non-Standard Model couplings. Constraints on non-Standard Model contributions to the Higgs to $Z$-boson coupling have been obtained with this approach from the Run-1 data set. A combination with an analogous measurement for $H \rightarrow WW^{*} \rightarrow e\nu\mu\nu$ decays has also been performed. The second approach combines the information on the CP structure of the Higgs boson from final state kinematic observables and from the Higgs boson production rate using part of the Run-2 data set. The first constraints on a CP-odd contribution to Higgs boson production in gluon fusion have been derived. All results are in agreement with the Standard Model predictions. The most stringent constraints have been obtained from the production rate with $-0.6 \leq c_{\alpha} \kappa_{\mathrm{HVV}} \leq 4.2$, $-4.4 \leq s_{\alpha} \kappa_{\mathrm{AVV}} \leq 4.4$ and $-0.68 \leq c_{\alpha} \kappa_{\mathrm{Agg}} \leq 0.68$ at $95\,\%$ CL.