Indirect probes to constrain the <em>CP</em> nature of the Higgs boson

Abstract

The measurements of the coupling, spin, and charge-parity CP properties of the Higgs boson are consistent with the Standard Model (SM) predictions. However, the corresponding uncertainties are still large, and alternative theories of electroweak symmetry breaking (EWSB) also fit the data. While the pure CP-odd hypothesis is already ruled out, general CP admixtures are still possible. This thesis studies the scope of possible CP violation in the Higgs sector using different Higgs boson interpretation frameworks. <br /> The tensor structure of the Higgs boson couplings to weak gauge bosons, gluons, and top quarks is measured in Higgs boson decays into four leptons, <var>H→ZZ^*→4ℓ</var> (<var>ℓ=e,μ</var>), using 139 fb^-1 of proton-proton collision data with a center-of-mass energy of 13 TeV recorded with the ATLAS detector at the Large Hadron Collider (LHC). Beyond the SM (BSM) effects in the Higgs couplings are measured at particle level and are interpreted in the Standard Model Effective Field Theory (SMEFT). Constraints have been derived on the CP-even and CP-odd BSM coupling parameters describing the Hgg (<var>c_HG</var> and <var>c_HG̃</var>), <var>Htt</var> (<var>c_uH</var> and <var>c_ũH</var>), and <var>HVV</var> (<var>c_HW, c_HB, c_HWB</var>, and <var>c_HW̃, c_HB̃, c_HW̃B</var>) interaction vertices. The most stringent constraints at 95% C.L. are -0.0083<<var>c_HG</var><0.0068 and |<var>c_HG̃</var>|<0.029. The limits on the CP-even parameter are stronger since the linear terms dominate in this case. The constraints on the CP-even coupling parameters to weak vector bosons are -3.4<<var>c_HW</var><2.1, -0.62<<var>c_HB</var><0.59, and -1.06<<var>c_HWB</var><0.99, while the CP-odd ones are |<var>c_HW̃</var>|<2.4, |<var>c_HB̃</var>|<0.56, and |<var>c_HW̃B</var>|<1.03. <br /> The CP nature of the Higgs boson is also probed by fitting CP-violating models to all inclusive and differential Higgs boson measurements provided by the ATLAS and CMS collaborations. This is done with the public computer code HiggsSignals. The first model targets the CP properties of the top-Yukawa interaction. The upper limit on a CP-violating phase is found to be <var>α_tt≤</var> 72°, which implies that the Higgs-top coupling is at last 25% CP-even. The second model describes the Higgs boson, <var>Φ=H</var>cos<var>α+A</var>sin<var>α</var>, as an admixture of CP-even (<var>H</var>) and CP-odd (<var>A</var>) components as described by the 2HDM. In this kind of models, <var>A</var> does not couple to weak gauge bosons, <var>V=W,Z</var>, at tree-level. For the CP-even component, no significant differences between the couplings to W and Z bosons are found. Therefore, a common coupling modifier <var>κ_V≡κ_W=κ_Z</var> is used. Constraints on the mixing parameter <var>α</var> are set in case <var>κ_V≤</var>1, which is a valid assumption in 2HDM-type models. Effective couplings of the CP-odd state to SM gauge bosons are considered in an effective field theory (EFT) approach. Since AVV couplings can only be realized through loop corrections of heavy new fields, they are strongly suppressed. By requiring that the EFT is perturbative, their impact is found to be marginal, and possible CP admixtures are constrained to <var>α≤</var>27.4°. This means that in the context of 2HDM-type models, the Higgs boson is at most 34% CP-odd. In addition to these studies, several performance tests of HiggsSignals with LHC Run-1 and Run-2 data are presented. <br /> The <var>H→ττ</var> channel is one of the most promising candidates for a direct CP measurement of a Higgs boson coupling. This measurement relies on the precise reconstruction of the tau decay products. For Run-2 of the LHC, the tau-lepton reconstruction at ATLAS has been significantly improved using the particle-flow concept. In this thesis, a new energy reconstruction method for hadronically decaying tau leptons has been developed, which improves the resolution of low-energetic taus by up to 25% (70%) in decays with (without) neutral pions. In addition, an energy correction of neutral pions in 1p1n, 1pXn, and 3pXn decays has been developed. The corresponding improvement of the neutral pion transverse momentum is about 20%, 30%, and 60%, respectively.