Higgs Boson CP Properties and Effective Field Theory Measurements from the ATLAS Experiment at the LHC

TL;DR

This work evaluates Higgs CP properties and EFT deviations using ATLAS Run 2 data at $\sqrt{s}=13$ TeV (up to $~140$ fb^-1) across fermionic and bosonic channels, employing SMEFT with dimension-6 operators in the Warsaw and HISZ bases. It leverages CP-sensitive observables such as $\phi^*_{CP}$, $OO$, and $\Delta\phi_{jj}$ and performs 1D/2D fits with an EFT expansion at $\Lambda = 1$ TeV to extract CP-mixing angles and Wilson coefficients. Across multiple final states, the results are consistent with a SM CP-even Higgs ($J^{CP}=0^{++}$), with no evidence of CP violation, and set the most stringent bound on the CP-odd Wilson coefficient $c_{H\tilde{W}}$ from VBF $H\to \tau\tau$ as $c_{H\tilde{W}} \in [-0.23, 0.70]$ (95% CL). The analyses demonstrate complementary channel sensitivities and establish a framework for global EFT fits, paving the way for HL-LHC constraints on Higgs self-coupling and CP structure.

Abstract

This proceedings presents a concise overview of the Higgs boson's charge-conjugation and parity (CP) properties and constraints on Effective Field Theory (EFT) operators, derived from the ATLAS experiment at the Large Hadron Collider (LHC). Using proton$\textendash$proton collision data with integrated luminosities of up to 140 fb$^{-1}$ at $\sqrt{s} = 13$ TeV, the ATLAS collaboration systematically probe the CP nature of the Higgs boson's couplings to fermions ($τ$ leptons, bottom quarks, and top quarks) and bosons ($W$, $Z$, and $γ$) across diverse decay final states. The EFT framework is used to parameterize Beyond the Standard Model (BSM) effects via dimension-6 operators, enabling model-independent constraints on CP violation and new physics scales. The main focus is comparison of measurement characteristics and sensitivity across different final states: (1) $H \to ττ$ (semileptonic/hadronic decays) for light fermion couplings; (2) $H \to γγ$ and $H \to bb$ in $t\bar{t}H/tH$ processes for heavy fermion couplings; (3) $H \to WW^* \to lνlν$, $H \to ZZ^* \to 4l$, and vector boson fusion (VBF) $H \to ττ/γγ$ for boson couplings; and (4) double Higgs ($HH$) production for self-couplings. All measurements are consistent with the Standard Model (SM) prediction of a CP-even Higgs boson ($J^{CP} = 0^{++}$), with no evidence of CP violation. The most stringent constraint on the CP-odd EFT parameter $c_{H\tilde{W}}$ is obtained from VBF $H \to ττ$ ($c_{H\tilde{W}} \in [-0.23, 0.70]$ at 95\% CL), highlighting the unique sensitivity of this channel. Complementary constraints from other final states reinforce the robustness of SM consistency and provide a foundation for future searches.

Figures (1)

• Figure 1: The expected and observed limits of the CP-odd Wilson coefficient $c_{H\tilde{W}}$ using ATLAS Run 2 data ATL-PHYS-PUB-2025-031.