CERN Accelerating science

ATLAS Note
Report number	ATL-PHYS-PROC-2025-118
Title	Higgs Boson CP Properties and Effective Field Theory Measurements from the ATLAS Experiment at the LHC
Corporate Author(s)	The ATLAS collaboration
Collaboration	ATLAS Collaboration
Publication	2025
Imprint	25 Nov 2025
Number of pages	5
In:	The 32nd International Symposium on Lepton Photon Interactions at High Energies (Lepton Photon 2025), Madison, Wisconsin, Us, 25 - 29 Aug 2025
Subject category	Particle Physics - Experiment
Accelerator/Facility, Experiment	CERN LHC ; ATLAS
Free keywords	Higgs CP ; Effective Field Theory ; New Physics ; ATLAS ; LHC ; HIGGS
Abstract	This paper presents a concise overview of the Higgs boson's CP properties and constraints on Effective Field Theory (EFT) operators, derived from the ATLAS experiment at the Large Hadron Collider (LHC). Using proton-proton collision data with integrated luminosities of up to 140 fb$^{-1}$ at $\sqrt{s} = 13$ TeV, we systematically probe the CP nature of the Higgs boson's couplings to fermions ($\tau$ leptons, bottom quarks, and top quarks) and bosons ($W$, $Z$, and $\gamma$) 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) $H \to \tau\tau$ (semileptonic/hadronic decays) for light fermion couplings, (2) $H \to \gamma\gamma$ and $H \to bb$ in $t\bar{t}H/tH$ processes for heavy fermion couplings, (3) $H \to WW^* \to l\nu l\nu$, $H \to ZZ^* \to 4l$, and vector boson fusion (VBF) $H \to \tau\tau/\gamma\gamma$ 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 $H \to \tau\tau$ ($c_{H\tilde{W}} \in [-0.23, 0.70]$ 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.
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