[Paper Review] Higgs Couplings in an Effective Theory Framework

Hermès Bélusca-Maïto|arXiv (Cornell University)|Jul 20, 2015

Particle physics theoretical and experimental studies14 references3 citations

TL;DR

This paper uses a dimension-6 effective field theory framework to study deviations in Higgs couplings to fermions and gauge bosons, including both CP-even and CP-odd operators, using LHC Higgs signal strengths from ATLAS and CMS and electroweak precision data. It finds strong constraints on CP-even couplings compatible with the Standard Model at 68% CL, while CP-odd couplings remain poorly constrained due to sign degeneracy in rate measurements.

ABSTRACT

The study of the properties of the scalar boson recently discovered at the LHC (ATLAS and CMS experiments) may allow us to know whether it is well described by the Standard Model. In the case where deviations from SM predictions are present, this would be an evidence for the presence of new physics. We focus on the study of the Higgs couplings to matter in a model-independent approach by introducing a dimension-6 effective Lagrangian that includes both CP-even and CP-odd effective couplings. Constraints are set on some of these coefficients using experimental data from ATLAS and CMS as well as electroweak precision measurements from LEP, SLC and Tevatron. These data meaningfully constrain CP-even and some CP-odd couplings.

Motivation & Objective

• To investigate potential new physics beyond the Standard Model through deviations in Higgs couplings.
• To apply a model-independent effective field theory approach using dimension-6 operators.
• To constrain both CP-even and CP-odd Higgs couplings using current LHC and electroweak precision data.
• To identify limitations in current data for probing CP-odd couplings and suggest future improvements.

Proposed method

• Constructs a dimension-6 effective Lagrangian including gauge-invariant, CP-even and CP-odd operators involving SM fields.
• Derives phenomenological Higgs couplings from the effective Lagrangian, including loop-induced couplings like hγγ.
• Performs a global fit to experimental signal strengths (μ^X) from ATLAS and CMS in multiple Higgs production and decay channels.
• Incorporates electroweak precision data from LEP, SLC, and Tevatron to constrain Wilson coefficients.
• Uses 2D likelihood functions in the μ^ggH+ttH–μ^VBF+VH plane to account for correlations between production modes.
• Applies χ² minimization to extract central values and 68% CL intervals for Wilson coefficients.

Experimental results

Research questions

• RQ1 To what extent do current LHC Higgs signal strengths constrain deviations in Higgs couplings to fermions and gauge bosons?
• RQ2 How well are CP-odd Higgs couplings constrained by current data, and what limitations exist due to sign degeneracy?
• RQ3 Can electroweak precision data from LEP, SLC, and Tevatron improve constraints on effective couplings?
• RQ4 What are the implications of the current data for the presence of new physics in the Higgs sector?
• RQ5 How can future data from the 14 TeV LHC run and electric dipole moment measurements improve sensitivity to CP-odd couplings?

Key findings

• The CP-even coupling to the W and Z bosons, cV, is measured as 1.04 ± 0.03, consistent with the Standard Model.
• The coupling to top quarks, cu, is found to be 1.31+0.10−0.34, indicating a possible deviation but within 68% CL uncertainty.
• The coupling to bottom quarks, cd, is 0.92+0.22−0.13, compatible with SM predictions.
• The CP-odd coupling to up-type quarks, ecu, has a large uncertainty and sign degeneracy, with a central value of ±(0.87+0.33−2.08).
• The CP-odd couplings ecγγ and ecgg are consistent with zero, with 68% CL intervals of ±(0.0033+0.0017−0.0028) and 0.0004+0.0038−0.0040, respectively.
• The χ² difference between the SM and best-fit model is 5.3, indicating a good fit to current data, with no significant evidence for new physics in the Higgs sector at this stage.