[Paper Review] Far from standard Higgs couplings

Bob Holdom|arXiv (Cornell University)|Jun 6, 2013

Particle physics theoretical and experimental studies7 references3 citations

TL;DR

This paper investigates the possibility that the 126 GeV boson observed at the LHC is not a standard model Higgs boson, but instead has significantly reduced couplings to W and Z bosons while exhibiting enhanced gluon fusion production. Using global fits to LHC data (as of April 2013), it shows that such non-standard couplings remain consistent with experimental constraints, especially if new physics—like heavy chiral fermions—enhances the gluon fusion rate while suppressing W/Z couplings, leaving open a viable, natural mechanism for electroweak symmetry breaking beyond the SM.

ABSTRACT

In this short note we consider a permitted region in the space of couplings of the 126 GeV boson that extends far away from the standard model Higgs couplings. This region is of interest for more natural models of electroweak symmetry breaking. Stronger evidence of vector boson fusion and/or associated production is needed to eliminate this possibility.

Motivation & Objective

• To assess whether the 126 GeV boson observed at the LHC could have couplings significantly different from the Standard Model (SM) values.
• To explore whether enhanced gluon fusion production and reduced W/Z couplings can coexist without contradicting current data.
• To evaluate the viability of a non-Higgs-like interpretation of the 126 GeV state, particularly one involving new physics such as heavy chiral fermions or CP-violating multi-Higgs models.
• To determine whether current LHC data can rule out large deviations in Higgs couplings, especially to W and Z bosons.
• To identify the role of vector boson fusion and associated production in breaking degeneracy in coupling fits.

Proposed method

• A global fit to ATLAS and CMS data from April 2013 was performed using HiggsSignals 1.0, with the Higgs mass fixed at 125.7 GeV.
• Two-parameter fits were conducted in the space of gluon fusion coupling factor $g_{gg}$ and a common factor $g_X$ for $\gamma\gamma$, $VV$, and $\tau\tau$ couplings.
• Three-parameter fits extended the analysis to include $g_{gg}$, $g_{\gamma\gamma}$, and $g_{VV}$ independently, with $g_{\tau\tau}$ set as the average of $g_{\gamma\gamma}$ and $g_{VV}$.
• Additional fits were performed with fixed $g_{gg}$ values (e.g., 1 and 3) to explore the $\Delta\chi^2$ distribution in planes of other couplings.
• Inclusive $H\to\gamma\gamma$ data was also used to compare with categorized results.
• A separate 3-parameter fit was performed with $g_{bb}$, $g_{gg}$, and $g_X$ to assess the impact of bottom quark coupling variations.

Experimental results

Research questions

• RQ1 Can the 126 GeV boson have significantly reduced couplings to W and Z bosons while still being consistent with LHC data?
• RQ2 To what extent can enhanced gluon fusion production coexist with suppressed $W$ and $Z$ couplings without violating experimental constraints?
• RQ3 Is there a degeneracy in the determination of Higgs couplings when gluon fusion dominates production, and can other production modes break this degeneracy?
• RQ4 What are the implications of such non-standard couplings for models of electroweak symmetry breaking, particularly those involving new heavy chiral fermions?
• RQ5 Can a light state that is mostly pseudoscalar, with a small scalar component, reproduce the observed Higgs-like properties at 126 GeV?

Key findings

• The global fit to ATLAS and CMS data yields a best-fit point at $g_{gg} = 0.87$ and $g_X = 0.98$, with $\chi^2/{\rm ndf} = 26.4/34$, indicating a region of parameter space far from the SM point that remains consistent with data.
• In a three-parameter fit with $g_{gg}$, $g_{\gamma\gamma}$, and $g_{VV}$, the global minimum occurs at $g_{gg} = 0.86$, $g_{\gamma\gamma} = 1.06$, and $g_{VV} = 0.96$, with $\chi^2/{\rm ndf} = 25.8/33$, showing that non-SM couplings are still viable.
• The 1σ contour in the 3-parameter fit includes values where $g_{VV}$ is as low as 0.85, indicating that W and Z couplings can be significantly reduced without contradicting data.
• When using inclusive $H\to\gamma\gamma$ data, the SM point lies on the 2σ contour, and the global minimum is at $g_{gg} = 0.88$, $g_{\gamma\gamma} = 1.22$, $g_{VV} = 0.95$, with $\chi^2/{\rm ndf} = 18.6/14$, further supporting the viability of non-SM couplings.
• A fit allowing for a free $b\bar{b}$ coupling yields a global minimum at $g_{bb} = 0.84$, $g_{gg} = 0.81$, and $g_X = 0.93$, with $\chi^2/{\rm ndf} = 26.3/33$, showing that bottom quark couplings can also be reduced.
• The results indicate that couplings far from SM values—particularly $W$ and $Z$ couplings at roughly one-third of SM strength—are not ruled out, suggesting a possible non-Higgs-like origin of the 126 GeV state.