[Paper Review] Global interpretation of LHC indications within the Georgi-Machacek Higgs model

François Richard|arXiv (Cornell University)|Mar 22, 2021

Particle physics theoretical and experimental studies4 citations

TL;DR

This paper presents a global interpretation of multiple LHC hints for new scalars—specifically a 96 GeV Higgs, a 400 GeV CP-odd boson (A(400)), and a 660 GeV heavy Higgs—within the Georgi-Machacek (GM) model. It shows that these anomalies, though not yet five-sigma, are consistently explainable in the GM framework, which also predicts enhanced rates for key processes at future $e^+e^-$ colliders like the ILC, suggesting strong motivation for HL-LHC and next-generation machines.

ABSTRACT

Following various LHC indications for new scalars, an interpretation of these is given in terms of the Georgi-Machacek (GM) model. On top of the confirmed SM Higgs boson, there are indications for a light Higgs at 96 GeV, for a CP-odd boson at 400 GeV, A(400), and for a heavy Higgs boson at 660 GeV. An extension of the GM is needed to interpret the fermion couplings of A(400). Potentially interesting deviations are also observed in the ttW cross-section measurement, which naturally fit into this picture. None of them crosses the fatidic five s.d. evidence but the addition of these effects, consistent with GM, suggest that there are good hopes for solid discoveries at HL-LHC, which should boost the motivation for future machines. The GM model also provides a useful framework to estimate the rates expected for various channels at an $e^+e^-$ collider, together with the range of energies needed. ILC performances are used for a quantitative estimate of these rates for the prominent channels.

Motivation & Objective

• To interpret multiple LHC anomalies—such as a 96 GeV Higgs, a 400 GeV CP-odd boson, and a 660 GeV heavy Higgs—within a single theoretical framework.
• To assess whether the Georgi-Machacek model can consistently explain the observed deviations in Higgs couplings and the ttW cross-section measurement.
• To evaluate the potential for discovering these new states at the High-Luminosity LHC and future $e^+e^-$ colliders like the ILC.
• To provide quantitative estimates of signal rates and required center-of-mass energies for key processes at future $e^+e^-$ colliders.

Proposed method

• The analysis uses a global fit to LHC data, including Higgs signal strengths, diboson resonances, and ttW production, within the Georgi-Machacek model's extended scalar sector.
• The model is extended to include fermion couplings for the A(400) state, allowing consistent interpretation of its observed properties.
• Theoretical predictions for $e^+e^-$ collider processes are derived using the GM model's scalar and gauge interactions, including Yukawa and gauge couplings.
• ILC performance simulations are employed to estimate signal rates for prominent channels such as $\gamma\gamma$, $ZZ$, $W^+W^-$, and $h\gamma$ at center-of-mass energies of 500 GeV and 1 TeV.
• The consistency of the observed anomalies with the GM model is evaluated through a combined analysis of signal strengths and cross-section deviations.

Experimental results

Research questions

• RQ1 Can the Georgi-Machacek model consistently explain the LHC hints for a 96 GeV Higgs, a 400 GeV CP-odd boson, and a 660 GeV heavy Higgs?
• RQ2 How do the observed deviations in the ttW production cross-section align with the predictions of the extended GM model?
• RQ3 What are the expected signal rates for key processes such as $\gamma\gamma$, $ZZ$, and $W^+W^-$ at an $e^+e^-$ collider like the ILC under the GM model?
• RQ4 At what center-of-mass energy and luminosity would the ILC achieve sufficient sensitivity to discover the proposed new scalars in the GM framework?

Key findings

• The observed LHC anomalies—particularly the 96 GeV Higgs, A(400) at 400 GeV, and the 660 GeV resonance—are consistently explained within the Georgi-Machacek model, suggesting a unified underlying structure.
• The model provides a natural explanation for the ttW cross-section deviation, which otherwise appears as a significant anomaly in the SM.
• The ILC is predicted to achieve high discovery potential for the new scalars, with signal rates for $\gamma\gamma$ and $ZZ$ channels reaching several hundred events per 1 ab⁻¹ at 500 GeV and up to 1 TeV.
• The model predicts that the 400 GeV A(400) state would be accessible via $\gamma\gamma$ and $W^+W^-$ final states, with cross-sections of order 10–100 fb at 500 GeV.
• The 660 GeV heavy Higgs state is expected to be produced with significant rates in $ZZ$ and $W^+W^-$ final states, especially at 1 TeV center-of-mass energy.