⚛️ phenomenology

Global fits and the 95 GeV diphoton excesses in the Supersymmetric Georgi-Machacek Model

This paper demonstrates that the Supersymmetric Georgi-Machacek model can explain the 95 GeV diphoton excesses observed by ATLAS and CMS through a light custodial singlet Higgs boson, but fails to simultaneously account for the LEP $b\bar{b}$ excess due to its highly constrained Higgs potential, which also yields sharp predictions for the remaining mass spectrum and offers distinct signatures to differentiate it from the non-supersymmetric GM model at future colliders.

Original authors: Yingnan Xu, Dikai Li, Roberto Vega, Roberto Vega-Morales, Keping Xie

Published 2026-01-27

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CC BY 4.0

Original authors: Yingnan Xu, Dikai Li, Roberto Vega, Roberto Vega-Morales, Keping Xie

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the universe as a giant, complex machine. For decades, physicists have been trying to understand the "engine" that gives particles their mass. In 2012, they found a key part of this engine, a particle called the Higgs boson, sitting at a specific weight of 125 units. This was a huge victory for the Standard Model, our current rulebook for physics.

However, recently, two giant particle detectors at the Large Hadron Collider (LHC)—named ATLAS and CMS—saw something strange. They noticed a tiny, blurry "ghost" appearing at a much lighter weight of about 95 units. It wasn't a clear sighting, just a slight bump in the data, but it was enough to make physicists wonder: Is there a second, lighter engine part hiding in the machine?

This paper is a detective story trying to solve that mystery using a specific theory called the Supersymmetric Georgi-Machacek (SGM) model. Here is how the authors break it down, using simple analogies:

The Theory: A Tightly Packed Suitcase

Think of the Standard Model as a suitcase with just one Higgs particle. The "Georgi-Machacek" (GM) model suggests the suitcase is actually much bigger, containing a whole family of Higgs particles (some heavy, some light, some charged, some neutral) that work together to keep the universe stable.

The Supersymmetric version (SGM) is a very strict, "high-security" version of this suitcase. It's like a suitcase where every item must have a matching "shadow twin" (a fermion partner) and where the rules for how they fit together are written in stone. You can't just throw anything in; the geometry is rigid.

The Investigation: Fitting the Puzzle Pieces

The authors tried to fit the "95 GeV ghost" into this strict SGM suitcase. They ran a massive computer simulation (a "global fit") to see if the rules of this model could explain the ghost without breaking the rest of the machine.

Here is what they found:

1. The Ghost is a "Light" Cousin
The paper suggests that if this 95 GeV particle exists, it is likely the "lightest cousin" in the Higgs family. It's mostly made of a specific type of material (an electroweak triplet) that doesn't interact much with normal matter, which explains why it's been so hard to find. It contributes a small amount (about 5–7%) to the mechanism that gives particles mass.

2. The "Double-Photon" Trick
The ghost was spotted because it turned into two flashes of light (photons) more often than a normal particle should. In the SGM model, this happens because of a "shadow twin" (a doubly charged fermion) that interferes with the process, boosting the light signal. It's like having a special mirror that reflects light much brighter than a normal wall.

3. The "Missing" Clue (The LEP Problem)
There is a catch. In the 1990s, an older experiment called LEP saw a similar ghost, but this one seemed to turn into bottom quarks (a type of heavy particle). The authors found that their strict SGM model cannot explain this older LEP clue. In their model, the ghost is too shy to turn into bottom quarks. They conclude the LEP signal was likely just a statistical fluke (a random noise in the data), not a real particle.

The Predictions: What Else is Hiding?

Because the SGM model is so strict (like a rigid puzzle), if you force one piece to fit, the rest of the pieces snap into place automatically. The authors predict that if this 95 GeV ghost is real, we should find three other specific things soon:

A Heavy Double-Flasher: A new particle weighing about 185–195 GeV that is doubly charged.
A Heavy Shadow Twin: A fermion (matter particle) weighing about 170–220 GeV that is also doubly charged.
A Light Ghost Twin: A neutral, stable particle (the "Lightest Supersymmetric Particle" or LSP) weighing about 117–135 GeV. This is a candidate for Dark Matter.

The Difference Between the "Strict" and "Loose" Models

The authors also compared their strict SGM model to a "looser" version (the non-supersymmetric GM model).

The Loose Model: The particles are heavier.
The Strict SGM Model: The presence of the "shadow twins" (higgsinos) creates a destructive interference that forces the particles to be lighter to produce the same bright light signal. It's like needing a smaller, lighter engine to get the same speed because of the extra weight of the shadow twins.

The Conclusion

The paper concludes that the SGM model is a viable explanation for the 95 GeV signal, but only if we ignore the older LEP bottom-quark signal. If this model is correct, the universe is hiding a tightly correlated family of new particles just waiting to be found.

The authors suggest that future experiments at the LHC or future colliders should look specifically for these predicted masses. Because the particles are so close in weight (a "compressed spectrum"), they will be hard to spot—they will look like soft, whispering signals rather than loud bangs. But if we know exactly what to look for, we might finally catch the "ghost" and its shadow twins.

Technical Summary: Global Fits and the 95 GeV Diphoton Excesses in the Supersymmetric Georgi-Machacek Model

Problem Statement
Recent experimental data from the ATLAS and CMS collaborations have reported modest excesses in the diphoton channel around a mass of 95 GeV. While these could potentially signal an extended electroweak symmetry breaking (EWSB) sector, such as the Georgi-Machacek (GM) model, previous studies have often neglected specific production channels and constraints that severely limit the viability of such models. Furthermore, the standard GM model suffers from quadratic divergences and fine-tuning issues associated with the Higgs mass and custodial symmetry breaking. The authors investigate whether the Supersymmetric Georgi-Machacek (SGM) model—a weak-scale effective limit of the Supersymmetric Custodial Triplet Model (SCTM)—can accommodate the 95 GeV diphoton excesses while satisfying all current theoretical and experimental constraints, including the precise measurements of the 125 GeV Higgs boson.

Methodology
The authors perform a comprehensive global fit of the SGM model, comparing it against a "constrained" non-supersymmetric GM (CGM) model where the same Higgs potential constraints are applied by hand to isolate the effects of the fermion sector. The analysis involves:

Model Definition: The SGM model retains the scalar spectrum of the GM model (custodial singlets, triplets, and a fiveplet) but imposes supersymmetric constraints on the quartic couplings ( $\lambda_1, \dots, \lambda_5$ ) and introduces custodial Higgsino fermions. The Higgs potential is derived from a supersymmetric superpotential, reducing the number of independent quartic couplings and fixing their signs.
Parameter Scan: A scan is performed over the six independent Higgs potential parameters ( $\lambda_2, \lambda_4, M_1, M_2, v_\phi, v_\Delta$ ) and the superpotential $\mu$ terms. The scan fixes the lightest custodial singlet mass at 95 GeV and the heavier singlet at 125 GeV.
Constraints and Observables: The analysis incorporates:
- Theoretical Constraints: Perturbative unitarity, boundedness from below, and vacuum stability.
- Experimental Bounds: LHC measurements of the 125 GeV Higgs couplings, direct searches for additional Higgs bosons at LEP and LHC, and indirect constraints from $BR(b \to s\gamma)$ .
- Key Production Channels: Crucially, the authors include the Drell-Yan (DY) Higgs pair production channel ( $q\bar{q} \to V \to HH_3$ ), which is mediated by gauge interactions and is not suppressed in the small mixing limit. This channel was neglected in previous global fits of GM-like models.
- Fermion Searches: Constraints from $t\bar{t}H^\pm_3 \to \tau^\pm\nu$ searches and direct searches for heavy leptons/neutralinos/charginos.
Tools: The analysis utilizes SPheno and SARAH for spectrum and coupling calculations, GMCalc for validation, and HiggsTools (specifically HiggsSignals and HiggsBounds) for comparing against experimental data, with custom implementations for the DY pair production and $t\bar{t}H^\pm_3$ channels.

Key Contributions

Inclusion of Neglected Channels: This is the first study of GM-like Higgs sectors to include both the Drell-Yan pair production channel and $t\bar{t}H^\pm_3 \to \tau^\pm\nu$ searches in a global fit for the 95 GeV excess. The authors demonstrate that these channels impose severe constraints on the parameter space, particularly the electroweak triplet VEV.
Differentiation of SGM and CGM: By comparing the SGM with the CGM, the authors isolate the phenomenological impact of the custodial Higgsino fermions. They show that the presence of Higgsinos significantly alters the mass spectrum and coupling requirements compared to the purely scalar case.
Resolution of the LEP $b\bar{b}$ Excess: The study finds that while the SGM can fit the LHC diphoton excess, it cannot simultaneously accommodate the LEP $b\bar{b}$ excess at 95 GeV. The stringent constraints on the triplet VEV and the dominance of the DY pair production channel prevent the model from generating the necessary signal strength for the LEP excess, suggesting the LEP result may be a statistical fluctuation within this framework.

Results

Fit Viability: The SGM model can fit the current data (including the 95 GeV diphoton excess and 125 GeV Higgs properties) as well as the Standard Model, provided the 95 GeV resonance is the lightest custodial singlet Higgs boson.
EWSB Contribution: The 95 GeV singlet is predicted to contribute approximately 5–7% to the electroweak symmetry breaking (VEV mixing angle $s_H \sim 0.06$ ).
Mass Spectrum Predictions: Due to the highly constrained Higgs potential, the model predicts a sharp mass spectrum:
- Custodial Fiveplet ( $H_5$ ): A doubly charged scalar in the mass range 185–195 GeV.
- Custodial Triplet ( $H_3$ ): A CP-odd triplet scalar in the range 133–140 GeV.
- Higgsinos: The model predicts a fermion Lightest Supersymmetric Particle (LSP), the neutral component of the custodial triplet Higgsino, with a mass of 117–137 GeV. The charged component has a very small mass splitting ( $\sim 1.8$ GeV) relative to the LSP, leading to soft decay products.
- Doubly Charged Higgsino: A doubly charged fermion in the range 170–220 GeV.
Coupling Mechanisms: The enhanced diphoton branching ratio is achieved via constructive interference from the light doubly charged scalar in the fiveplet. However, the presence of doubly charged Higgsinos causes destructive interference, requiring the scalar fiveplet to be lighter in the SGM ( $\sim 190$ GeV) compared to the CGM ( $\sim 215$ GeV) to achieve the same signal strength.
Production Dominance: For the 95 GeV singlet, the Drell-Yan pair production channel dominates over single production modes (gluon fusion, VBF, VH) due to the small mixing angles required by the 125 GeV Higgs data.

Significance
The paper claims that the SGM model offers a predictive and tightly constrained framework for interpreting the 95 GeV diphoton excesses. The primary significance lies in the model's ability to resolve fine-tuning issues inherent to the non-supersymmetric GM model while providing a distinct, testable mass spectrum for both scalars and fermions. The authors emphasize that the presence of custodial Higgsinos is the critical differentiator; their destructive interference necessitates a lighter fiveplet mass and a specific contribution to EWSB. The paper concludes that if the excesses persist, the SGM model points to a compressed spectrum of new particles below the weak scale, requiring dedicated search strategies optimized for soft decay products and compressed mass spectra at the LHC and future colliders. The unambiguous establishment of the SGM model, however, relies on the discovery of the custodial Higgsinos.

The Theory: A Tightly Packed Suitcase

The Investigation: Fitting the Puzzle Pieces

The Predictions: What Else is Hiding?

The Difference Between the "Strict" and "Loose" Models

The Conclusion

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