Interpretation of LHC excesses at 95 GeV and 152 GeV in an extended Georgi-Machacek model

This paper demonstrates that a minimally extended Georgi-Machacek model can naturally explain the observed LHC and LEP excesses at 95 GeV and 152 GeV through specific features like light scalar masses and enhanced diphoton decays, while predicting additional light scalars and offering promising prospects for future discovery at the HL-LHC and $e^+e^-$ colliders.

The Gist

Imagine the Standard Model of particle physics as a perfectly tuned orchestra. For years, this orchestra has played a beautiful, predictable symphony, with one specific note—the Higgs boson at 125 GeV—serving as the conductor's baton, holding everything together.

But recently, the audience (the scientists at the Large Hadron Collider, or LHC) started hearing some strange, faint whispers in the music.

The Mystery Whispers

There are two main whispers causing a stir:

1. The 95 GeV Whisper: A faint signal suggesting a new, lighter particle exists. Both the CMS and ATLAS experiments heard it, and when they combined their notes, it sounded like a real possibility (about 3 times louder than random noise).
2. The 152 GeV Whisper: A slightly heavier signal, spotted in the "sidebands" of the data (the quiet spaces between the main notes), suggesting another new particle might be hiding there.

For a long time, the "Standard Model Orchestra" couldn't explain these whispers. It's like trying to explain a ghost in a machine that was built to be ghost-free.

The New Conductor: The "meGM" Model

The authors of this paper propose a new conductor for the orchestra: an Extended Georgi-Machacek (meGM) model.

Think of the original Georgi-Machacek (GM) model as a slightly expanded orchestra that already included some extra instruments (like "doubly charged" Higgs bosons, which are like super-violins that can play two notes at once). However, the original GM model was too rigid; it insisted that certain instruments had to play in perfect lockstep (a symmetry called "custodial symmetry").

The meGM model is a "minimal extension." It's like taking that expanded orchestra and loosening the rules just a tiny bit. It allows the instruments to play slightly differently from one another, breaking the perfect symmetry just enough to let those strange whispers be heard clearly, without ruining the main symphony.

How the New Model Solves the Mystery

The paper argues that this new model is a perfect fit for the data because of four "magic tricks":

1. The Natural Family: In this model, if you have a 125 GeV Higgs and a 95 GeV Higgs, the math naturally forces the other family members (the other particles) to be light, too. It's like if you have a father and a son, and the family tree says the brother must be a teenager. The model predicts a whole family of light particles, including the mysterious 152 GeV one.
2. The Super-Violin (Doubly Charged Higgs): The model includes a special particle called a "doubly charged Higgs." Think of this as a super-instrument that amplifies the sound of the 95 GeV particle when it decays into two photons (light particles). This amplification makes the whisper loud enough to be heard by the detectors.
3. The Asymmetric Dancers: In the old model, a particle would dance with "W" bosons and "Z" bosons in a perfectly balanced way. In this new model, the symmetry is slightly broken. The 152 GeV particle prefers to dance with W bosons much more than Z bosons. This explains why we see it in some channels but not others.
4. The Perfect Balance Sheet: Despite breaking the symmetry, the model is careful not to break the universe's "balance sheet" (the $\rho$ parameter). It keeps the fundamental math of the universe stable, ensuring the model doesn't collapse under its own weight.

The Results: A Better Tune-Up

When the authors ran the numbers, they found that this new model fits the data much better than the old Standard Model.

• The 95 GeV Whisper: The model explains it perfectly.
• The 152 GeV Whisper: The model explains this one significantly better than the Standard Model does, reducing the "tension" (the mismatch between theory and data) by about 23%.

What's Next? The Future Concert

The paper doesn't just solve the mystery; it predicts where to look next.

• More Light Particles: The model predicts a whole zoo of other light particles (charged and "odd" particles) that should be hiding just below the 170 GeV mark.
• The Next Big Show: The authors suggest that the High-Luminosity LHC (a supercharged version of the current collider) and future electron-positron colliders (like the ILC) will be able to "see" these particles clearly. It's like upgrading from a radio to a high-definition television; the static will disappear, and the picture will be crystal clear.

The Bottom Line

This paper suggests that the universe might be a bit more crowded and colorful than we thought. Those faint whispers at 95 and 152 GeV aren't just random noise; they might be the sound of a hidden family of particles, waiting for us to tune our instruments just right to hear them. The "meGM" model is the sheet music that tells us exactly how to listen.

Technical Summary

Here is a detailed technical summary of the paper "Interpretation of LHC excesses at 95 GeV and 152 GeV in an extended Georgi-Machacek model".

1. Problem Statement

The paper addresses the simultaneous interpretation of several persistent experimental anomalies in Higgs boson searches:

• The 95 GeV Excess: CMS and ATLAS have reported a combined local significance of $\sim3.1\sigma$ in the diphoton ($\gamma\gamma$) channel at 95.4 GeV. This is compatible with a previous LEP excess of $2.3\sigma$in the$b\bar{b}$ channel at the same mass.
• The 152 GeV Excess: Phenomenological re-analyses of sideband data from SM Higgs searches suggest a potential narrow resonance at $\sim152$ GeV, particularly in diphoton and associated production channels, though no official excess was claimed in the original ATLAS/CMS publications.
• The Theoretical Challenge: Standard Model (SM) extensions often struggle to explain both the 95 GeV and 152 GeV excesses simultaneously while maintaining consistency with the 125 GeV Higgs properties and electroweak precision observables. Specifically, the Georgi-Machacek (GM) model naturally predicts light scalars but enforces custodial symmetry, which forces a fixed ratio between $WW$ and $ZZ$ couplings. This prevents the model from accommodating a 152 GeV state that couples much more strongly to $WW$ than to $ZZ$, a feature suggested by the 152 GeV data hints.

2. Methodology

The authors propose and analyze a Minimally Extended Georgi-Machacek (meGM) model to resolve these tensions.

• Model Framework:

  • The model extends the SM with a complex triplet ($\chi$, $Y=1$) and a real triplet ($\xi$, $Y=0$), in addition to the standard doublet ($\phi$, $Y=1/2$).
  • Unlike the original GM model which imposes a global $SU(2)_L \times SU(2)_R$ symmetry (leading to exact custodial symmetry), the meGM model introduces "soft" symmetry-breaking terms in the scalar potential. This allows for mild custodial symmetry breaking, enabling asymmetric couplings to $WW$ and $ZZ$ bosons.
  • The model preserves the tree-level prediction of the electroweak $\rho$ parameter ($\rho \approx 1$) to satisfy precision constraints.
  • It predicts a rich spectrum of physical scalars: two CP-even neutral ($H_1, H_2, H_3, h$), one CP-odd neutral, singly charged ($H_1^\pm, H_2^\pm$), and doubly charged ($H^{\pm\pm}$) Higgs bosons.

• Numerical Analysis Strategy:

  • Parameter Space: The authors perform a global fit using the input parameters $\{v_\chi, v_\xi, \lambda_2, \lambda_3, \sigma_4, \mu_{\phi\chi}, \mu_{\phi\xi}, \mu_{\chi\xi}\}$.
  • Constraints:
    • Theoretical: Vacuum stability, boundedness from below, and perturbative unitarity (tree-level).
    • Experimental:
      • 125 GeV Higgs signal rates (using HiggsSignals).
      • LEP and LHC exclusion limits on additional Higgs bosons (using HiggsBounds).
      • The 95 GeV excesses in $\gamma\gamma$ and $b\bar{b}$ channels.
      • The 152 GeV excess: A custom $\chi^2$ analysis was performed on sideband data from ATLAS (Refs. [83, 84]) covering 20 signal regions (SRs) in the $m_{\gamma\gamma}$ range [150, 155] GeV.
  • Simulation: Event generation was performed using MadGraph5_aMC@NLO, parton showering with Pythia 8, and detector simulation with Delphes 3.

3. Key Contributions

• Resolution of the 152 GeV Tension in GM Models: The paper demonstrates that the meGM model naturally accommodates a 152 GeV resonance with asymmetric couplings ($BR(H_3 \to WW) \gg BR(H_3 \to ZZ)$), a feature forbidden in the original GM model due to custodial symmetry.
• Mechanism for Enhanced Diphoton Rates: The model utilizes the doubly charged Higgs boson ($H^{\pm\pm}$) loop contribution to significantly enhance the $H_1 \to \gamma\gamma$ decay rate, explaining the 95 GeV diphoton excess without requiring large deviations in the 125 GeV Higgs couplings.
• Natural Mass Hierarchy: The model predicts a specific mass hierarchy where fixing $m_{H_1} \approx 95$ GeV and $m_{H_3} \approx 152$ GeV naturally constrains other scalars (including $H_2$ and charged states) to be below $\sim200$ GeV, avoiding fine-tuning.
• Comprehensive Global Fit: The study provides the first simultaneous fit of the 95 GeV, 125 GeV, and 152 GeV excesses within a single, theoretically consistent BSM framework.

4. Results

• Fit Quality: The meGM model significantly improves the fit to experimental data compared to the SM.
  • $\Delta\chi^2_{95} = -11.0$ (improvement over SM).
  • $\Delta\chi^2_{152} = -8.72$ (improvement over SM).
  • Total improvement $\Delta\chi^2_{total} = -19.2$.
• Best-Fit Point Properties:
  • Masses: $m_{H_1} = 95$ GeV, $m_{H_2} = 147$ GeV, $m_{H_3} = 152$ GeV, $m_{H_1^\pm} = 100$ GeV, $m_{H_2^\pm} = 156$ GeV, $m_{H^{\pm\pm}} = 163$ GeV.
  • Couplings: The 125 GeV Higgs ($h$) couplings remain close to SM values ($\kappa \in [0.94, 0.97]$). The 95 GeV Higgs ($H_1$) has suppressed couplings to fermions and gauge bosons ($\kappa \approx 0.2-0.3$) but an enhanced diphoton branching ratio due to $H^{\pm\pm}$ loops.
  • 152 GeV State ($H_3$): Exhibits a large branching ratio to $WW$ ($\sim71\%$) and a very small branching ratio to $ZZ$ ($\sim2.8\%$), satisfying the requirement to explain the sideband excesses without conflicting with $ZZ$ searches.
• Phenomenology: The model predicts a "light" scalar spectrum with multiple charged and neutral states accessible at current and future colliders. The dominant decay of the heavier charged Higgs ($H_2^\pm$) is to $H_1^\pm Z$, while the doubly charged Higgs decays significantly to $H_1^\pm W^\pm$.

5. Significance and Future Prospects

• Validation of Light Scalar Spectra: The results suggest that if the 95 GeV and 152 GeV excesses are confirmed, the Higgs sector is likely extended with a light, non-decoupled spectrum of scalars, rather than a heavy, decoupled BSM sector.
• Role of Custodial Symmetry Breaking: The work highlights that mild breaking of custodial symmetry is a necessary ingredient for GM-type models to explain the specific pattern of LHC excesses (specifically the $WW/ZZ$ asymmetry at 152 GeV).
• Future Discovery Potential:
  • HL-LHC: The production cross-sections for $H_2^\pm$ and $H^{\pm\pm}$ are enhanced compared to the standard GM model, making them prime targets for discovery in $WZ$ and $WW$ channels.
  • Future $e^+e^-$ Colliders (ILC/CLIC/CEPC): The model predicts that the 95 GeV Higgs ($H_1$) can be produced abundantly at $\sqrt{s}=250$ GeV. Precision measurements of its couplings (specifically $H_1 \to b\bar{b}$ and $H_1 \to \tau\tau$) at the ILC could distinguish the meGM interpretation from other models with a $\sim5\sigma$ significance.
  • 152 GeV State: While kinematically challenging for a 250 GeV collider, a 500 GeV machine (ILC500) would be required to directly produce and study the 152 GeV resonance.

In conclusion, the paper establishes the meGM model as a highly motivated and predictive framework that naturally unifies the 95 GeV and 152 GeV anomalies while preserving the success of the 125 GeV Higgs discovery, offering clear pathways for experimental verification in the upcoming LHC runs and future lepton colliders.