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

TL;DR

The paper investigates simultaneous explanations for a $\sim95$ GeV di-photon excess and a possible $\sim152$ GeV signal within a minimally extended Georgi–Machacek (meGM) model. By allowing mild custodial-symmetry breaking, the model yields a light scalar spectrum with a 95 GeV state, a 125 GeV SM-like Higgs, and a 152 GeV neutral state, while a doubly charged Higgs enhances $\mathrm{BR}(\gamma\gamma)$ channels. A rigorous numerical analysis combining LHC excess data, Higgs-coupling fits, and theoretical constraints finds a best-fit region with a consistent mass hierarchy and improved global $\chi^2$ relative to the SM, predicting additional light CP-odd and charged scalars accessible in future runs. The study also outlines HL-LHC and future $e^+e^-$ collider prospects, showing measurable deviations in Higgs couplings and promising discovery channels for the extended scalar sector.

Abstract

We analyze the excesses at 95 GeV in the light Higgs-boson searches in the di-photon decay channel reported by CMS and ATLAS, which combined are at the level of three standard deviations and are compatible with the excess in the $b\bar{b}$ final state observed at LEP, together with an excess in the di-photon channel at around 152 GeV reported based on a sideband analysis. We demonstrate that these excesses can be well described in a minimally extended Georgi-Machacek (meGM) model. This is enabled by four key features of the meGM model: (1) a natural prediction for scalar boson masses of $\lesssim$200 GeV arising from the condition to describe both the Higgs boson signal at 125 GeV and the excesses at 95 GeV, (2) the prediction for a doubly charged Higgs boson that can potentially enhance the di-photon decay rates, (3) asymmetric $WW$ and $ZZ$ couplings to neutral scalar bosons that are induced by mild custodial symmetry breaking, and (4) the approximate preservation of the electroweak $ρ$ parameter to be 1 at tree level. We show in our numerical analysis that the meGM model naturally improves the fit to the LHC data around 152 GeV when describing the excesses at 95 GeV. At the same time, the model also predicts additional light CP-odd and charged scalar bosons that can be potentially probed in future experiments, which motivates dedicated searches in the upcoming LHC runs. We also present the results of sensitivity studies for the 95 and 125 GeV Higgs-boson couplings at the HL-LHC and future $e^+e^-$ colliders, which demonstrate very interesting prospects for probing the meGM model at future colliders.

Figures (10)

• Figure 1: A representative Feynman diagram of the charged Drell-Yan production of the scalar boson pair $H_3$ and $H_{1,2}^\pm$, which then decay to $\gamma\gamma$ and $XY=WZ,tb,cs,\tau\nu_\tau$, respectively. For $H_2^\pm$ furthermore the decay modes into $H_1^\pm Z$, $H_1W^\pm$, and $hW^\pm$ can occur.
• Figure 2: Left: Sample distributions in the $v_\chi$--$v_\xi$ plane, where the dashed line indicates the custodial-symmetric limit, i.e. , $v_\chi=v_\xi$. Right: Sample distributions in the $\mu_{\gamma\gamma,95}$--$\mu_{b\bar{b},95}$ plane. The elliptic contour denotes the 1 $\sigma$ bounds of the corresponding 95-GeV excess measurements. For the color coding, see the main text.
• Figure 3: Sample distributions in the ( left) $m_{H_2}$--$m_{H^{\pm\pm}}$ and ( right) $m_{H_1^\pm}$--$m_{H_2^{\pm}}$ planes. The color coding is the same as in Fig. \ref{['fig:vev-mu_H1']}.
• Figure 4: Sample distributions in the ( left) $\kappa_{h\gamma\gamma}$--$\kappa_{H_1\gamma\gamma}$ and ( right) $\tilde{\kappa}_{h\gamma\gamma}$--$\tilde{\kappa}_{H_1\gamma\gamma}$ planes (see text for details). The color coding is the same as in Fig. \ref{['fig:vev-mu_H1']}.
• Figure 5: Sample distributions in the ( left) ${\rm BR}(H_3\to WW)$--${\rm BR}(H_3\to ZZ)$ and ( right) ${\rm BR}(H_3\to \gamma\gamma)$--$\Delta\chi^2_{152}$ planes. The color coding is the same as in Fig. \ref{['fig:vev-mu_H1']}. It should be noted that we did not include the blue points in the right plot since we do not perform the 152-GeV excess analysis on them.