Investigating the 95 GeV Higgs Boson Excesses within the I(1+2)HDM

TL;DR

The paper investigates whether the I(1+2)HDM Type-I, a CP-conserving three-Higgs-doublet framework with an inert doublet, can explain the 95 GeV excesses observed in $\gamma\gamma$ and $b\bar{b}$ channels, while remaining compatible with the 125 GeV SM-like Higgs measurements and other constraints. By scanning a 12-parameter space under perturbativity, unitarity, vacuum stability, EWPOs, Higgs-precision data, flavor observables, and exclusion limits, the authors identify viable regions with a light CP-even state $h$ in the $[94,97]$ GeV range and a SM-like $H$ near 125 GeV. They show that inert charged scalars $\chi^{\pm}$ can modify $h\to\gamma\gamma$ at one loop, enabling constructive interference that helps fit the observed diphoton rate, while correlations among $\mu_{\gamma\gamma}$, $\mu_{bb}$, and $\mu_{\tau\tau}$ constrain $\tan\beta$ and mixing through $\sin(\beta-\alpha)$. The best-fit scenario yields modest diphoton enhancement with compatible $bb$ and $\tau\tau$ signals, and the work also highlights future collider prospects (HL-LHC and ILC) for testing the 95 GeV state via both Higgs precision measurements and direct production channels at $\sqrt{s}=250$–$500$ GeV. Overall, the results indicate that the I(1+2)HDM Type-I can simultaneously address multiple 95 GeV anomalies within a consistent BSM framework, with distinctive predictions for upcoming experiments.

Abstract

In this work, we explore how the 2-Higgs Doublet Model (2HDM) Type-I, extended by an inert doublet, can provide an explanation for the recently observed excesses at the Large Hadron Collider (LHC) in the $γγ$ and $τ^+ τ^- $ final states. Hence, by imposing theoretical constraints and experimental bounds on the model parameter space, our findings show that a light CP-even Higgs boson, $h$, with a mass around 95 GeV, can account for these anomalies. This result aligns with the excess in $b\bar b$ signatures reported in earlier data from the Large Electron-Positron (LEP) collider.

Figures (6)

• Figure 1: Correlations among the signal strengths in Eqs. (\ref{['eq:mubb']})--(\ref{['eq:mutautau']}), with the $\chi^2_{\gamma\gamma+\tau^+\tau^-+b\bar{b}}$ value represented by the colour bar. The red star marks the $\min(\chi^2_{\gamma\gamma+\tau^+\tau^-+b\bar{b}})$ best-fit point, while the green star indicates the $\min(\chi^2_{\gamma\gamma+\tau^+\tau^-+b\bar{b}+125})$ best-fit point including the SM-like Higgs data. The ATLAS and CMS results, along with their corresponding 1$\sigma$ bands, are also depicted through the ellipse contours, which indicate the regions corresponding to the excess observed at such a C.L. (We use the $\chi^2_x+\chi^2_y=2.30$ relation, where the indexes $x$ and $y$ represent each possible pair of the three signal channels: $\gamma\gamma$, $\tau^+\tau^-$ and $b\bar{b}$.)
• Figure 2: Upper panel: squared moduli $|C_{H^\pm}|^2$ and $|C_{\chi^\pm}|^2$ as a function of $\sin(\beta-\alpha)$, with the colour code indicating $m_{H^\pm}$ and $m_{\chi^\pm}$. Lower panel: interference terms $2 \, \Re(C_{\chi^\pm} C^*_{H^\pm})$, $2 \, \Re(C_{\chi^\pm} C^*_{W^\pm})$, and $2 \, \Re(C_{\chi^\pm} C^*_{t})$ projected onto the $(\mu_{\gamma\gamma},\,\mu_{bb})$ plane. The modified top and $W^\pm$ contributions are also shown for comparison.
• Figure 3: Comparison of $\mu_{\gamma\gamma}-\mu_{bb}$ correlation with and without the $\chi^\pm$ contribution.
• Figure 4: Correlations among the signal strengths ($\mu_{bb}$ - $\mu_{\gamma\gamma}$), ($\mu_{\tau^+\tau^-}$ - $\mu_{\gamma\gamma}$) and ($\mu_{\tau^+\tau^-}$ - $\mu_{bb}$), with the $\tan\beta$ value represented by the colour bar . The ATLAS and CMS results, as described in the previous figure, are also included.
• Figure 5: Scattered points in the plane ($|c_{h_{125}\tau^+\tau^-}|$, $|c_{h_{125}VV}|$) following our scan, with the colour bar indicating the combined value of $\chi^2$ for the three discussed excesses. Projections from the HL-LHC and an ILC running at 500 GeV are also sketched.