Probing Type-I 2HDM light Higgs in the top-pair-associated diphoton channel

Motivated by the possible 95 GeV diphoton excess, we investigate the capability of the Type-I Two-Higgs-Doublet Model (2HDM-I) to explain this signal under current theoretical and experimental constraints. Using full Monte Carlo (MC) simulations for the process of $pp \to t(\to W^+ b)\bar{t}(\to W^- \bar{b})h(\to γγ)$, we evaluate the discovery potential of a 95 GeV Higgs boson at future colliders. Direct Higgs searches strongly constrain the parameter $α$, excluding the region with $α\lesssim 0.95$. Monte Carlo results indicate that a minimum cross section of 0.3 fb is required to achieve a $5σ$ signal statistical significance at the HL-LHC with $L = 3~\mathrm{ab}^{-1}$. For the same luminosity, HE-LHC and FCC-hh require 0.67 fb and 2.36 fb, respectively. At the 14 TeV HL-LHC with an integrated luminosity of $3~\mathrm{ab}^{-1}$, parameter regions with $\sin(β-α) \gtrsim 0.4$ and $\sin(β-α) \gtrsim 0.25$ can be probed at the $5σ$ and $2σ$ significance levels, respectively. At the 27 TeV HE-LHC with $L = 10~\mathrm{ab}^{-1}$, the sensitivity improves to $\sin(β-α) \gtrsim 0.25$ ($5σ$) and $\gtrsim 0.15$ ($2σ$). For the 100 TeV FCC-hh with $L = 30~\mathrm{ab}^{-1}$, even regions with $\sin(β-α) \gtrsim 0.1$ or $\sin(β-α) \lesssim -0.05$ can be covered at the $5σ$ level. Parameter regions near $\sin(β-α) \approx 0$ remain challenging to probe in the diphoton channel, even with increased energy or luminosity.

💡 Research Summary

The paper investigates whether the Type‑I Two‑Higgs‑Doublet Model (2HDM‑I) can account for the recurring diphoton excess observed near 95 GeV by CMS and ATLAS. In this framework the lighter CP‑even scalar h is taken to have a mass of 95 GeV, while the heavier CP‑even scalar H is identified with the 125 GeV SM‑like Higgs. The authors perform a comprehensive scan over the relevant parameters: the mixing angle α (−π/2 ≤ α ≤ π/2), tan β (0 ≤ tan β ≤ 20), the soft‑breaking term m₁₂² (−10 TeV² ≤ m₁₂² ≤ 10 TeV²), and the common mass of the CP‑odd and charged Higgs bosons (150 GeV ≤ m_A = m_{H±} ≤ 1000 GeV).

A series of theoretical and experimental constraints are imposed. Flavor observables (B_d,s → μ⁺μ⁻, B → X_sγ) are evaluated with SuperIso, requiring tan β ≳ 2.6 and limiting the size of |m₁₂|. Direct Higgs searches at LEP and the LHC are incorporated via HiggsBounds, which exclude essentially all points with α ≲ 0.9. Compatibility with the measured signal strengths of the 125 GeV Higgs is enforced using HiggsSignals (χ² < 124.3 for 107 d.o.f.). Electroweak precision data (S, T, U) are required to lie within the latest global fit, and theoretical consistency (unitarity, vacuum stability, perturbativity) is checked with SARAH/SPheno.

After applying all constraints, the surviving region is characterized by α≈0.95–1.55, corresponding to sin(β−α) ≳ 0.4, and tan β ≳ 2.6. In this domain the h‑W‑W coupling (proportional to sin(β−α)) is sizable, while the h‑fermion couplings (∝ cos α/ sin β) are moderately suppressed, leading to an enhanced branching ratio Br(h → γγ). The diphoton width is dominated by the W‑loop; the charged‑Higgs contribution is negligible because the charged Higgs is heavy and its coupling to h is small. When sin(β−α) approaches zero, the h‑W‑W coupling vanishes, the diphoton width drops to ∼10⁻⁶, and the branching ratio becomes essentially unobservable.

The phenomenological focus is the process pp → t t̄ h with h → γγ, which benefits from the presence of top‑pair tags that suppress QCD backgrounds. The authors generate signal events with MadGraph5_aMC@NLO, shower them with Pythia 8, and simulate detector effects with Delphes. They apply realistic object selections: two isolated photons with p_T > 25 GeV, |η| < 2.5, and an invariant‑mass window 93–97 GeV; b‑tagging efficiency of 70 % and a fake rate of 1 % are assumed. Main backgrounds include t t̄ γγ, t t̄ h_SM (h → γγ), and t t̄ γj (jet faking a photon). After cuts, the signal efficiency is about 5 %, yielding roughly 30–40 signal events for an integrated luminosity of 3 ab⁻¹ at the HL‑LHC.

Statistical significance is evaluated using the standard formula Z = √{2