A 95 GeV Higgs boson and spontaneous CP-violation at the finite temperature

TL;DR

This work investigates a complex singlet extension of the two-Higgs-doublet model (2HDM) to address the 95.4 GeV diphoton excess and the LEP $b\bar{b}$ excess, by realizing a light Higgs $h_1$ from mixing among three CP-even states. It simultaneously embeds spontaneous CP violation at finite temperature to enable electroweak baryogenesis (EWBG) and generate the observed baryon asymmetry, while naturally avoiding stringent EDM constraints. The analysis combines collider and flavor constraints with a finite-temperature phase-transition study, identifying viable parameter regions and three-step phase transition histories that yield BAU without conflicting with current data. The work predicts additional features, such as a light CP-odd state around 70–80 GeV and specific patterns in Higgs couplings, that provide concrete avenues for experimental testing in future runs. Overall, the paper offers a cohesive scenario in which a light scalar explains collider anomalies and a thermal history of the Universe accounts for the BAU within a single, testable framework.

Abstract

The ATLAS and CMS collaborations reported a diphoton excess in the invariant mass distribution around the 95.4 GeV with a local significance of $3.1σ$. Moreover, there is another $2.3σ$ local excess in the $b\bar{b}$ final state at LEP in the same mass region. A plausible solution is that the Higgs sector is extended to include an additional Higgs boson with a mass of $95.4$ GeV. We study a complex singlet scalar extension of the two-Higgs-doublet model in which the 95.4 GeV Higgs is from the mixing of three CP-even Higgs fields. In addition, the extended Higgs potential can achieve spontaneous CP-violation at the finite temperature and restore CP symmetry at the present temperature of the Universe. We find that the model can simultaneously explain the baryon asymmetry of the Universe, the diphoton and $b\bar{b}$ excesses around the 95.4 GeV while satisfying various relevant constraints including the experiments of collider and electric dipole moment.

Figures (8)

• Figure 1: The samples accommodating the $b\bar{b}$ excess at the 95.4 GeV. The excess is explained within $1\sigma$ and $2\sigma$ ranges for the squares and circles, respectively.
• Figure 2: Same as the Fig. \ref{['figbb']}, but for the diphoton excess at the 95.4 GeV.
• Figure 3: The circles (green) are excluded by the constraints of "pre-95" while the pluses (blue) and squares (red) are allowed. The $b\bar{b}$ and diphoton excesses are simultaneously explained within $1\sigma$ and $2\sigma$ ranges for squares (red) and the pluses (blue).
• Figure 4: Phase histories for the BP1. $\phi(i)$ denotes the VEV of $\phi$ at the i-th phase, where $\phi=\varphi_1,\varphi_2,\chi,\varphi_3,\eta_s$. The black dot represents the occurrence of a second-order PT.
• Figure 5: Same as Fig. 1, but for the BP2.