95 GeV Higgs boson and nano-Hertz gravitational waves from domain walls in the N2HDM

TL;DR

This work investigates whether the observed 95.4 GeV Higgs-like excess and nano-Hertz gravitational waves from domain walls can be jointly explained within the N2HDM, a model that adds a real singlet to the 2HDM under a $Z_2$ symmetry. It analyzes two regimes: Scenario A, where the 95.4 GeV state is singlet-dominated yet mixes with doublets to satisfy SM-like Higgs constraints, and Scenario B, where it is mainly from doublet CP-even components. Under comprehensive theoretical and experimental constraints, Scenario A can fully account for both the diphoton and $bar{b}$ excesses at 95.4 GeV (within $1σ$) and predicts a nano-Hertz GW signal with a peak amplitude up to about $2 imes10^{-12}$ at $f_{ m peak} obreak obreak hickspace obreak $ for $v_s=100$ TeV; Scenario B yields a larger GW amplitude up to $ obreak 6 imes10^{-8}$ but only marginally explains the 95 GeV excess. The domain-wall dynamics produce a SGWB that can be probed by current PTA datasets (e.g., NANOGrav) and future campaigns (e.g., SKA). The results map viable parameter spaces and emphasize the synergy between collider signals and gravitational-wave observations as a probe of the extended Higgs sector.

Abstract

We explore the diphoton and $b\bar{b}$ excesses at 95.4 GeV, as well as nano-Hertz gravitational waves originating from domain walls, within the framework of the next-to-two-Higgs-doublet model (N2HDM), which extends the two-Higgs-doublet model by introducing a real singlet scalar subject to a discrete $Z_2$ symmetry. The $Z_2$ symmetry is spontaneously broken by the non-zero vacuum expectation value of the singlet scalar, $v_s$, which leads to the formation of domain walls. We discuss two different scenarios: in scenario A, the 95.4 GeV Higgs boson predominantly originates from the singlet field, while in scenario B, it arises mainly from the CP-even components of the Higgs doublets. Taking into account relevant theoretical and experimental constraints, we find that scenario A can fully account for both the diphoton and $b\bar{b}$ excesses at 95.4 GeV within the $1σ$ range. Moreover, the peak amplitude of the gravitational wave spectrum at a peak frequency of $10^{-9}$ Hz can reach $2 \times 10^{-12}$ for $v_s = 100$ TeV. Scenario B only marginally accounts for the diphoton and $b\bar{b}$ excesses at the $1σ$ level, but the peak amplitude of the gravitational wave spectrum at the peak frequency of $10^{-9}$ Hz can reach $6\times 10^{-8}$ for $v_s=100$ TeV. The nano-Hertz gravitational wave signals predicted in both scenarios can be tested by the current and future pulsar timing array projects.

Figures (7)

• Figure 1: In scenario A, the parameter space is progressively constrained by sequentially imposing the following: theoretical requirements and the oblique parameters; the signal data of the 125 GeV Higgs boson; searches for additional Higgs bosons at the collider and flavour observables; and finally the condition $\chi_{95}^2<6.18$. The surviving parameter points at each stage are represented by green bullets, cyan squares, blue-edged squares, and red pluses, respectively.
• Figure 2: In scenario A, the surviving parameter points satisfy $\chi_{95}^2<6.18$ while remaining consistent with the theoretical constraints, the oblique parameters, the Higgs signal data at 125 GeV, searches for additional Higgs bosons at the collider, and flavor observables.
• Figure 3: Same as the Fig. \ref{['figalhc']}, but for scenario B.
• Figure 4: Same as the Fig. \ref{['figachi']}, but for scenario B.
• Figure 5: In scenario A, the parameter points satisfy $\Omega_{\rm GW}^{\rm peak} h^2>10^{-15}$ and $\chi_{95}^2<6.18$ while remaining consistent with relevant theoretical and experimental constraints mentioned above.