Strongly coupled inert scalar sector with radiative neutrino masses

TL;DR

This work develops a beyond-Standard-Model framework with a strongly coupled inert scalar sector under the gauge group $SU(2)_2\times SU(2)_1\times U(1)_Y$, featuring two inert Higgs doublets, a bidoublet $\Sigma$, and a singlet $\sigma$ to realize radiative neutrino masses and a universal seesaw for lighter-than-top fermions. Electroweak symmetry breaking proceeds through a hierarchical pattern, yielding a SM-like Higgs, heavy vector resonances, and a rich scalar spectrum; experimental constraints from Higgs properties, oblique parameters, LFV, and diphoton rates are satisfied while providing distinctive signals. Neutrino masses arise radiatively at one loop, whereas charged-fermion masses are generated by a universal seesaw with high-scale $\Lambda$, enabling a natural fermion mass hierarchy. The model also accommodates leptogenesis and a potentially observable 95 GeV diphoton resonance, offering multiple testable predictions for future collider and flavor experiments. Overall, it constitutes a coherent, testable BSM scenario tying together neutrino physics, Higgs phenomenology, and baryogenesis in a strongly coupled scalar framework.

Abstract

We explore the phenomenological consequences of a model with an extended scalar sector, incorporating strongly coupled inert Higgs doublets. The model introduces three Higgs doublets: one that interacts with the $SU(2)$ symmetry of the Standard Model, and two inert doublets belonging to a strongly interacting sector, embedded in the $SU(2)_2 \times SU(2)_1 \times U(1)_Y$ electroweak gauge symmetry. This symmetry structure is further supplemented by a spontaneously broken $\mathbb{Z}_2$ and a preserved $\mathbb{Z}_2'$ discrete symmetry. Our approach harnesses the exotic scalar fields emerging from this sector to implement mass-generation mechanisms. In particular, a one-loop seesaw mechanism accounts for the smallness of neutrino masses, while a universal seesaw mechanism naturally explains the hierarchy of charged fermion masses below the top quark mass. The model is shown to be consistent with current experimental constraints, including those from charged lepton flavor violation, electroweak precision observables, the Higgs diphoton decay rate, and the Higgs trilinear self-coupling. Notably, it also provides a viable interpretation of the 95~GeV diphoton excess, offering a distinctive signature of new physics beyond the Standard Model.

Figures (19)

• Figure 1: Fermion mass mechanism. In (a) Universal seesaw mechanism for quarks and charged lepton $\psi_L=\{ Q_{nL},e_{iL}\}$, $\psi_R=\{u_{nR},d_{nR}, e_{iR} \}$, $\Psi_L=\{T_{nL},B_{iL},E_{iL}\}$ and $\Psi_R=\{T_{nR},B_{iR},E_{iR}\}$, for $n=1,2$, $i=1,2,3$ (excluding top quark). In (b) One-loop radiative seesaw mechanism for neutrino sector.
• Figure 2: Panel (a): Behavior of the mixing angle $\theta_{CH}$ as a function of the charged vector resonance mass $M_{\rho^\pm}$ for different values of the $c$ parameter. The horizontal black dashed-dotted line indicates the exclusion region arising from the electroweak precision data and is denoted as "EW" Pankov:2021vzs. Panel (b): Behavior of the mixing angle $\theta_{N}$ as a function of the neutral vector resonance mass $M_{\rho_0}$ for different values of the $c$ parameter. The horizontal black dashed-dotted line indicates the exclusion region resulting from the electroweak precision data (EW) Pankov:2021vzs.
• Figure 3: One-loop Feynman diagrams that contribute to the mass of the visible charged scalars. (a) Diagrams contributing to the diagonal entries of the mass matrix. Here $\alpha=1,2$. (b) Diagrams contributing to the off-diagonal entries of the mass matrix. Here $\alpha,\beta=1,2$ and $\alpha\neq\beta$.
• Figure 4: The one-loop running of the charged scalar masses around the pole mass is shown. Panel (a) displays the running of $m_{\eta_1^\pm}$ in the range $\left[0.5m_{\eta_1^\pm}^{(\mathrm{pole})}, 2m_{\eta_1^\pm}^{(\mathrm{pole})}\right]$ and the lower bound for collider searches Workman:2022ynf. Panel (b) presents the running of $m_{\eta_2^\pm}$ in the range $\left[0.5m_{\eta_2^\pm}^{(\mathrm{pole})}, 2m_{\eta_2^\pm}^{(\mathrm{pole})}\right]$, compared to its tree-level value.
• Figure 5: (a) The correlation parameter matrix of the quark sector. The color scale corresponds to the degree of correlation between two parameters. (b, c, d) Correlation plot between the mixing angles of the quarks and the Jarlskog invariant obtained with our model. The dashed and dot-dashed lines correspond to the experimental values.