Leptogenesis and Neutrino Oscillations in the Classically Conformal Standard Model with the Higgs Portal

TL;DR

The paper demonstrates that electroweak symmetry breaking and the baryon asymmetry of the universe can be simultaneously explained in a classically conformal Standard Model extended by a Higgs portal to a Coleman-Weinberg hidden sector, specifically a U(1)_{B-L} realization. In this setup, a CW scalar ⟨φ⟩ triggers the hidden sector dynamics and, via the Higgs portal, induces EWSB without explicit mass terms, while Majorana masses for right-handed neutrinos arise as M ∼ Y^M⟨φ⟩ and generate light active neutrinos through the see-saw mechanism. Leptogenesis proceeds through CP-violating oscillations of GeV-scale sterile neutrinos, treated with a density-matrix formalism (ARS/DG), and is converted into the observed BAU by electroweak sphalerons; thermal corrections and φ-induced mass terms are incorporated to adapt the DG framework to this class of models. The analysis identifies viable parameter regions with ⟨φ⟩ ≈ 10^4–10^5 GeV, Majorana masses in the GeV–few hundred GeV range, and Higgs portal couplings λ_P ∼ 10^{-6}–10^{-3}, predicting a TeV-scale Z' that can be probed by collider and precision experiments and a spectrum of sterile neutrinos accessible to future searches. Overall, the work provides a natural, testable route to address both the origin of the electroweak scale and the matter-antimatter asymmetry within a scale-invariant framework.

Abstract

The Standard Model with an added Higgs portal interaction and no explicit mass terms is a classically scale-invariant theory. In this case the scale of electroweak symmetry breaking can be induced radiatively by the Coleman-Weinberg mechanism operational in a hidden sector, and then transmitted to the Standard Model through the Higgs portal. The smallness of the generated values for the Higgs vev and mass, compared to the UV cutoff of our classically scale-invariant effective theory, is naturally explained by this mechanism. We show how these classically conformal models can generate the baryon asymmetry of the Universe without the need of introducing mass scales by hand or their resonant fine-tuning. The minimal model we consider is the Standard Model coupled to the Coleman-Weinberg scalar field charged under the $U(1)_{B-L}$ gauge group. Anomaly cancellation requires automatic inclusion of three generations of right-handed neutrinos. Their GeV-scale Majorana masses are induced by the Coleman-Weinberg field and lead to the generation of active neutrino masses through the standard see-saw mechanism. Leptogenesis occurs via flavour oscillations of right-handed sterile neutrinos and is converted to the baryon asymmetry by electroweak sphalerons.

Figures (6)

• Figure 1: Left panel shows the effective thermal mass squared difference $\Delta M^2(T)$ given by \ref{['eq:subs']} with smoothened theta function (and the initial value taken to be $\Delta M^2_0=3\, {\rm GeV}^2$) as the function of the temperature over $\langle |\phi |\rangle$. On the right panel, the blue curve sketches the initial temperature $T_{\rm osc}$ as the function of $\langle |\phi |\rangle$ showing the transition between the unbroken ($T_{\rm osc}>\langle |\phi |\rangle$) and the broken ($T_{\rm osc}<\langle |\phi |\rangle$) phase. The horizontal green line gives the value of $T_{\rm osc}$ computed in the regime of Drewes:2012ma via \ref{['eq:Tosc']}. On the right of the plot, the blue and green curves coincide.
• Figure 2: Left panel shows maximal values of Majorana masses in GeV for which the wash-out bound in Eq. \ref{['eq:washout']} can be achieved. The panel on the right shows contours for the baryon asymmetry produced, normalised to the observed value. Majorana masses used in \ref{['fig:1']}(b) are taken from \ref{['fig:1']}(a) for each value of $Re[\omega_{23}]$ and $Im[\omega_{23}]$. In both plots we vary $Re[\omega_{23}]$ and $Im[\omega_{23}]$ keeping other parameters of the model fixed at indicative values as in Ref. Drewes:2012ma, detailed in the Tables \ref{['tab:1']} and \ref{['tab:2']}.
• Figure 3: Superposition of the Majorana mass contours in GeV satisfying the wash-out bound with the baryon asymmetry produced with shaded regions denoting the required baryon asymmetry from Fig. \ref{['fig:1']}
• Figure 4: The wash-out rate (left panel) and the normalised baryon asymmetry computed in the classically conformal $B-L$ model. The values of model parameters are defined in the text.
• Figure 5: Superposition of the wash-out rate $\le 1$(inside the shaded ellipse) with the baryon asymmetry produced from Fig. \ref{['fig:2']}