Universal Seesaw Leptogenesis

TL;DR

The paper presents a novel leptogenesis mechanism in a left-right symmetric model with Universal Seesaw, where CP violation arises from decays of gauge-singlet mediators $N_{L}$ and $N_{R}$, enabled by parity doubling. It demonstrates that CP violation can be generated with a single mediator generation and that, unlike conventional LR models, the usual gauge-induced washout constraints can be evaded, allowing both high-scale and low-scale LR symmetry breaking. The analysis identifies viable parameter regions, including $v_R$ as low as current collider limits (around a few-TeV to tens of TeV) under certain parity-breaking assumptions, and predicts a sub-eV lightest RH neutrino that can contribute as dark radiation with controlled $\Delta N_{\text{eff}}$. The work also explores cosmological implications of the light RHN and discusses the strong CP problem, domain walls, and higher-dimensional operator constraints within generalized parity frameworks, highlighting the interplay between particle physics requirements and cosmological consistency.

Abstract

We study the implications for leptogenesis in a class of left-right symmetric model, where all fermion masses are induced through the Universal Seesaw mechanism. Unlike conventional analyses, we do not use the decays of the neutrino embedded in the right-chiral lepton doublet, but rather those of the gauge-singlet mediators required for neutrino mass generation in the canonical Type-I seesaw. Due to the generalized parity symmetry that doubles the fermionic degrees of freedom in this model, we can generate the required $CP$ violation in the heavy fermion decays with only a single generation of mediators. One of the distinct features of our scenario is that the bounds from thermalization or washout via gauge interactions typically encountered in the canonical left-right symmetric models do not apply here. Moreover, the heavy mediators can decay to both the left and the right-chiral neutrinos, leading to a cancellation in the resulting baryon asymmetry for decays above the left-right symmetry breaking scale. We discuss ways to avoid this cancellation and show that low scale left-right symmetry breaking above the current collider limits is viable. The right chiral neutrinos also obtain their masses from the seesaw mechanism, and the lightest one turns out to have a sub-eV scale mass. We find that its abundance is consistent with standard cosmology, and it acts as potentially observable dark radiation.

Figures (4)

• Figure 1: Tree-level (left), one-loop vertex (middle) and self-energy (right) diagrams for the decays of heavy fermions $N_i \rightarrow l_k H_k$ with $\{i,j,k,m\} = L,R$ and $i \neq j$.
• Figure 2: Contours of the observed baryon asymmetry (95% CL) in the $M_N$ - $|Y|$ plane for decays after the high LRS breaking. Left: Varying $r$ with fixed $\delta$ and $\varphi = \pi/4$. Right: Varying $\varphi$ for a fixed value of $r$ and $\delta$. In the shaded gray region, the lightest neutrino mass is larger than the solar mass splitting $\sqrt{\Delta m_{21}^2}$, whereas the shaded light brown and red regions are excluded from $2 \leftrightarrow 2$ scatterings and $L \leftrightarrow R$ conversion for $v_R =10^7$ GeV, respectively (see Eqs. \ref{['eq:22scat']} and \ref{['eq:weak']}). The left and the right wings of all the boomerang shaped contours correspond to the strong and weak washout regimes, respectively.
• Figure 3: Same as Fig. \ref{['fig:scanpar1']}, but for decays before LRS breaking. Left: Varying $\omega$ with fixed $r$ and $\varphi$, and setting $\chi=0$. Right: Varying $\varphi$ with fixed $r$ and $\omega$. The shaded regions correspond to the strongest constraints, obtained for $\varphi=\pi/16$.
• Figure 4: Cosmologically allowed parameter space for the lightest generation of $\nu_R$ in the plane spanned by the lightest active neutrino mass $m_{\nu_L}$ and the VEV $v_R$. In the gray region our assumption of an effectively massless lightest active neutrino (see the discussion around Eq. \ref{['eq:light']}) breaks down. The red and blue regions are excluded from the limits on $v_R$ from the quality problem (see Eq. \ref{['eq:qual2']}) and collider searches (see Eq. \ref{['eq:18']}). In the brown region, the condition on the relic abundance in Eq. \ref{['eq:frac']} is violated. Above the magenta line, the CMB limit on the effective $\nu_R$ mass in Eq. \ref{['eq:meff']} is violated, which coincides with the aforementioned limit on the relic abundance. We depict iso-contours for $m_{\nu_R}=10keV,\; 10eV,\; 10meV$ and for $\Delta N_\text{eff.}=0.06,\;0.14.$