Universal Seesaw Leptogenesis

TL;DR

This paper explores leptogenesis within a left-right symmetric model utilizing the Universal Seesaw mechanism, highlighting novel decay processes of gauge-singlet mediators and implications for baryon asymmetry and dark radiation.

Contribution

It introduces a leptogenesis scenario based on gauge-singlet mediator decays, avoiding typical washout constraints and enabling low-scale symmetry breaking.

Findings

Heavy mediators decay to both chiral neutrinos, causing potential asymmetry cancellation.

Low-scale left-right symmetry breaking is feasible without conflicting with collider bounds.

The lightest right-chiral neutrino acts as dark radiation with sub-eV mass.

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.