# Low-scale leptogenesis assisted by a real scalar singlet

## TL;DR

This paper shows that adding a real scalar singlet to the Higgs sector enables low-scale leptogenesis with lighter right-handed neutrinos, making the scenario more experimentally accessible and less fine-tuned.

## Contribution

It introduces a model where a real scalar singlet enhances leptogenesis, allowing for lighter RHNs and stronger CP violation without degeneracy, and provides a semi-analytical tool for parameter analysis.

## Key findings

- Leptogenesis can occur with RHN masses as low as 500 GeV.
- The scalar singlet coupling enhances CP violation and out-of-equilibrium conditions.
- The model's parameter space can be probed in current and future experiments.

## Abstract

Standard thermal leptogenesis in the type-I seesaw model requires very heavy right-handed neutrinos (RHNs). This makes it hard to probe this scenario experimentally and results in large radiative corrections to the Higgs boson mass. In this paper, we demonstrate that the situation is considerably different in models that extend the Higgs sector by a real scalar singlet. Based on effective-theory arguments, the extra scalar is always allowed to couple to the heavy neutrinos via singlet Yukawa terms. This opens up new RHN decay channels leading to larger $CP$ violation as well as to a stronger departure from thermal equilibrium during leptogenesis. As a consequence, the baryon asymmetry can be generated for a lightest RHN mass as low as 500 GeV and without the need for a highly degenerate RHN mass spectrum. In fact, the requirement of successful leptogenesis via the Higgs portal coupling singles out an interesting parameter region that can be probed in on-going and future experiments. We derive a semianalytical fit function for the final baryon asymmetry that allows for an efficient study of parameter space, thus enabling us to identify viable parameter regions. Our results are applicable to a wide range of models featuring an additional real scalar singlet.