Electromagnetic leptogenesis with light-heavy sterile neutrinos

TL;DR

This paper introduces a new leptogenesis mechanism using electromagnetic dipole operators involving sterile neutrinos, linking neutrino mass generation, dark matter, and potential experimental signatures at TeV scales.

Contribution

It proposes a novel leptogenesis scenario with dimension-5 dipole operators enabling two-body decay of heavy right-handed neutrinos, distinct from previous three-body decay models.

Findings

Leptogenesis at around 10 TeV scale is feasible with this mechanism.

Predicted keV-scale sterile neutrinos could serve as warm dark matter.

Potential observable monochromatic photon signatures at experiments.

Abstract

We propose a novel leptogenesis scenario utilising the two-body decay of heavy right handed neutrino (RHN) via the electromagnetic dipole operator. While the requirement of the standard model (SM) gauge invariance requires such dipole operator only at dimension-6 forcing the generation of non-zero CP asymmetry from three-body decay with two-loop corrections, we write down dimension-5 dipole operators involving heavy RHN $N_R$ and its lighter counterpart $\nu_R$. This allows the generation of lepton asymmetry in $\nu_R$ from two-body decay of heavy RHN which later gets transferred to left handed leptons via sizeable Yukawa coupling with a neutrinophilic Higgs doublet. The asymmetry in left handed leptons is then converted to baryon asymmetry via electroweak sphalerons. The lepton number violation by heavy RHN also induces a one-loop Majorana mass of $\nu_R$ rendering the light neutrinos to be Majorana fermions. While smallness of the Majorana mass of $\nu_R$ prevents additional sources or washout of lepton asymmetry, it also constrains the scale of leptogenesis. For $\mathcal{O}(10)$ TeV scale leptogenesis, this leads to additional sterile neutrinos lighter than a few keV with interesting implications for a variety of observables including neutrino oscillation, warm dark matter as well as effective degrees of freedom within reach of future cosmic microwave background experiments. Additionally, heavy RHN can lead to observable monochromatic photon signatures at terrestrial experiments.