Effects of Lightest Neutrino Mass in Leptogenesis

TL;DR

This paper investigates how the lightest neutrino mass influences flavoured leptogenesis, showing that even with CP-conservation in the Yukawa sector, the Majorana and Dirac phases can generate sufficient baryon asymmetry.

Contribution

It demonstrates that the lightest neutrino mass can significantly enhance baryon asymmetry in leptogenesis, even when the Yukawa coupling matrix is CP-conserving, highlighting the role of neutrino phases.

Findings

Baryon asymmetry can be increased by a factor of ~100 for certain neutrino masses.

Successful leptogenesis is possible with lightest neutrino mass in the range 5×10^{-6} to 5×10^{-2} eV.

Majorana and Dirac phases alone can drive leptogenesis without CP-violation in Yukawa couplings.

Abstract

The effects of the lightest neutrino mass in ``flavoured'' leptogenesis are investigated in the case when the CP-violation necessary for the generation of the baryon asymmetry of the Universe is due exclusively to the Dirac and/or Majorana phases in the neutrino mixing matrix U. The type I see-saw scenario with three heavy right-handed Majorana neutrinos having hierarchical spectrum is considered. The ``orthogonal'' parametrisation of the matrix of neutrino Yukawa couplings, which involves a complex orthogonal matrix R, is employed. Results for light neutrino mass spectrum with normal and inverted ordering (hierarchy) are obtained. It is shown, in particular, that if the matrix R is real and CP-conserving and the lightest neutrino mass m_3 in the case of inverted hierarchical spectrum lies the interval 5 \times 10^{-4} eV < m_3 < 7 \times 10^{-3} eV, the predicted baryon asymmetry can be larger by a factor of \sim 100 than the asymmetry corresponding to negligible m_3 \cong 0. As consequence, we can have successful thermal leptogenesis for 5 \times 10^{-6} eV < m_3 < 5 \times 10^{-2} eV even if R is real and the only source of CP-violation in leptogenesis is the Majorana and/or Dirac phase(s) in U.