Effects of Lightest Neutrino Mass in Leptogenesis

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.