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 m3m3 in the case of inverted hierarchical spectrum lies the interval 5×10−4 eV≲m3≲7×10−3 eV5×10−4 eV≲m3≲7×10−3 eV, the predicted baryon asymmetry can be larger by a factor of ∼100 than the asymmetry corresponding to negligible m3≅0m3≅0. As consequence, we can have successful thermal leptogenesis for 5×10−6 eV≲m3≲5×10−2 eV5×10−6 eV≲m3≲5×10−2 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.