Soft breaking of $L_μ-L_τ$ symmetry: Light neutrino spectrum and Leptogenesis

Abstract

Continuous $U(1)_{L_μ-L_τ}$ symmetry can generate quasi degenerate mass spectrum for both left handed light and right handed heavy Majorana neutrinos assuming that the symmetry preserving non zero parameters are nearly same. There is an accidental $μτ$ exchange symmetry in the light and heavy neutrino Majorana mass terms. This implies $θ_{13}=0$ and $θ_{23}=\fracπ{4}$. In addition it generates another zero mixing angle and one zero mass difference. We restrict ourselves to type-I See-Saw mechanism for generation of light neutrino mass. We have found that under $U(1)_{L_μ-L_τ}$ symmetry cosmological lepton asymmetry vanishes. We break $U(1)_{L_μ-L_τ}$ such a way that the $μτ$ exchange symmetry preserves in the neutrino sector. We have seen that light neutrino phenomenology can be explained under soft breaking of this symmetry. We have observed that softness of this symmetry breaking depends on the degeneracy of the light neutrino mass spectrum. Quasi-degeneracy of right handed neutrino mass spectrum opens an option for resonant leptogenesis. The degeneracy of the right handed neutrino mass spectrum is restricted through light neutrino data. We observed that for generation of right sized baryon asymmetry common neutrino mass scale $m_0$ have to be of the order of $\sqrt{Δm^2_{\rm atm}}$ and corresponding right handed neutrino mass scale have to be nearly $10^{13}$ GeV. We also have discussed the effect of RG evolution on light neutrino spectrum and also on baryon asymmetry.