Flavoured leptogenesis and CP\u03bc\u03c4 symmetry

Rome Samanta,Manibrata Sen

doi:10.1007/jhep01(2020)193

Abstract

We present a systematic study of leptogenesis in neutrino mass models with μτ-flavoured CP symmetry. In addition to the strong hierarchical N1-dominated scenario (N1DS) in the ‘two flavour regime’ of leptogenesis, we show that one may choose the right-handed (RH) neutrino mass hierarchy as mild as M2 ≃ 4.7M1 for a perfectly valid hierarchical N1DS. This reduces the lower bound on the allowed values of M1, compared to what is stated in the literature. The consideration of flavour effects due to the heavy neutrinos also translate into an upper bound on M1. It is only below this bound that the observed baryon-to-photon ratio can be realized for a standard N1 domination, else a substantial part of the parameter space is also compatible with N2DS. We deduce conditions under which the baryon asymmetry produced by the second RH neutrino plays an important role. Finally, we discuss another scenario where lepton asymmetry generated by N2 in the two flavour regime faces washout by N1 in the three flavour regime. Considering a hierarchical light neutrino mass spectrum, which is now favoured by cosmological observations, we show that at the end of N1-leptogenesis, the asymmetry generated by N2 survives only in the electron flavour and about 33% of the parameter space is consistent with a pure N2-leptogenesis.

Highlights

The CPμτ symmetry, which is a CP transformation [43,44,45] on the left-handed (LH) neutrino fields with μτ interchange symmetry as the CP generator in the low energy effective neutrino Lagrangian, predicts a co-bimaximal mixing [46]: θ23 = π/4 and δ = π/2, 3π/2, along with arbitrary non-zero values of θ13
Considering a hierarchical light neutrino mass spectrum, which is favoured by cosmological observations, we show that at the end of N1-leptogenesis, the asymmetry generated by N2 survives only in the electron flavour and about 33% of the parameter space is consistent with a pure N2-leptogenesis
In the N1-dominated scenario (N1DS), leptogenesis has been studied with a strong hierarchical scenario [22, 35, 69], e.g., M2/M1 = 103; i.e., assuming other heavy neutrinos are not produced at all, or if produced, the lepton asymmetry due to N2 faces a significant washout by the N1-interactions and is negligible, whereas that produced by N1 does not encounter a N2-washout

Summary

CPμτ symmetry and its variants in seesaw model

We discuss some aspects of the CPμτ symmetry in neutrino mass models. The μτ symmetry (G) and the μτ antisymmetry (G) have completely different predictions when they are used as an ordinary field transformation, i.e., νLl → GlmνLm or νLl → GlmνLm [85] In their CP-transformed versions, along with the diagonalization condition U T MνU = Md, where Md = diag(m1, m2, m3), both the symmetries (eq (2.4) and eq (2.6)) lead to the same predictions [22, 35]. Using eq (2.3), the effective light neutrino mass matrix Mν can be written as. In (2.12), new real parameters x1,2 and y1,2 are defined by scaling a1,2 and b1,2 with the square roots of the respective RH neutrino masses M1,2, i.e. A few comments on the matrix MνCP μτA are in order. We use this redefined phase θ for both MνCP μτA as well as mD

Validity of N1DS in one flavour thermal leptogenesis

Flavour effects and importance of N2-leptogenesis

Leptogenesis in the CPμτ symmetric model

Two flavour regime

Findings

Summary