Flavoured leptogenesis and ${\rm CP}^{\mu\tau}$ symmetry

TL;DR

This paper systematically studies leptogenesis with $$-flavoured CP symmetry, revealing new bounds on right-handed neutrino masses and conditions for different leptogenesis scenarios, especially considering hierarchical neutrino masses.

Contribution

It introduces a detailed analysis of flavoured leptogenesis with $$ symmetry, including bounds on RH neutrino masses and the role of $N_2$ in baryon asymmetry generation.

Findings

Mild hierarchy $M_2 \u2248 4.7 M_1$ is viable for $N_1$-dominated leptogenesis.

Lower bounds on $M_1$ are reduced compared to previous literature.

Approximately 33% of parameter space supports $N_2$-leptogenesis with hierarchical neutrino masses.

Abstract

We present a systematic study of leptogenesis in neutrino mass models with $\mu\tau$-flavoured CP symmetry. In addition to the strong hierarchical $N_1$-dominated scenario ($N_1$DS) in the `two flavour regime' of leptogenesis, we show that one may choose the right-handed (RH) neutrino mass hierarchy as mild as $M_2\simeq 4.7 M_1$ for a perfectly valid hierarchical $N_1$DS. This in turn reduces the lower bound on the allowed values of $M_1$, 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 $M_1$. It is only below this bound that the observed baryon-to-photon ratio can be realized for a standard ${ N_1}$ domination, else a substantial part of the parameter space is also compatible with $N_2$DS. We deduce conditions under which the baryon asymmetry produced by the second RH neutrino plays an important role. Finally, we discuss another interesting scenario where lepton asymmetry generated by $N_2$ in the two flavour regime faces washout by $N_1$ 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 $N_1$-leptogenesis, the asymmetry generated by $N_2$ survives only in the electron flavour and around $33\%$ of the parameter space is consistent with a pure $N_2$-leptogenesis.