Aspects of High Scale Leptogenesis with Low-Energy Leptonic CP Violation

TL;DR

This paper investigates the conditions and regimes of high-scale leptogenesis driven by low-energy CP violation, highlighting the importance of flavor effects and the limitations of Boltzmann equations in accurately predicting baryon asymmetry.

Contribution

It provides a detailed analysis of flavor regime transitions in leptogenesis, identifies when Boltzmann equations fail, and links low-energy CP phases to the baryon asymmetry sign.

Findings

Flavor effects persist beyond 10^{12} GeV.

Boltzmann equations often fail to describe correctly the asymmetry.

Successful leptogenesis depends on the CP phases and the neutrino mass hierarchy.

Abstract

Using the density matrix equations (DME) for high scale leptogenesis based on the type I seesaw mechanism, in which the CP violation (CPV) is provided by the low-energy Dirac or/and Majorana phases of the neutrino mixing (PMNS) matrix, we investigate the 1-to-2 and the 2-to-3 flavour regime transitions, where the 1, 2 and 3 leptogenesis flavour regimes in the generation of the baryon asymmetry of the Universe $\eta_B$ are described by the Boltzmann equations. Concentrating on the 1-to-2 flavour transition we determine the general conditions under which $\eta_B$ goes through zero and changes sign in the transition. Analysing in detail the behaviour of $\eta_B$ in the transition in the case of two heavy Majorana neutrinos $N_{1,2}$ with hierarchical masses, $M_1 \ll M_2$, we find, in particular, that i) the Boltzmann equations in many cases fail to describe correctly the generation of $\eta_B$ in the 1, 2 and 3 flavour regimes, ii) the 2-flavour regime can persist above (below) $\sim 10^{12}$ GeV ($\sim 10^9$ GeV), iii) the flavour effects in leptogenesis persist beyond the typically considered maximal for these effects leptogenesis scale of $10^{12}$ GeV. We further determine the minimal scale $M_{1\text{min}}$ at which we can have successful leptogenesis when the CPV is provided only by the Dirac or Majorana phases of the PMNS matrix as well as the ranges of scales and values of the phases for having successful leptogenesis. We show, in particular, that when the CPV is due to the Dirac phase $\delta$, there is a direct relation between the sign of $\sin \delta$ and the sign of $\eta_B$ in the regions of viable leptogenesis in the case of normal hierarchical light neutrino mass spectrum; for the inverted hierarchical spectrum the same result holds for $M_1 \lesssim 10^{13}$ GeV. The considered scenarios of leptogenesis are testable and falsifiable in low-energy neutrino experiments.