Flavored leptogenesis with quasi degenerate neutrinos in a broken cyclic symmetric model

TL;DR

This paper explores how breaking cyclic symmetry in a neutrino mass model enables resonant leptogenesis, leading to nonzero neutrino mass differences and baryon asymmetry, with detailed flavor-dependent CP asymmetry calculations.

Contribution

It introduces a small symmetry-breaking parameter in a cyclic symmetric neutrino model to generate realistic neutrino masses and leptogenesis, including generalized flavor-dependent CP asymmetry expressions.

Findings

Resonant leptogenesis is possible with small symmetry breaking.

Flavor-dependent Boltzmann equations are solved to estimate baryon asymmetry.

Bound on right-handed neutrino mass from baryon asymmetry constraints.

Abstract

Cyclic symmetry in the neutrino sector with the type-I seesaw mechanism in the mass basis of charged leptons and right chiral neutrinos ($N_{iR}$, $i=e,\mu,\tau$) generates two fold degenerate light neutrino and three fold degenerate heavy neutrino mass spectrum. Consequently, such scheme, produces vanishing one light neutrino mass squared difference and lepton asymmetry. To circumvent such unphysical outcome, we break cyclic symmetry in the diagonal right chiral neutrino mass term by a small breaking parameter. Nonzero mass squared differences and mixing angles are generated with the help of the small breaking parameter. Smallness of the breaking parameter opens up a possibility of resonant leptogenesis. Assuming complex Yukawa couplings, we derive generalized expressions flavor dependent CP asymmetry parameters ($\varepsilon^\alpha_i$) which are valid for quasi degenerate as well as hierarchical mass spectrum of right handed neutrinos. There after we set up the chain of coupled Boltzmann equations (which are flavor dependent too) which have to be solved in order to get the final lepton asymmetries. Depending upon the temperature regime the CP asymmetries and the Boltzmann equations may also be flavor independent. As our goal is to study the enhancement of CP asymmetry due to quasi degeneracy of right handed neutrinos, we select only the lowest allowed (by neutrino oscillation data) value of breaking parameter (and other corresponding Lagrangian parameters) and estimate the baryon asymmetry parameter $Y_B$. Experimental constraint of $Y_B$ introduces a bound on right handed neutrino mass which remained unrestricted by neutrino oscillation data.