[Paper Review] Resonant leptogenesis and TM$_1$ mixing in minimal Type-I seesaw model with S$_4$ symmetry

Bikash Thapa, Ng. K. Francis|arXiv (Cornell University)|Jul 5, 2021

Neutrino Physics Research69 references12 citations

TL;DR

This paper proposes an S4 flavor-symmetric minimal Type-I seesaw model that realizes TM1 neutrino mixing and resonant leptogenesis at the TeV scale. By introducing a flavon field ψ to satisfy orthogonality conditions, the model achieves normal neutrino mass hierarchy and predicts a baryon asymmetry of |ηB| = 6.3 × 10⁻¹⁰ via flavor-corrected Boltzmann equations, while the effective Majorana mass for neutrinoless double beta decay is predicted to be between 2.6–3.6 meV.

ABSTRACT

We present an S$_4$ flavour symmetric model within a minimal seesaw framework resulting in mass matrices that leads to TM$_1$ mixing. Minimal seesaw is realized by adding two right-handed neutrinos to the Standard Model. The model predicts Normal Hierarchy (NH) for neutrino masses. Using the constrained six-dimensional parameter space, we have evaluated the effective Majorana neutrino mass, which is the parameter of interest in neutrinoless double beta decay experiments. The possibility of explaining baryogenesis via resonant leptogenesis is also examined within the model. A non-zero, resonantly enhanced CP asymmetry generated from the decay of right-handed neutrinos at the TeV scale is studied, considering flavour effects. The evolution of lepton asymmetry is discussed by solving the set of Boltzmann equations numerically and obtain the value of baryon asymmetry to be $\lvert \eta_B vert = 6.3 imes 10^{-10}$.

Motivation & Objective

• To construct a minimal seesaw model with S4 flavor symmetry that realizes TM1 mixing in the neutrino sector.
• To explain the observed baryon asymmetry of the universe (BAU) via resonant leptogenesis at the TeV scale.
• To predict the effective Majorana neutrino mass for neutrinoless double beta decay (0νββ) and assess its detectability.
• To ensure consistency with 3σ neutrino oscillation data, including non-zero θ13 and CP phase.

Proposed method

• Extends the Standard Model with two right-handed neutrinos and flavon fields (ϕl, φl, ϕν, φν, ψ, β, ρ) transforming under S4 × Z3 × Z2 symmetry.
• Imposes vacuum expectation values (vevs) on flavon fields to generate charged lepton and neutrino mass matrices, with ψ vev chosen to satisfy orthogonality conditions for TM1 mixing.
• Derives the light neutrino mass matrix via the seesaw formula mν = −mD m⁻¹R mᵀD, leading to TM1 mixing pattern.
• Solves the coupled Boltzmann equations numerically for RH neutrino and lepton number densities, including flavor effects and washout processes.
• Uses the ULYSSES package to compute baryon asymmetry ηB from CP-asymmetry εiα and decay rates.
• Evaluates the effective Majorana mass |⟨mee⟩| as the (1,1) element of mν, and maps its predicted range against experimental sensitivities.

Experimental results

Research questions

• RQ1 Can a minimal seesaw model with S4 symmetry realize the TM1 mixing pattern compatible with current neutrino oscillation data?
• RQ2 Does resonant leptogenesis at the TeV scale generate a baryon asymmetry consistent with the observed value |ηB| ≈ 6.3 × 10⁻¹⁰?
• RQ3 What is the predicted range of the effective Majorana neutrino mass |⟨mee⟩| for neutrinoless double beta decay in this model?
• RQ4 How do flavor effects and non-degenerate right-handed neutrino masses (via higher-dimensional terms) influence CP asymmetry and baryogenesis?

Key findings

• The model predicts a normal neutrino mass hierarchy with total neutrino mass sum in the range 0.0576 eV < Σmi < 0.0599 eV.
• The effective Majorana neutrino mass |⟨mee⟩| is predicted to lie between 2.6 meV and 3.6 meV, below the sensitivity reach of current and near-future 0νββ experiments.
• Resonant leptogenesis with a splitting parameter d ≈ 10⁻⁸ generates a non-zero, resonantly enhanced CP asymmetry from out-of-equilibrium decay of TeV-scale right-handed neutrinos.
• Numerical solution of the Boltzmann equations yields a baryon asymmetry of |ηB| = 6.3 × 10⁻¹⁰, consistent with the observed value.
• The model successfully accommodates 3σ ranges of neutrino oscillation parameters, including θ13 ≠ 0 and δCP ≈ 1.09π.
• The orthogonality condition ⟨ψ⟩·⟨φl⟩ = 0 and ⟨ψ⟩·⟨φν⟩ = 0 is crucial for realizing the TM1 mixing pattern via a 23-rotation of the TBM matrix.