[Paper Review] Color Octet Leptogenesis

M. Losada, Sean Tulin|ArXiv.org|Sep 3, 2009

Particle physics theoretical and experimental studies3 citations

TL;DR

This paper proposes a mechanism for low-scale leptogenesis via decays of color octet fermions and scalars in a one-loop neutrino mass model, where the CP asymmetry is not constrained by neutrino masses and can be large. It shows that lepton flavor violation bounds from $μ\to e\gamma$ strongly constrain the CP asymmetry, requiring octet masses near the TeV scale for viability, while gauge interactions enhance thermal production and testability at the LHC.

ABSTRACT

We study the implications for generating the cosmological baryon asymmetry through leptogenesis in the recent model of Fileviez Perez and Wise, which provides a new mechanism for generating neutrino masses at one-loop by introducing new color octet fermion and scalar fields. We find that there are significant differences with respect to other models for leptogenesis: low scale leptogenesis can occur naturally and the CP asymmetry can be large as there is no upper bound arising from neutrino masses. The CP asymmetry is insensitive to the phases in the neutrino mixing matrix. We investigate in detail the minimal model that can simultaneously fit low scale neutrino physics, the $μ o e γ$ bound and leptogenesis. The model can provide outstanding collider signatures and the value of the CP-asymmetry can be more constrained from lepton flavour violating processes than from neutrino physics.

Motivation & Objective

• To explore leptogenesis in the Fileviez Perez and Wise model, which generates Majorana neutrino masses at one-loop via new color octet fermions and scalars.
• To determine whether low-scale leptogenesis is viable in this framework, given the absence of constraints from small neutrino masses.
• To assess the impact of lepton flavor violating processes, particularly $\mu\to e\gamma$, on the allowed parameter space for CP asymmetry.
• To evaluate the role of strong SU(3) gauge interactions in maintaining lepton asymmetry despite thermalization.
• To identify collider signatures and constraints from electroweak precision observables in the minimal model.

Proposed method

• The model introduces a new SU(3) color octet fermion $F$ and scalar $S$, transforming as $\mathbf{8}$ under $SU(3)_c$, with Yukawa couplings $y_{iab}$ to lepton doublets $L_i$ and $S$.
• Leptogenesis is driven by two-body decays $F_1 \to S \ell_i$, with the CP asymmetry $\epsilon_{F_1}$ computed from interference of tree-level and one-loop self-energy diagrams.
• The CP asymmetry is derived from the imaginary part of a product of Yukawa couplings: $\epsilon_{F_1} \propto \textrm{Im}\left[(y_1 y_2^*)^2\right]$, independent of neutrino mixing phases.
• The model uses a parameter scan to relate $\epsilon_{F_1}$ and the branching ratio $\textrm{Br}(\mu\to e\gamma)$, applying the experimental bound $\leq 1.2\times10^{-11}$.
• The efficiency factor $\eta$ is not computed numerically but argued to be $\mathcal{O}(1)$ due to strong gauge interactions that maintain thermal equilibrium without excessive washout.
• An extended model with $\mathcal{N}_F=2$, $\mathcal{N}_S=3$ is explored to suppress lepton flavor violation while preserving large CP asymmetry via non-diagonal $\lambda_{bc}$ in the neutrino mass matrix.

Experimental results

Research questions

• RQ1 Can large CP asymmetries for leptogenesis be achieved at the electroweak scale in the color octet model, without being constrained by small neutrino masses?
• RQ2 How do experimental bounds on $\mu\to e\gamma$ constrain the magnitude of the CP asymmetry $\epsilon_{F_1}$ in this model?
• RQ3 Can strong SU(3) gauge interactions prevent excessive washout of the lepton asymmetry despite thermalization?
• RQ4 Is it possible to achieve large CP asymmetry while suppressing lepton flavor violating processes through model extension?
• RQ5 What are the key collider signatures and precision constraints in the minimal color octet leptogenesis model?

Key findings

• The CP asymmetry $\epsilon_{F_1}$ in the color octet model is not bounded by neutrino masses, allowing for large values even at the electroweak scale.
• The CP asymmetry is insensitive to the CP-violating phases in the neutrino mixing matrix, decoupling it from low-energy neutrino observables.
• The $\mu\to e\gamma$ decay branching ratio provides a stronger constraint on $\epsilon_{F_1}$ than neutrino physics, requiring $\textrm{Br}(\mu\to e\gamma) \leq 1.2\times10^{-11}$.
• For viable parameter space, octet masses must be at least a few TeV to satisfy $\mu\to e\gamma$ bounds, unless the model is extended to suppress lepton flavor violation.
• The efficiency factor $\eta$ is expected to be $\mathcal{O}(1)$ due to strong SU(3) gauge interactions, which maintain thermal equilibrium without excessive washout.
• The model allows for distinctive LHC signatures, including same-sign dilepton final states from $F$ pair production and decay, enhancing testability.