Probing $0νββ$ and $μ\to eγ$ via Fully Determined Dirac Mass Terms in LRSM with Double Seesaw

TL;DR

This paper investigates neutrinoless double beta decay and charged lepton flavor violation within a TeV-scale Left-Right Symmetric Model augmented by a double seesaw mechanism. It analyzes two Dirac-mass textures—Case I with $M_D \propto \mathbb{1}$ and Case II with model-determined Dirac masses—to map regions where $0\nu\beta\beta$ and $\mu\to e\gamma$ are enhanced by heavy right-handed and sterile neutrinos. The authors derive analytic expressions for light/heavy neutrino masses and mixings, compute GEMM and loop-induced BR$(\mu\to e\gamma)$, and identify RHN hierarchies that maximize signals in the cLFV-relevant regime while remaining consistent with current collider bounds. They show that, in the cLFV-focused parameter space, non-standard $0\nu\beta\beta$ contributions can substantially boost the effective Majorana mass and that upcoming experiments like MEG-II and LEGEND will probe significant portions of the viable parameter space. The work provides optimistic, testable benchmarks linking low-energy neutrino data to heavy-sector phenomenology and collider prospects distinct to LRSM with a double seesaw.

Abstract

Neutrinoless double beta decay ($0νββ$) and charged lepton flavor violation (cLFV) experiments provide promising avenues to probe new physics contributions from extended neutrino sectors in beyond Standard Model (BSM) scenarios. We consider a Left-Right Symmetric Model (LRSM) extended with three generations of sterile neutrinos to realize a double type-I seesaw mechanism for light neutrino mass generation. The double seesaw induces maximal lepton number violation in the right-handed sector and facilitates enhanced Majorana masses for right-handed neutrinos, thereby leading to their dominant contributions in both cLFV and $0νββ$ processes. We perform a comprehensive exploration of the parameter space for new-physics contributions to the cLFV decay $μ\to e γ$ and to $0νββ$, considering two different textures for the Dirac mass matrices: (i) a symmetry-motivated limit with $M_D \propto \mathbb{1}$, and (ii) a texture fully determined by the model framework. A detailed analysis of the common parameter regions accessible to current experiments like KamLAND-Zen and LEGEND-200, and upcoming experiments, such as MEG-II and LEGEND-1000, is presented, underscoring the phenomenological relevance of this framework. Our results aim to provide optimistic benchmarks for future searches targeting right-handed current-mediated neutrino interactions.

Figures (13)

• Figure 1: Representative Feynman diagrams contributing to neutrinoless double beta decay: light neutrino exchange (left), heavy right-handed neutrino exchange (middle), and sterile neutrino exchange (right).
• Figure 2: One-loop diagrams contributing to the charged lepton flavor violating process $\mu \rightarrow e\gamma$ in the LRSM with double seesaw. The left diagram involves the exchange of heavy right-handed neutrinos $N_{R_i}$, while the right diagram involves sterile neutrinos $S_{L_i}$. The amplitudes are governed by the neutrino mixing matrices $V^{NN}$ and $V^{NS}$, which enter through the vertices and control the strength of lepton flavor violation in each diagram, leading to sizeable cLFV contributions.
• Figure 3: Variation of $\text{BR}(\mu \to e\gamma)$ with the lightest active neutrino mass for both normal ordering (left, $m_{N_1}$ fixed) and inverted ordering (right, $m_{N_3}$ fixed) scenarios. In the NO case, achieving branching ratios within the MEG II sensitivity range requires large $m_{N_1}$ and $\delta = 0$, while in the IO case, the dependence on $\delta$ is negligible, and large values of $m_{N_3}$ are already excluded by existing bounds.
• Figure 4: Region plots of BR$(\mu \to e\gamma)$ in the plane of heaviest RHN mass and lightest active neutrino mass for Case I. The left and right panels correspond to the normal ordering (NO) and inverted ordering (IO) of the active neutrino mass spectrum, respectively. The plots highlight regions within the reach of MEG II (light green), currently excluded by MEG (faint orange), and ruled out by the PLANCK constraint on the sum of neutrino masses (translucent grey band). Various branching ratio ranges are indicated using the color bar on the right.
• Figure 5: Variation of the generalized effective Majorana mass $|m_{\beta\beta}^{\nu+N+S}|$ (left) and the corresponding $0\nu\beta\beta$ decay half-life $T_{1/2}^{\nu+N+S}$ (right) as functions of the lightest active neutrino mass, considering contributions from light active neutrinos, right-handed neutrinos, and sterile neutrinos for Dirac mass matrix in Case I. The cyan and magenta bands represent the NO and IO spectra, respectively, with their overlap indicating the quasi-degenerate region. The light-pink, light-blue, and grey horizontal bands denote current and projected experimental limits from KamLAND-Zen, LEGEND-200, and LEGEND-1000, respectively. Vertical orange and yellow bands indicate bounds from KATRIN and PLANCK on the absolute neutrino mass scale.