Linear & Inverse Seesaw Mechanisms

• Linear and inverse seesaw mechanisms are neutrino mass models that use TeV-scale new physics and extended neutral-fermion sectors to generate sub-eV masses.
• They rely on hierarchical 9×9 mass matrices with suppressed lepton number–violating parameters, using either small Majorana or Dirac portal terms.
• Experimental implications include pseudo-Dirac heavy neutrinos, lepton flavor violation, and potential dark matter candidates, testable in collider and low-energy experiments.

Linear and inverse seesaw mechanisms are extensions of the canonical seesaw paradigm developed to explain the sub-eV masses of active neutrinos via TeV-scale new physics, circumventing the need for ultra-high mass right-handed states or extremely suppressed Yukawa couplings. Both mechanisms are realized in models featuring enlarged neutral-fermion sectors—typically including sterile singlets and additional symmetries—and rely on either small lepton number–violating entries or Dirac portal insertions to generate phenomenologically viable masses and mixings.

1. Block Structure and Seesaw Formulas

The defining feature of the linear and inverse seesaw mechanisms is the presence of a $9\times9$ mass matrix for the neutral lepton fields, typically arranged as basis $(\nu_L,\,N_R,\,S_R)$—with $\nu_L$ the SM doublets, $N_R$ right-handed neutrinos, $S_R$ gauge-singlet fermions. The general mass matrix is

$$\mathcal M = \begin{pmatrix} 0 & M_D & \varepsilon \ M_D^T & M_R & M_S \ \varepsilon^T & M_S^T & \mu \end{pmatrix}$$

where $M_D$, $M_S$, and $\varepsilon$ are Dirac-type blocks and $M_R$, $\mu$ are Majorana terms.

• Inverse seesaw: Set $M_R=0$, $\varepsilon=0$, leaving Majorana mass only on the singlet block ($\mu$), typically $\|\mu\|\ll\|M_S\|$.
• Linear seesaw: Set $M_R=0$, $\mu=0$, and introduce a small Dirac portal term $\varepsilon$ linking active and sterile sectors.

Block-diagonalizing in the hierarchical regime ($M_S\gg M_D,\varepsilon,\mu$), the effective light-neutrino mass matrices are:

• Inverse seesaw:

$$m_\nu^{\text{inv}} \simeq M_D\,M_S^{-1}\,\mu\,(M_S^{-1})^T\,M_D^T$$

• Linear seesaw:

$$m_\nu^{\text{lin}} \simeq - M_D\,M_S^{-1}\,\varepsilon^T - \varepsilon\,(M_S^{-1})^T\,M_D^T$$

The relative size and symmetry of the $\mu$ and $\varepsilon$ matrices dictate the flavor structures and phenomenology (Han et al., 2021, Gu et al., 2010).

2. Lagrangian Realizations and Origin of Small Parameters

The construction of realistic seesaw models imposes stringent constraints on the field content and allowed couplings—these are often enforced by additional gauge and/or discrete symmetries.

• Inverse seesaw: The crucial lepton number–violating parameter $\mu$ is naturally taken to be small (keV–MeV), technically natural in the ’t Hooft sense as lepton number is restored as $\mu\to0$. In models where tree-level $\mu$ is forbidden (e.g., by a U(1)$_D$ or lepton number symmetry), $\mu$ can be generated radiatively, typically at two loops, yielding values compatible with sub-eV $m_\nu$ and alleviating fine-tuning issues (Guo et al., 2012, Abada et al., 12 Dec 2025).
• Linear seesaw: The small parameter $\varepsilon$ arises from Dirac-type couplings, typically suppressed by symmetry-protected selection rules or small vacuum expectation values (VEVs) of extra scalar fields. The induced VEV $v_L$ in left-right models controls $\varepsilon$ and can be hierarchically small compared to other scales (Gu et al., 2010).

A summary table illustrates the dependence of light $m_\nu$ on mediating parameters:

Seesaw type	Controlling parameter	Required value for $m_\nu\sim 0.05$ eV with $M_D\sim 100$ GeV, $M_S\sim 1$ TeV
Inverse	$\mu$	keV–MeV (by hand or radiatively)
Linear	$\varepsilon$	~5 keV

3. Symmetry Embeddings and Flavored Texture Structures

Many models introduce flavor symmetries (e.g., $A_4$, $S_3$) and extended scalar sectors to structure the Yukawa couplings and enforce the suppression of lepton-number or flavor-violating terms (Devi et al., 2021, Sinha et al., 2015):

• Texture zeros and flavor alignments imposed via discrete symmetries (e.g., $A_4 \times Z_4 \times Z_5$) lead to highly predictive textures for $m_\nu$ and can restrict the allowed number of independent zero entries.
• In minimal inverse seesaw models, up to seven viable two-zero $m_\nu$ textures are realizable, whereas linear seesaw constructions with maximal texture zeros typically yield only a single viable texture (Sinha et al., 2015).
• Flavon VEV alignments specify the resulting mass matrices, enabling detailed matching with observed PMNS parameters (mixing angles, mass-squared splittings, and CP phases).

4. Radiative and Dynamical Realizations

Several recent models achieve the linear or inverse seesaw structure radiatively via loop diagrams governed by extra fermions and scalars and supported by discrete or gauge symmetries (Guo et al., 2012, Abada et al., 12 Dec 2025):

• Two-loop models not only account for neutrino masses and mixing but can also correlate with dark matter stability and properties via an unbroken (residual) symmetry.
• Dynamical scotogenic models generate both $\varepsilon$ and $\mu$ at two loops, yielding small values without fine-tuned input, and naturally explain the atmospheric/solar mass-squared ratio hierarchy. In these scenarios, the atmospheric scale is controlled by the inverse seesaw (via $\mu$), and the solar scale arises from the linear seesaw (via $\varepsilon$) (Abada et al., 12 Dec 2025).

5. Phenomenology and Experimental Signatures

Both mechanisms yield pseudo-Dirac heavy neutral leptons (HNLs) at the TeV scale, suitable for phenomenological study at colliders and intensity-frontier experiments.

• Inverse seesaw: Small $\mu$ generates quasi-Dirac HNLs with tiny ($\sim$keV) Majorana splittings, leading to distinctive long-lived signatures, suppressed lepton-number-violating rates, and prospects for displaced-vertex events at colliders. Lepton-flavor-violating (LFV) decays (e.g., $\mu\to e\gamma$) probe the mixing angles and heavy mass scales (Agashe et al., 2018, Abada et al., 2014).
• Linear seesaw: Induced $v_L$ modifies charged-current interactions via $W_L$–$W_R$ mixing, impacting $0\nu\beta\beta$ decay and rare LFV decays in a model-dependent fashion. The absence of ultra-small Majorana masses enables higher predictivity for the low-energy spectrum (Gu et al., 2010).
• Neutrinoless double beta decay: Both mechanisms suppress $m_{ee}$ in $0\nu\beta\beta$ due to the quasi-Dirac character of heavy neutral fermions, though indirect contributions may be significant for particular benchmark points (Abada et al., 2014).
• Dark matter: In radiative inverse seesaw models, scalar or fermionic DM candidates arise naturally, with Higgs-portal couplings and relic densities compatible with LUX/LZ and XENON100 bounds. Collider production cross sections for vector-like leptons and triplet scalars are in the fb regime at LHC energies (Guo et al., 2012, Abada et al., 12 Dec 2025).

6. Parametric Ranges and Theoretical Constraints

The viable parameter range for the linear and inverse seesaw mechanisms depends on the detailed model realization but shares characteristic features:

• Mass scales: $M_S$ (Dirac mass between sterile and right-handed neutrinos) is in the $\sim$TeV range; $\mu$ (inverse seesaw) or $\varepsilon$ (linear seesaw) are typically in the (keV–MeV) range (Han et al., 2021, Gu et al., 2010).
• Mixings: The active-heavy mixing $|R_{i\alpha}|$ can reach up to $10^{-2}$–$10^{-1}$ without conflicting with electroweak precision tests or LFV constraints.
• Hierarchies: In dynamical models, two-loop suppression of $\mu$ and $\varepsilon$ yields the observed $\Delta m_{\rm atm}^2 / \Delta m_\odot^2 \sim 30$ without tuning (Abada et al., 12 Dec 2025).
• Fine-tuning: Conventional setups demand tree-level insertion of small parameters; radiative/dynamical mechanisms naturally suppress $\mu$ or $\varepsilon$ via loop factors, enhancing naturalness (Guo et al., 2012, Abada et al., 12 Dec 2025).

7. Model Variants and Outlook

• Minimal and extended realizations: The minimal ISS with $3$ generations each of $N_R$ and $S$ suffices for three light Majorana neutrinos and two heavy pseudo-Dirac pairs (Abada et al., 2014). Augmented models offer additional sterile eV-scale states or dark matter in the keV range (Rojas et al., 2017).
• Hybridization: Models embedding both high-scale (Type I) and TeV-scale ISS modules connect leptogenesis to observable low-scale signatures, possibly resolving shortcomings in conventional ISS via additional structure or symmetry breaking (Agashe et al., 2018).
• Texture-zero and flavor symmetry analyses: The constraints on CP phases and mass orderings derived in maximal zero-texture studies provide powerful discriminants; only certain two-zero textures are viable for each scenario (Sinha et al., 2015).
• Experimental prospects: Next-generation cLFV, $0\nu\beta\beta$, and collider searches will further constrain or illuminate the seesaw parameter space, especially in radiative/dynamical models with rich new-physics spectra (Abada et al., 12 Dec 2025, Guo et al., 2012).

In summary, linear and inverse seesaw mechanisms constitute a broad and technically natural framework for neutrino mass generation at testable energy scales, with diverse model-building, phenomenological, and cosmological implications that are actively explored in the literature (Gu et al., 2010, Han et al., 2021, Abada et al., 12 Dec 2025, Guo et al., 2012, Sinha et al., 2015, Abada et al., 2014).