Low-Scale Inverse Seesaw Mechanism

• Low-scale inverse seesaw is a neutrino mass generation framework that introduces heavy neutrino states and an additional sterile fermion alongside a small lepton-number-violating parameter.
• It utilizes a dynamically or radiatively suppressed μ term and large neutrino Yukawa couplings to achieve sub-eV masses at experimentally accessible TeV–GeV scales.
• The mechanism leads to distinctive phenomenological signatures such as pseudo-Dirac heavy neutrinos and lepton flavor violation, offering clear targets for collider and dark matter investigations.

A low-scale inverse seesaw mechanism is an extension of the Standard Model (SM) neutrino mass generation paradigm that achieves sub-eV active neutrino masses with new heavy states at experimentally accessible (TeV–GeV) scales. In contrast to the canonical high-scale seesaw, where light neutrino masses are suppressed by ultra-heavy right-handed Majorana masses, the inverse seesaw lifts the scale of new physics to the TeV regime by introducing additional sterile fermions and a small lepton-number-violating (LNV) parameter, typically denoted μ, whose natural smallness arises dynamically or radiatively. This framework allows large neutrino Yukawa couplings, enhances testability at colliders, and is compatible with various grand unified, supersymmetric, radiative, and flavor-extended models.

1. Essential Structure of the Low-Scale Inverse Seesaw

The canonical low-scale inverse seesaw introduces, for each generation, three types of neutral fermions: the SM neutrino (ν_L), a right-handed neutrino (ν^c, or N_R), and an additional gauge-singlet sterile state (S). The mass Lagrangian in the flavor basis is

$$\mathcal{L}_{\rm mass} = \tfrac{1}{2} (\nu_L,\, \nu^c,\, S) \begin{pmatrix} 0 & m_D & 0 \ m_D^T & 0 & M \ 0 & M^T & \mu \end{pmatrix} \begin{pmatrix} \nu_L \ \nu^c \ S \end{pmatrix} + \textrm{h.c.}$$

where:

• $m_D$ is the Dirac mass ($\sim$ 10–100 GeV),
• $M$ is a large Dirac mass ($\sim$ 0.5–2 TeV),
• $\mu$ is a symmetric Majorana mass block for S, typically $\textrm{keV–MeV}$.

Block-diagonalization for $\mu \ll m_D \ll M$ yields light active neutrino masses: $m_\nu \approx m_D M^{-1} \,\mu\, (M^{-1})^T m_D^T$ This structure enables eV-scale $m_\nu$ at low $M$ if $\mu$ is sufficiently suppressed, in contrast to type-I seesaw, which would require $M \gg 10^{14}$ GeV for similar $m_D$ (Hernández et al., 2019, Bazzocchi, 2010, Hernández et al., 2018).

2. Dynamical and Radiative Origins for the Small LNV Parameter

Unlike simply setting $\mu$ by hand, multiple models realize the "naturalness" of a small μ dynamically:

• Radiative Generation: In several constructions, $\mu$ arises exclusively at the one- or two-loop level, with lepton number conservation maintained at tree level by extended gauge or discrete symmetries. For instance, in the model of (Bazzocchi, 2010), $\mu$ is generated dynamically at two loops via scalar mixing and trilinear couplings involving singlet scalars. The result is

$$\mu\sim y_S^2 y_{\nu^c} \frac{A^3 v^2_{\tilde \Delta}}{(16\pi^2)^2 M^4}$$

with $A$ representing trilinear scalar couplings and $M$ a TeV-scale mass, leading to $\mu$ naturally of order keV.

• Discrete and Gauge Symmetries: Implementing discrete symmetries (e.g., $\mathbb{Z}_4$, $\mathbb{Z}_5\times\mathbb{Z}_2$, or cyclic symmetries with flavor alignments) can forbid or suppress $\mu$ at tree level, diverting its appearance to higher order (loop or nonrenormalizable) operators (Ahriche et al., 2016, Abada et al., 12 Dec 2025, Dias et al., 2011).
• Planck/High-Scale Induced Tadpoles: In supergravity or inflation frameworks, the smallness of μ can result from Planck-suppressed operators or soft supersymmetry-breaking tadpoles (Moursy, 2021, Dias et al., 2011). For instance, after SUSY breaking and inflation, scalar singlet VEVs proportional to $m_{3/2}/\kappa_1$ feed into $$\mu_S \sim (m_{3/2}^2/\kappa_1^2 M_P)(\lambda_2 - \kappa_2)$$ with $m_{3/2}$ TeV-scale, giving $\mu_S$ in the sub-keV to MeV range.

3. Realizations in Extended Gauge and Flavor Models

Low-scale inverse seesaw mechanisms integrate into a wide range of BSM constructions:

Framework/Model Class	Typical Features	References
Extended Gauge Models (e.g., 3-3-1 with Δ(27), SU(3)ₗ×U(1)ₓ, U(1)_B-L)	TeV-scale extra gauge bosons (Z', W'), new heavy neutral leptons, symmetry-enforced μ	(Hernández et al., 2018, Palcu, 2014, Bazzocchi, 2010)
Flavor Symmetry Models (A₄, T', cyclic symmetries)	Texture zeros, natural Yukawa hierarchy, suppressed μ from high-dimension flavon terms	(Hernández et al., 2019, Hernández et al., 2019)
Radiative/Irradiative ISS (one/two-loop mechanisms)	μ generated at 1- or 2-loop, dark sectors, DM candidates	(Guo et al., 2012, Ahriche et al., 2016, Abada et al., 12 Dec 2025)
SUSY/Inflation/SO(10) Embeddings	μ induced by SUSY breaking, Planck-scale physics, nonrenormalizable R-symmetry breaking terms	(Moursy, 2021, Awasthi et al., 2011, Altin et al., 2017)

In all cases, a systematic symmetry structure ensures the technical naturalness of a small μ: lepton number is restored in the $\mu\to0$ limit (`'t Hooft naturalness'), and the scale separation between $M$ and μ is justified without extreme fine-tuning.

4. Experimental and Phenomenological Implications

Low-scale inverse seesaw models are distinctive for their collider, flavor, and cosmological phenomenology:

• Heavy Neutrino Signatures: The (N, S) states form pseudo-Dirac pairs at the TeV scale, with masses $M_\pm \approx M \pm \frac12\mu$. Owing to $m_D/M$ mixing, pseudo-Dirac heavy neutrino production at hadron colliders (LHC, future FCC-ee/hh) via $pp \to Z'/W' \to NN$ is viable for $M \sim 0.5$–$5$ TeV if mixing angles $U_{\alpha N} \sim 10^{-2}$–$10^{-1}$ are not excessively suppressed (Hernández et al., 2018, Palcu, 2014, Bazzocchi, 2010, Hernández et al., 2019).
• Lepton-Number Violation and LFV: μ controls lepton-number–violating (LNV) processes such as $0\nu\beta\beta$, and cLFV transitions ($\mu\to e\gamma$, $\tau\to e\gamma$). Typically, branching ratios scale as $(m_D/M)^2$ or $(\mu/M)$, and can be within reach of forthcoming experiments for not-too-small μ and moderate mixing (Hernández et al., 2019, Abada et al., 12 Dec 2025, Ahriche et al., 2016).
• Dark Matter and Baryogenesis: In several models, either a scalar or a fermionic dark matter candidate is stabilized by discrete or gauge symmetries. The μ-generating sector sometimes contains DM candidates, as in the "minimal dynamical ISS" with a stable MeV-scale sterile neutrino. Extended gauge sectors also enable resonant leptogenesis or baryogenesis scenarios, with μ serving as a crucial CP-violating and out-of-equilibrium ingredient (Bazzocchi, 2010, Ahriche et al., 2016, Gu, 2019).
• Electroweak and Higgs Sector Impacts: Large neutrino Yukawa couplings, allowed at low $M$, can affect the Higgs mass via radiative threshold corrections in supersymmetric and scale-invariant scenarios, lowering the necessary superpartner or scalar masses and ameliorating the naturalness problem (Romeri et al., 2018, Ahmed et al., 17 Apr 2025).

5. Benchmark Scales and Parameter Relations

Across models, the relevant scales enabling sub-eV neutrino masses with TeV-scale $M$ are:

• $m_D \sim 10$–$100$ GeV (Dirac neutrino mass)
• $M \sim 0.5$–$10$ TeV (sterile neutrino Dirac block)
• $\mu \sim 0.01$–$1$ keV (from loops, suppressed VEVs, or higher-dimensional operators).

Quantitatively (in one-family notation): $m_\nu \simeq \mu \left(\frac{m_D}{M}\right)^2$ With $m_D = 30$ GeV, $M = 1$ TeV, and $\mu = 1$ keV, $m_\nu \simeq 0.9 \times 10^{-1}$ eV (Hernández et al., 2019).

6. Distinctive Model Features and Variants

• Texture Zeros and Flavor Predictivity: Family symmetry constructions can predict one- or two-zero textures in $m_\nu$, correlating mass orderings, sum rules, and CP-violating phases with measurable quantities. Examples include U(1)$_{L_\mu-L_\tau}$ enforcing $(m_\nu)_{\tau\tau}=0$ or $(m_\nu)_{\mu\mu}=0$, selectively favoring inverted ordering and connecting neutrino and muon $g-2$ anomalies (Araki et al., 2019).
• Radiative and Linear Seesaw Interplay: In models with combined linear and inverse seesaw (e.g., (Abada et al., 12 Dec 2025), the atmospheric and solar mass-squared splittings can be attributed separately to the inverse and linear contributions, offering new perspectives on the observed neutrino mass hierarchy.
• Embedding in GUT and SUGRA Frameworks: Nonsupersymmetric SO(10) and no-scale supergravity models permit comprehensive RG analyses, detailed unification patterns, and predictions for proton lifetime, non-unitarity parameters like $\eta_{\mu\tau}$, or links to inflationary reheating consistent with neutrino mass constraints (Moursy, 2021, Awasthi et al., 2011).

7. Outlook and Experimental Probes

Low-scale inverse seesaw mechanisms offer several testable predictions accessible to current and near-future experiments:

• LHC and Future Collider Searches: Pseudo-Dirac heavy neutrinos at the TeV scale can be probed via Drell–Yan, multi-lepton signatures, and displaced vertices. Associated Z', W', and extra inert scalars are present in extended gauge realizations (Bazzocchi, 2010, Hernández et al., 2018).
• Lepton Flavor Violating Decays: $\mathrm{Br}(\mu\rightarrow e\gamma)$, $\mu\to eee$, and flavor-violating $\tau$ decays are within reach for realistic parameter choices (Hernández et al., 2019, Abada et al., 12 Dec 2025).
• Dark Matter Direct and Indirect Detection: Scalar or fermion DM candidates stabilized by $\mathbb{Z}_2$ or gauge symmetries, with correct relic density and accessible direct detection cross sections (Ahriche et al., 2016, Guo et al., 2012).
• Neutrinoless Double Beta Decay: Contributions dominated by light neutrino masses, directly linked to the μ parameter and mixing angles, with effective mass predictions potentially observable in forthcoming searches (Araki et al., 2019, Awasthi et al., 2011).

Low-scale inverse seesaw models thus establish a theoretically well-motivated, phenomenologically rich, and experimentally accessible alternative to high-scale Majorana seesaw scenarios. Their defining feature is the dynamical or radiative suppression of the LNV parameter $\mu$, enabling sub-eV neutrino masses at the TeV scale and opening a window onto physics beyond the Standard Model (Bazzocchi, 2010, Dias et al., 2011, Abada et al., 12 Dec 2025, Ahriche et al., 2016, Hernández et al., 2019, Hernández et al., 2019, Hernández et al., 2018, Palcu, 2014, Awasthi et al., 2011, Altin et al., 2017, Ahmed et al., 17 Apr 2025, Romeri et al., 2018).