Inverse Seesaw Mechanism

Updated 8 February 2026

Inverse Seesaw Mechanism is a framework that generates sub-eV Majorana neutrino masses via a doubly suppressed neutrino mass matrix, leveraging TeV-scale physics and softly broken lepton symmetry.
It allows large neutrino Yukawa couplings and notable active–sterile mixing, leading to rich collider, flavor, and dark matter phenomenology that can be tested in current experiments.
Embedding in both non-supersymmetric and supersymmetric models, such as left–right symmetric theories and the NMSSM, provides natural ultraviolet completions with testable predictions like lepton flavor violation and neutrinoless double beta decay.

The inverse seesaw mechanism is a framework for generating small Majorana neutrino masses at the sub-eV scale while keeping all new physics at or near the TeV scale. It achieves the necessary suppression via a doubly suppressed structure in the neutrino mass matrix, exploiting an approximate lepton-number symmetry that is softly broken by a small parameter. This architecture is technically natural in the sense of ’t Hooft, allows for large neutrino Yukawa couplings and sizable active–sterile mixing, and admits a rich phenomenology accessible to collider, flavor, and dark matter experiments. Numerous realizations exist in both non-supersymmetric and supersymmetric settings, and the framework has been embedded into broader contexts such as left–right symmetry, $U(1)_{B-L}$ gauge extensions, the Next-to-Minimal Supersymmetric Standard Model (NMSSM), 3-3-1 models, and radiative dark sector models.

1. Structural Principles of the Inverse Seesaw

The canonical inverse seesaw extends the Standard Model (SM) by adding, per generation, right-handed neutrinos $N_R$ and new SM-singlet fermions $S$ . The most general renormalizable Lagrangian, in the basis $\Psi^T = (\nu_L,\,N_R^c,\,S)$ , contains the terms

$\mathcal{L}_{\nu} \supset Y_\nu\,\overline{L}\,\tilde H\,N_R + M\,\overline{N_R^c}\,S + \frac{1}{2}\mu_S S S + \mathrm{H.c.}$

yielding, after electroweak symmetry breaking, a Majorana mass matrix of the form: $\mathcal{M}_\nu = \begin{pmatrix} 0 & m_D^T & 0 \ m_D & 0 & M^T \ 0 & M & \mu_S \end{pmatrix}$ where $m_D = Y_\nu v/\sqrt{2}$ , $M$ is a large Dirac mass (typically $\gtrsim 100$ GeV–TeV), and $\mu_S$ is a small Majorana term, softly breaking lepton number by $\Delta L=2$ .

Block-diagonalization in the regime $\mu_S \ll m_D \ll M$ yields, for the light-neutrino sector,

$m_\nu \simeq m_D^T\, (M^T)^{-1} \mu_S\, M^{-1}\, m_D$

This “doubly suppressed” structure enables $m_\nu \lesssim 0.1$ eV for $m_D \sim 10-100$ GeV, $M \sim 500-1000$ GeV, and $\mu_S \sim 1$ keV, all with $\mathcal{O}(1)$ Yukawa couplings (Carvajal et al., 2015, Dias et al., 2012, Das et al., 2017, Khalil, 2010). In the limit $\mu_S \to 0$ , total lepton number is restored, naturally stabilizing the smallness of $\mu_S$ (Das et al., 2017, Mandal et al., 2020).

2. Origin and Naturalness of the Small Parameter

The technical naturalness of the inverse seesaw stems from the fact that $\mu_S$ is the only lepton-number–violating parameter in the theory, protected by an approximate global or gauge symmetry. Various ultraviolet completions generate a suppressed $\mu_S$ :

Planck-suppressed operators: $\mu_S \sim \langle \phi \rangle^n / M_{\mathrm{Pl}}^{n-1}$ , with $\langle \phi \rangle$ at an intermediate scale and $n \geq 3$ (Carvajal et al., 2015, Khalil, 2010).
Spontaneous breaking: via vacuum expectation values of SM-singlet scalars (Mandal et al., 2020, Carvajal et al., 2015).
Radiative generation: as in dark-sector and “scotogenic” constructions, where $\mu_S$ arises at one or two loops (Ahriche et al., 2016, 0904.4450).
Seesaw among singlets: $\mu_S$ generated by a mini-seesaw structure, further suppressing its scale (Aoki et al., 2015).

In dynamical models promoting the lepton-number violation to the vacuum expectation value of a singlet (Majoron), $\mu_S$ is replaced by $Y_S \langle \sigma \rangle$ with $\langle \sigma \rangle \ll$ electroweak scale, offering additional naturalness and a phenomenologically safe Majoron (Mandal et al., 2020, Carvajal et al., 2015).

3. Mass Eigenstates, Mixing, and Phenomenological Scales

The inverse seesaw generically produces:

Three light mostly-active neutrinos with sub-eV masses set by the formula above.
Six (for three generations) heavy states forming three quasi-Dirac (“pseudo-Dirac”) pairs with masses $M \pm \mu_S/2$ . The splitting is $\sim \mu_S \ll M$ (Khalil, 2010, Awasthi et al., 2013, Aoki et al., 2015).
Active–sterile mixing angles of order $\theta \sim m_D / M \sim 10^{-2}$ – $10^{-1}$ , sufficiently large to induce potentially observable collider and flavor signals (Das et al., 2017, Khalil, 2010, Pires et al., 2018).
Heavy neutrino decay modes that, for $\theta$ sizable, are dominated by two-body decays to charged leptons and $W$ bosons ( $N \to \ell W$ ). This contrasts with the three-body decays of the type-I seesaw where $\theta$ is minuscule (Arun et al., 2021).

Scale choices giving neutrino masses in accord with oscillation data are: | Parameter | Scale | Role | |-----------|----------------|---------------| | $m_D$ | 10–100 GeV | Dirac mass | | $M$ | 0.1–10 TeV | Pseudo-Dirac heavy | | $\mu_S$ | keV – MeV | L-number violation |

(Carvajal et al., 2015, Das et al., 2017, Mandal et al., 2020)

4. Model Embeddings and Variants

Non-Supersymmetric and Gauge Extensions

$U(1)_{B-L}$ and left-right symmetric models: Inverse seesaw implemented with TeV-scale right-handed neutrinos and additional singlets, rendering $\mu_S$ technically natural via matter parity or higher-dimensional operators (Khalil, 2010, Delepine et al., 8 Jan 2026, Arun et al., 2021).
3-3-1 and 3-3-1 with RHN: The required fermion content arises naturally; $\mu_S$ is generated via a singlet scalar and discrete symmetries (Dias et al., 2012, Pires et al., 2018).
Inverse type-II seesaw: TeV-scale scalar triplets, no new fermions; $\mu$ parameter controls the small $\langle \Delta^0 \rangle$ , yielding distinctive doubly-charged scalar signatures (Freitas et al., 2014, Pires et al., 2018).

Supersymmetric Embeddings

NMSSM, MSSM, and compact SUSY: Embedding the inverse seesaw allows large $Y_\nu$ Yukawas, which enhance the lightest Higgs mass via radiative corrections (by 2-3 GeV), enabling lighter sparticle spectra compatible with current LHC bounds. This framework supports sneutrino dark matter with isosinglet–isodoublet mixing (Gogoladze et al., 2014, Cao et al., 2017, Romeri et al., 2018).
Inflationary models: Models exist where inflation is connected with $B-L$ breaking and the generation of $\mu_S$ via SUSY breaking and Planck-suppressed operators (Moursy, 2021).

Universal and Radiative Constructions

Universal inverse seesaw: Applied to explain the entire SM fermion mass hierarchy and charged-lepton masses by extending the inverse seesaw to the charged sector (Hernández et al., 2021).
Radiative inverse seesaw: $\mu_S$ generated by multi-loop diagrams involving new dark sector fields, leading to low-scale dark matter and testable signals at colliders and in precision flavor experiments (Ahriche et al., 2016, 0904.4450).

5. Phenomenological Implications and Signatures

Neutrinoless double beta decay: The pseudo-Dirac nature of heavy neutrinos suppresses the rate; however, for heavy neutrino masses near the scale of the nuclear virtuality momentum, contributions can be resonantly enhanced, possibly dominating over the standard light-neutrino contribution (Awasthi et al., 2013).
Lepton flavor violation (LFV): Enhanced rates of $\mu \rightarrow e \gamma$ , $\tau \rightarrow \mu \gamma$ are predicted due to sizable active–sterile mixing, e.g., $\mathrm{Br}(\mu \to e\gamma)\sim10^{-13}$ – $10^{-12}$ , within reach of upcoming experiments (Awasthi et al., 2013, Dias et al., 2012).
LHC signals: Heavy pseudo-Dirac neutrinos can be produced via $W'$ or $Z'$ bosons, with dominant decay $N \to \ell W$ . Signatures include multi-lepton+jet states, opposite-sign dileptons with a boosted $W$ -fatjet, and distinctive four-lepton signals from doubly-charged scalar pair production in type-II variants (Arun et al., 2021, Freitas et al., 2014, Pires et al., 2018).
Electroweak vacuum stability: Large $Y_\nu$ couplings can destabilize the SM Higgs potential at scales $\sim10^7$ – $10^9$ GeV. However, in “dynamical” inverse seesaw with a Majoron, additional scalar couplings can ensure vacuum stability to the Planck scale (Mandal et al., 2020).
Dark matter: Extensions naturally predict new dark matter candidates (e.g., singlet fermions or scalars stabilized by discrete symmetries), with cross sections near current direct-detection limits (Ahriche et al., 2016, Thongyoi et al., 18 Feb 2025). Sneutrino dark matter is viable in the supersymmetric versions (Cao et al., 2017, Romeri et al., 2018).

6. Leptogenesis and Cosmology

In standard inverse seesaw, $\mathcal{O}(1)$ Yukawa couplings lead to excessive washout of produced lepton asymmetry in thermal leptogenesis scenarios. Non-thermal mechanisms, such as right-handed neutrino production via decay of an extra $B-L$ Higgs, allow successful baryogenesis provided that the scalar spectrum is appropriately tuned to keep the reheating temperature low and washout under control. The resonance condition $\Delta M \sim \Gamma/2$ for pseudo-Dirac heavy neutrino pairs can enhance the CP asymmetry, enabling resonant leptogenesis at the TeV scale (Aoki et al., 2015, Delepine et al., 8 Jan 2026).

7. Table: Core Structure of the Inverse Seesaw

Matrix Block	Coupling	Role
$(\nu_L,N_R)$	$m_D$	Dirac mass (EW)
$(N_R,S)$	$M$	Large Dirac (TeV)
$(S,S)$	$\mu_S$	Small Majorana (keV–MeV)

Light neutrino masses: $m_\nu \propto (m_D/M)^2\mu_S$ .
Heavy sector: three quasi-Dirac pairs at $M$ , split by $\mu_S$ .

References

(Carvajal et al., 2015) Axion Like Particles and the Inverse Seesaw Mechanism
(Dias et al., 2012) A Simple Realization of the Inverse Seesaw Mechanism
(Das et al., 2017) Probing sterile neutrinos in the framework of inverse seesaw mechanism through leptoquark productions
(Khalil, 2010) TeV Scale Gauged B-L With Inverse Seesaw Mechanism
(Aoki et al., 2015) A model realizing inverse seesaw and resonant leptogenesis
(Freitas et al., 2014) Inverse type II seesaw mechanism and its signature at the LHC and ILC
(Delepine et al., 8 Jan 2026) Non-Thermal Leptogenesis in the BLSM with Inverse Seesaw Mechanism
(Cao et al., 2017) Sneutrino DM in the NMSSM with inverse seesaw mechanism
(Pires et al., 2018) Implementing the inverse type-II seesaw mechanism into the 3-3-1 model
(Awasthi et al., 2013) Neutrinoless double beta decay and pseudo-Dirac neutrino mass predictions through inverse seesaw mechanism
(Thongyoi et al., 18 Feb 2025) Inverse Seesaw Mechanism and Axion Portal Fermionic Dark Matter
(Ahriche et al., 2016) Dark Radiative Inverse Seesaw Mechanism
(Arun et al., 2021) Testing left-right symmetry with an inverse seesaw mechanism at the LHC
(Moursy, 2021) No-scale gauge non-singlet inflation inducing TeV scale inverse seesaw mechanism
(Gogoladze et al., 2014) Effects of Neutrino Inverse Seesaw Mechanism on the Sparticle Spectrum in CMSSM and NUHM2
(Mandal et al., 2020) Electroweak symmetry breaking in the inverse seesaw mechanism
(Romeri et al., 2018) Inverse seesaw mechanism with compact supersymmetry: enhanced naturalness and light super-partners
(Hernández et al., 2021) Universal Inverse seesaw mechanism as a source of the SM fermion mass hierarchy
(0904.4450) Radiative Inverse Seesaw: Verifiable New Mechanism of Neutrino Mass

The inverse seesaw mechanism provides a technically natural, experimentally testable explanation for small neutrino masses, with rich connections to collider phenomenology, lepton flavor violation, dark matter, baryogenesis, and vacuum stability in the context of both non-supersymmetric and supersymmetric frameworks.