Majorana Neutrino Masses: Mechanisms & Phenomenology

• Majorana neutrino masses are generated by ΔL=2 interactions, making neutrinos their own antiparticles with a complex symmetric mass matrix.
• Seesaw mechanisms, radiative loops, and higher-dimensional operators offer diverse methods to achieve small neutrino masses with distinct signatures.
• Experimental searches such as neutrinoless double beta decay and lepton flavor violation, alongside cosmological constraints, probe these mass generation models.

Majorana neutrino masses characterize the paradigm in which neutrino mass eigenstates are their own antiparticles, leading to lepton number violation by two units (ΔL=2). Unlike Dirac masses, which preserve lepton number, Majorana masses arise from effective dimension-five or higher operators built from Standard Model fields, and are realized in a vast range of ultraviolet completions—from high-scale tree-level seesaw models to TeV-scale radiative mechanisms and new topological effects in string theory and extra-dimensional frameworks. The generic Majorana mass matrix is complex symmetric, incorporates both Dirac and Majorana CP-violating phases, and manifests in distinctive signatures such as neutrinoless double beta decay and lepton-flavor violation.

1. General Framework for Majorana Neutrino Mass Generation

Majorana neutrino masses originate from lepton number–violating interactions, generically encapsulated by the effective dimension-five Weinberg operator: $$\mathcal{O}_5 = \frac{1}{\Lambda} (\overline{L^c} i\tau_2 H)(H^T i\tau_2 L),$$ where $L$ is the SU(2) lepton doublet, $H$ the Higgs doublet, and $\Lambda$ the scale of new physics. After electroweak symmetry breaking, this operator induces a Majorana mass term for the light neutrinos: $m_\nu = \frac{v^2}{\Lambda},$ with $v \approx 246$ GeV. The minimal realization at tree level leads to the classic seesaw models:

• Type I: exchange of right-handed singlet neutrinos $N_R$ with large Majorana masses.
• Type II: scalar SU(2) triplet $\Delta$ acquiring a small VEV.
• Type III: SU(2) triplet fermions.

Loop-level and higher-dimensional realizations extend this framework, involving radiative diagrams, extra scalar or fermionic multiplets, and higher-dimension operators ($d \geq 7$), often with small couplings or suppressed vacuum expectation values to accommodate $m_\nu \ll v$ (Sierra, 2011, Cepedello et al., 2019).

A general model-independent formula universally encapsulating explicit Majorana mass generation is the "master formula" (Cordero-Carrión et al., 2019, Cordero-Carrión et al., 2018, García et al., 21 Oct 2025): $$m_{\alpha\beta} = f \left[ y_1^T M y_2 + y_2^T M^T y_1 \right]_{\alpha\beta},$$ where $y_{1,2}$ are model-specific Yukawa matrices, $M$ contains the masses and, if relevant, loop functions of mediating fields, $f$ a dimensionless factor (±1 or loop/SM couplings), and $\alpha,\beta$ label lepton flavors.

2. Standard Model Extensions Realizing Majorana Masses

2.1. Seesaw Mechanisms

Type I Seesaw:

The Lagrangian includes terms

$$-\mathcal{L}_{\rm seesaw} = \overline{L} Y \tilde H N + \frac{1}{2} \overline{N^c} m_R N + \textrm{h.c.},$$

leading—after integrating out heavy $N$—to the effective neutrino mass

$m_\nu = -m_D m_R^{-1} m_D^T,$

with $m_D = Y v/\sqrt{2}$ (García et al., 21 Oct 2025).

Type II and III Seesaw:

Scalar triplet exchange (Type II) yields $m_\nu = Y_\Delta \langle \Delta^0 \rangle$, while SU(2) triplet fermion exchange (Type III) gives a similar formula to Type I (Sierra, 2011, Cepedello et al., 2019).

2.2. Radiative and Higher-Dimensional Models

Radiative Models:

Radiative Majorana neutrino masses arise via loop diagrams, which can be classified by loop order ($\ell=1,2,3$). Examples include:

• Zee Model: One-loop charged-scalar exchange.
• Ma Scotogenic Model: One-loop, new fermionic singlets/inert scalars, dark matter candidate (Anda et al., 2021).
• Babu-Zee Model: Two-loop, doubly-charged scalar.

The generic scaling is

$$m_\nu \sim \left(\frac{1}{16\pi^2}\right)^\ell \frac{v^2}{\Lambda},$$

where higher loop number, smaller couplings, or additional symmetry suppression permit TeV-scale mediators (Sierra, 2011, Cepedello et al., 2019, Hall et al., 2023, Chao, 2010).

Higher-Dimensional Operators:

$LLHH$ at $d=5$ is unique, but extensions at $d=7,9$ etc. allow for new topologies and fields to induce Majorana masses without tree-level or one-loop lower-dimensional contributions (Cepedello et al., 2019).

Extra Generations and Composite Dynamics:

Additional heavy generations or partial compositeness yield Majorana masses via radiative corrections, naturally small mixings, or inverse seesaw-like structures (Chacko et al., 2020, Aparici et al., 2011).

D-brane Instantons and String Realizations:

Intersecting D-brane models generate right-handed Majorana masses via stringy instanton effects; mass matrices exhibit residual cyclic flavor symmetries leading to specific mixing patterns (Hamada et al., 2014).

3. Structure and Parameterization of the Majorana Mass Matrix

The general Majorana mass matrix $M_\nu$ for three light active neutrinos is complex symmetric: $$M_\nu = \begin{pmatrix} M_{ee} & M_{e\mu} & M_{e\tau} \ M_{e\mu} & M_{\mu\mu} & M_{\mu\tau} \ M_{e\tau} & M_{\mu\tau} & M_{\tau\tau} \end{pmatrix},$$ with 12 real parameters. It is diagonalized by the PMNS matrix $U$, yielding physical masses, three mixing angles, one Dirac CP phase, and two Majorana phases—parameterized in the standard PDG convention. All observables (oscillation, $\beta\beta0\nu$, etc.) are functions of these parameters (Adhikary et al., 2013, Hu et al., 2021).

Universal parameterizations for the Yukawa matrices in any ultraviolet model, ensuring automatic agreement with low-energy mass and mixing data, are given by "master parameterization" methods (Cordero-Carrión et al., 2019, Cordero-Carrión et al., 2018) and the generalized Casas–Ibarra prescription (García et al., 21 Oct 2025). These allow efficient analytic and numerical scans of the model parameter space under phenomenological constraints.

4. Phenomenological Implications and Experimental Signatures

Neutrinoless Double Beta Decay ($0\nu\beta\beta$):

A signature of Majorana masses is lepton number violation via $0\nu\beta\beta$, with the decay rate controlled by the effective mass

$|m_{ee}| = \left| \sum_i U_{ei}^2 m_i \right|.$

In normal ordering, cancellations among components can render $|m_{ee}|$ arbitrarily small due to phase tuning, while in inverted ordering a lower bound $\gtrsim 10$ meV exists (0711.4993, Hu et al., 2021).

Lepton Flavor Violation (LFV):

Majorana masses induce $m_\nu^2$-suppressed but log-enhanced rates for LFV processes such as $\mu \to e \gamma$, with branching ratios typically far below current or near-future sensitivity unless additional new physics enhances the signal (Davidson et al., 2018).

Collider Signatures:

At the TeV scale, radiative or low-scale seesaw realizations predict accessible signals via same-sign dileptons, displaced vertices, or lepton-number–violating decays of heavy neutral or charged leptons (Chao, 2010, Hall et al., 2023, Aparici et al., 2011).

Cosmological and Astrophysical Constraints:

The absolute mass scale and heavy sector parameters are constrained by cosmological bounds on the sum of neutrino masses ($\Sigma m_i$), supernova cooling via Majorana neutrino emission, and CMB spectral distortions from late decays (Chacko et al., 2020, Anda et al., 2021).

5. Advanced Model Realizations and Flavor Structures

TeV-Scale Loop-Induced Seesaws:

A concrete model with loop-induced $m_D$ and TeV-scale $M_R$ incorporates additional Higgs doublets, singly-charged scalars, and vectorlike charged leptons, yielding sub-eV $m_\nu$ without high-scale new physics (Chao, 2010).

Gauge-Higgs Unification (GHU):

In GHU models, Majorana masses emerge via higher-dimensional gauge-invariant operators. Type I + Type III seesaw admixtures and double-seesaw (inverse seesaw) structures are possible, with mass scales determined by the compactification and symmetry-breaking structure (Hasegawa et al., 2018, Hasegawa, 2019).

String Theory and D-brane Instantons:

Intersecting D-brane compactifications generate unique forms of the Majorana mass matrix, often enforcing flavor symmetries and trimaximal mixing patterns. The mass matrix is determined by geometric data and theta functions, connecting topological string effects with phenomenology (Hamada et al., 2014).

Low-Scale Partial Compositeness:

If neutrinos are partially composite, the Majorana mass arises from IR-confined strong dynamics coupled via irrelevant operators, leading to an inverse-seesaw structure parameterized by scaling dimensions of CFT operators, with strong implications for laboratory, cosmological, and astrophysical observables (Chacko et al., 2020).

6. Outlook: Classification, Parametrization, and Unification

Comprehensive model-building frameworks for Majorana masses reduce to classifying the possible structures for the heavy-sector block matrices $M_{ij}$ in the master formula, and parameterizing the corresponding Yukawa sector in terms of physical observables plus orthogonal/free matrices. This is achieved systematically in the generalized Casas–Ibarra and master parametrizations, which encompass tree-level, loop-level, and mixed hierarchy models, and accommodate arbitrary flavor textures and CP violation (Cordero-Carrión et al., 2019, García et al., 21 Oct 2025).

Experimental progress in $0\nu\beta\beta$ decay, direct searches for heavy neutral leptons, precise oscillation data, LFV limits, and cosmological probes will continue to sharpen parameter space and test the broad landscape of Majorana neutrino mass generation, potentially distinguishing between the wide variety of mechanisms and ultraviolet completions discussed above.