Zee model
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| Zee model | |
|---|---|
| Name | Zee model |
| Field | Particle physics |
| Introduced | 1980 |
| Authors | A. Zee |
| Components | charged scalar, Higgs doublet |
| Related | Two-Higgs-Doublet Model, Standard Model |
Zee model is a radiative neutrino mass generation mechanism proposed to explain small neutrino masses via one-loop diagrams. It extends the Standard Model with additional scalar fields and lepton-number-violating interactions, linking ideas from the Two-Higgs-Doublet Model, Majorana fermion mass generation, and charged-scalar phenomenology. The proposal influenced subsequent frameworks such as the Ma model, Zee–Babu model, and studies of lepton flavor violation at experiments like MEG and Belle II.
Introduction
The Zee model was introduced by A. Zee in 1980 to generate neutrino masses without right-handed neutrino singlets, employing an extra Higgs field structure and a singly charged scalar. It contrasts with the see-saw mechanism approaches associated with Grand Unified Theory scenarios like SO(10), SU(5), and with radiative schemes such as the Scotogenic model. The construction embeds charged-scalar interactions that violate lepton number and induce Majorana masses through loop processes, connecting to experimental programs at Super-Kamiokande, SNO, and reactor experiments like Daya Bay.
Theoretical Framework
The model augments the Standard Model scalar sector by introducing a second Higgs doublet and a singly charged SU(2)-singlet scalar. Yukawa-type couplings connect the new charged scalar to lepton doublets, breaking flavor symmetries present in minimal constructions. The scalar potential mixes the two Higgs field doublets and the charged singlet, reminiscent of potentials studied in Two-Higgs-Doublet Model literature and analyses of electroweak symmetry breaking at the Large Hadron Collider. Gauge invariance under SU(2)_L×U(1)_Y and discrete symmetries are often imposed to control tree-level flavor-changing neutral currents, as in models motivated by Glashow–Weinberg conditions.
Radiative Neutrino Mass Generation
Neutrino masses arise at one-loop order via diagrams containing charged leptons, the second Higgs doublet, and the charged singlet scalar circulating in the loop. The generated Majorana mass matrix is antisymmetric in flavor indices at leading order, producing predictive textures studied in fits to oscillation data from Super-Kamiokande, SNO, KamLAND, and T2K. Loop integrals depend on scalar masses and mixing parameters akin to computations performed in quantum field theory treatments used for radiative corrections in electroweak precision observables measured at LEP and SLD.
Variants and Extensions
Multiple variants extend the original setup: the Zee–Babu extension adds a doubly charged scalar to generate neutrino masses at two loops, linking to analyses from Muon g-2 studies and doubly charged Higgs searches. Other extensions embed the mechanism into supersymmetric contexts such as Minimal Supersymmetric Standard Model variants or into grand-unified frameworks like SO(10), often combining with sterile-neutrino portals studied in LSND and MiniBooNE anomalies. Discrete symmetry implementations borrow ideas from flavor model-building groups such as A4 (group), S4 (group), and U(1)_L global symmetries to obtain realistic mixing patterns compatible with data from NOvA and RENO.
Phenomenology and Experimental Constraints
Predictions include modified lepton-flavor-changing rates, altered scalar spectra accessible at colliders, and contributions to electroweak precision parameters constrained by LEP, Tevatron, and LHC measurements. Searches for charged scalars constrain parameter space through processes analogous to charged-Higgs searches in CMS and ATLAS analyses, while low-energy probes such as MEG set limits on radiative decays. Global fits incorporate neutrino oscillation results from Super-Kamiokande and reactor experiments alongside collider bounds, affecting viable mass ranges and coupling strengths compared with predictions in the Higgs boson sector uncovered by ATLAS and CMS.
Implications for Lepton Flavor Violation
The model generically induces lepton-flavor-violating processes like l_i → l_j γ, l_i → l_j l_k l_l, and μ–e conversion in nuclei, connecting to experimental programs at MEG II, Mu2e, and COMET. Flavor textures originating from antisymmetric Yukawa couplings produce characteristic correlations among branching ratios, leveraged in reinterpretations of limits from BaBar and Belle. Constraints from charged-lepton processes complement neutrino oscillation fits from T2K and NOvA, shaping viable coupling hierarchies in parameter scans often performed using tools developed for Global fits and statistical analyses utilized by collaborations such as Particle Data Group.
Collider Signatures and Searches
Collider signals include pair production and associated production of charged scalars, decays into charged leptons and neutrinos, and modifications of Higgs decay channels measurable at LHC experiments ATLAS and CMS. Dedicated searches target same-sign dilepton signatures similar to those used in doubly charged scalar hunts at CMS and ATLAS, and final states with missing energy paralleling slepton searches in Supersymmetry analyses. Prospects at future facilities like the High-Luminosity LHC, International Linear Collider, and proposed muon colliders motivate continued reinterpretation of results from CMS, ATLAS, and flavor experiments including Belle II.