Abstract
A plausible explanation for the lightness of neutrino masses is that neutrinos are massless at tree level, with their mass (typically Majorana) being generated radiatively at one or more loops. The new couplings, together with the suppression coming from the loop factors, imply that the new degrees of freedom cannot be too heavy (they are typically at the TeV scale). Therefore, in these models there are no large mass hierarchies and they can be tested using different searches, making their detailed phenomenological study very appealing. In particular, the new particles can be searched for at colliders and generically induce signals in lepton-flavor and lepton-number violating processes (in the case of Majorana neutrinos), which are not independent from reproducing correctly the neutrino masses and mixings. The main focus of the review is on Majorana neutrinos. We order the allowed theory space from three different perspectives: (i) using an effective operator approach to lepton number violation, (ii) by the number of loops at which the Weinberg operator is generated, (iii) within a given loop order, by the possible irreducible topologies. We also discuss in more detail some popular radiative models which involve qualitatively different features, revisiting their most important phenomenological implications. Finally, we list some promising avenues to pursue.
Highlights
The discovery of neutrino oscillations driven by mass mixing is one of the crowning achievements of experimental high-energy physics in recent decades
In addition to the νL that resides inside the doublet L, and the standard RH neutrino of Equation (8), we introduce a vectorlike heavy neutral fermion NL,R ∼ (1, 1, 0) and impose total lepton-number conservation with νL,R and NL,R assigned lepton numbers of 1
Based on the general classification of 1-loop models [100], the authors of Restrepo et al [111] performed a systematic study for models compatible with dark matter (DM) stabilized by a discrete Z2 symmetry
Summary
The discovery of neutrino oscillations driven by mass mixing is one of the crowning achievements of experimental high-energy physics in recent decades. Future experiments and observational programs have excellent prospects to determine the mass ordering, discover leptonic CP violation, observe neutrinoless double beta-decay (0νββ) and the violation of lepton number by two units, and measure the absolute neutrino mass scale. Before turning to a discussion of neutrino mass models, we should review some interesting experimental anomalies that may imply the existence of light sterile neutrinos in addition to the active flavors νe,μ,τ (see Gariazzo et al [32], Kopp et al [33] for phenomenological fits). The Gallium anomaly arose from neutrino calibration source measurements by the Gallex and SAGE radiochemical solar neutrino experiments, indicating a deficit [42,43,44,45] Both deficits are consistent with very short baseline transitions driven by eV-scale sterile neutrinos, and a significant number of experiments are underway to test the oscillation explanation. Appendix gives further details on the relative contributions of the different operators to neutrino masses
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