# From the trees to the forest: a review of radiative neutrino mass models

## TL;DR

This review discusses radiative neutrino mass models, emphasizing their testability at colliders, their phenomenological implications, and categorization based on loop order and operator approach, focusing mainly on Majorana neutrinos.

## Contribution

It provides a comprehensive classification of radiative neutrino mass models from multiple perspectives and revisits key models with detailed phenomenological analysis.

## Key findings

- Models predict new particles at TeV scale accessible at colliders.
- Radiative models naturally explain small neutrino masses without large hierarchies.
- Lepton-flavor and lepton-number violating signals are key experimental signatures.

## 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.