From the trees to the forest: a review of radiative neutrino mass models (1706.08524v3)

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.

• The paper presents a comprehensive review and classification of radiative neutrino mass models generated by 1-loop to 3-loop processes, distinguishing them from traditional seesaw mechanisms.
• The paper analyzes key models, including Zee, Zee-Babu, scotogenic, and KNT, detailing their specific particle content and associated phenomenological signatures.
• The paper discusses the experimental implications by highlighting testable predictions, such as dark matter stability and lepton-flavor violation processes, to guide future research.

A Review of Radiative Neutrino Mass Models

The paper under review provides an extensive examination of radiative neutrino mass models, highlighting their significance in the landscape of particle physics and their intricate relationship with dark matter and lepton-flavor violating processes. Radiative neutrino mass models stand out as they propose a mechanism for the smallness of neutrino masses without introducing very high mass scales associated with traditional seesaw mechanisms. They achieve this by generating neutrino masses at loop levels, thereby offering new insights into the subatomic field that are potentially testable with ongoing and future experiments.

Summary of Radiative Neutrino Mass Models

The central theme of the paper is the classification and systematic analysis of radiative neutrino mass models. These models are distinguished from tree-level seesaw models by the generation of neutrino masses predominantly through loop processes rather than through direct mass terms. This feature makes them more naturally aligned with the observed smallness of neutrino masses due to additional suppression factors, including loop and Yukawa coupling suppressions.

The models are categorized primarily based on how effectively they "open up" the Weinberg operator, $LLHH$, at different loop orders. This generates a hierarchy of models characterized by the loop order at which the dominant contribution to neutrino masses is generated. The paper presents 1-loop, 2-loop, and even up to 3-loop models, examining their underlying particle content and the symmetries that prevent tree-level mass generation.

Key Models and Their Features

1. Zee and Zee-Babu Models: These are among the pioneering models discussed, where neutrino masses emerge at 1-loop and 2-loop levels, respectively. The Zee model introduces a singly-charged scalar, while the Zee-Babu model adds a doubly-charged scalar, leading to distinct phenomenological consequences such as lepton-flavor violating processes.
2. Scotogenic Model: This model innovatively ties neutrino mass generation with dark matter, using a minimally extended particle content that includes a new scalar doublet and singlet fermions stabilized by a $Z_2$ symmetry. This symmetry not only results in neutrino masses at the 1-loop level but also ensures the stability of a dark matter candidate.
3. Krauss-Nasri-Trodden (KNT) Model: This example of a 3-loop radiative model further showcases the complexity and richness of these frameworks. It combines new exotic fermions and scalars, operating in the lepton and scalar sectors, leading to a single neutrino mass matrix eigenvalue due to its minimal field content.

Phenomenological Implications

The implications of radiative models extend beyond neutrino masses alone. These models often predict significant observables, including lepton-flavor violation (LFV), which is manifest in rare processes like $\mu \rightarrow e\gamma$ and $\mu-e$ conversion in nuclei. The paper emphasizes the strong experimental effort in probing these processes as part of searches for new physics. Additionally, the models frequently involve extended Higgs sectors or leptoquarks, predicting exotic signatures at collider experiments, further motivating their paper.

Dark Matter and Neutrino Mass Connection

Many radiative models suggest a linkage between neutrino masses and dark matter, proposing candidates that are stable due to additional imposed symmetries like $Z_2$ parity. These candidates are often part of the particle content required for radiative mass generation, implying a direct path from neutrino phenomenology to cosmological observations related to dark matter.

Challenges and Prospects

Despite the promising framework that radiative neutrino mass models offer, they face challenges, notably in the precise predictions of neutrino masses and mixing parameters. These models also need to be evaluated against results expected from other searches, such as the non-observation of $0\nu\beta\beta$ decay, potential signals in collider physics, and cosmological constraints.

In conclusion, the paper provides a thorough review and classification of radiative neutrino mass models, reinforcing their theoretical attractiveness and experimental relevance. As both theoretical constructs and as practical models for new physics, they remain a vibrant area of research with the potential to unveil profound insights into the universe's fundamental structure. Future experimental advances could provide critical tests, validating these models or reshaping our understanding of neutrino physics and beyond.