The see-saw mechanism: neutrino mixing, leptogenesis and lepton flavor violation

TL;DR

This paper reviews the see-saw mechanism for neutrino mass generation, exploring its phenomenology, variants, and indirect tests like leptogenesis and lepton flavor violation, highlighting the challenges in reconstructing the mechanism from low-energy data.

Contribution

It provides a comprehensive overview of see-saw models, their variants, and phenomenological implications, including the dependence of lepton flavor violation on neutrino parameters and model specifics.

Findings

Lepton flavor violation depends on the dominant see-saw component.

Absence of certain decays implies CP conservation in neutrino oscillations.

Tau -> mu gamma has the largest branching ratio in specific models.

Abstract

The see-saw mechanism to generate small neutrino masses is reviewed. After summarizing our current knowledge about the low energy neutrino mass matrix we consider reconstructing the see-saw mechanism. Low energy neutrino physics is not sufficient to reconstruct see-saw, a feature which we refer to as ``see-saw degeneracy''. Indirect tests of see-saw are leptogenesis and lepton flavor violation in supersymmetric scenarios, which together with neutrino mass and mixing define the framework of see-saw phenomenology. Several examples are given, both phenomenological and GUT-related. Variants of the see-saw mechanism like the type II or triplet see-saw are also discussed. In particular, we compare many general aspects regarding the dependence of LFV on low energy neutrino parameters in the extreme cases of a dominating conventional see-saw term or a dominating triplet term. For instance, the absence of mu -> e gamma or tau -> e gamma in the pure triplet case means that CP is conserved in neutrino oscillations. Scanning models, we also find that among the decays mu -> e gamma, tau -> e gamma and tau -> mu gamma the latter one has the largest branching ratio in (i) SO(10) type I see-saw models and in (ii) scenarios in which the triplet term dominates in the neutrino mass matrix.