Neutrino Physics (theory)

TL;DR

Nonzero neutrino masses necessitate physics beyond the Standard Model, motivating a survey of Majorana vs. Dirac masses, mass hierarchies, mixing, and the number of neutrinos. The paper integrates experimental status (oscillations, MSW effects, and bounds from $ββ_{0ν}$ and cosmology) with theoretical frameworks, detailing seesaw and triplet mechanisms and the particular challenges of embedding them in string constructions. It highlights how string theory constraints can suppress Majorana masses and alter the expected mass textures, while still allowing Dirac masses, extended seesaws, or triplet scenarios. The discussion emphasizes that experimental advances (ββ_{0ν}$ searches, KATRIN, cosmology, and precision oscillation measurements) will be decisive in distinguishing among normal, inverted, and degenerate spectra and in identifying the underlying mass-generation mechanism.

Abstract

Nonzero neutrino masses are the first definitive need to extend the standard model. After reviewing the basic framework, I describe the status of some of the major issues, including tests of the basic framework of neutrino masses and mixings; the question of Majorana vs. Dirac; the spectrum, mixings, and number of neutrinos; models, with special emphasis on constraints from typical superstring constructions (which are not consistent with popular bottom-up assumptions); and other implications.

Figures (5)

• Figure 1: Dirac and Majorana masses.
• Figure 2: The normal and inverted hierarchies.
• Figure 4: $2+2$ and $3+1$ patterns.
• Figure 5: The ordinary seesaw. $N$ is a heavy Majorana neutrino.
• Figure 6: The triplet seesaw