Helicity oscillations of Dirac and Majorana neutrinos

TL;DR

This paper reviews how magnetic fields and polarized media induce helicity oscillations in Dirac and Majorana neutrinos, highlighting the role of in-medium dispersion effects and differences based on neutrino type.

Contribution

It provides a unified analysis of helicity oscillations for both neutrino types considering magnetic and medium polarization effects.

Findings

Magnetic fields cause helicity oscillations in Dirac neutrinos due to their magnetic dipole moment.

Polarized or anisotropic media induce similar effects for both Dirac and Majorana neutrinos.

Medium polarization and matter-antimatter asymmetry influence the occurrence of helicity oscillations.

Abstract

The helicity of a Dirac neutrino with mass $m$ evolves under the influence of a $B$-field because it has a magnetic dipole moment proportional to $m$. Moreover, it was recently shown that a polarized or anisotropic medium engenders the same effect for both Dirac and Majorana neutrinos. Because a $B$-field polarizes a background medium, it instigates helicity oscillations even for Majorana neutrinos unless the medium is symmetric between matter and antimatter. Motivated by these observations, we review the impact of a $B$-field and of an anisotropic or polarized medium on helicity oscillations for Dirac and Majorana neutrinos from the common perspective of in-medium dispersion.