A Mechanism for Ordinary-Sterile Neutrino Mixing

TL;DR

The paper investigates how ordinary and sterile neutrinos can mix significantly without fine-tuning. It argues that this requires both Dirac and Majorana masses to be small and comparable, which is hard to realize in standard models. The author then presents a string-inspired framework where higher-dimensional operators, suppressed by an intermediate-scale singlet vev, naturally yield suppressed $m_D$ and $m_M$, and identifies a specific operator-dimension relation $P_D = P_M - K$ that can make $m_M$ and $m_D$ comparable. While not universal across all compactifications, this mechanism provides a plausible route to ordinary-sterile mixing and highlights potential phenomenological and cosmological implications, albeit with challenging experimental verification.

Abstract

Efficient oscillations between ordinary (active) and sterile neutrinos can occur only if Dirac and Majorana mass terms exist which are both small and comparable. It is shown that this can occur naturally in a class of string models, in which higher-dimensional operators in the superpotential lead to an intermediate scale expectation value for a scalar field and to suppressed Dirac and Majorana fermion masses.