Sterile Neutrinos

Abstract

Neutrinos, being the only fermions in the Standard Model of Particle Physics that do not possess electromagnetic or color charges, have the unique opportunity to communicate with fermions outside the Standard Model through mass mixing. Such Standard Modelsinglet fermions are generally referred to as "sterile neutrinos". In this review article, we discuss the theoretical and experimental motivation for sterile neutrinos, as well as their phenomenological consequences. With the benefit of hindsight in 2020, we point out potentially viable and interesting ideas. We focus in particular on sterile neutrinos that are light enough to participate in neutrino oscillations, but we also comment on the benefits of introducing heavier sterile states. We discuss the phenomenology of eVscale sterile neutrinos in terrestrial experiments and in cosmology, we survey the global data, and we highlight various intriguing anomalies. We also expose the severe tension that exists between different data sets and prevents a consistent interpretation of the global data in at least the simplest sterile neutrino models. We discuss non-minimal scenarios that may alleviate some of this tension. We briefly review the status of keV-scale sterile neutrinos as dark matter and the possibility of explaining the matter-antimatter asymmetry of the Universe through leptogenesis driven by yet heavier sterile neutrinos.

Low-energy probes of sterile neutrino transition magnetic moments

“…In what follows, for our analysis involving reactor neutrino experiments (ν e source) or 51 Cr experiments (ν e source), the effective neutrino magnetic moment will be given by Eq. (9). Similarly, for the analysis of the COHERENT experiment involving pion-decay-at-rest neutrinos, Eq.…”

“…From this viewpoint, probing the existence of such sterile neutrinos is extremely interesting. Depending on the mass and mixing parameters characterizing the light sterile states, there can be stringent restrictions following from laboratory, astrophysical and cosmological observations [7][8][9][10].…”

Results from the Baksan Experiment on Sterile Transitions (BEST)

“…The possibility of the existence of light sterile neutrinos (ν s ) is presently a major field of inquiry. The literature on this topic is extensive but has been summarized well in a number of recent reviews [1][2][3][4][5][6][7]. Much of the evidence for ν s 's comes from oscillation experiments that search for the conversion of an active neutrino into a sterile state.…”

A Search for Electron Neutrino Transitions to Sterile States in the BEST Experiment

“…Sterile neutrinos (ν s ) are hypothetical fermions which are singlets with respect to the Standard Model (SM) gauge groups [1][2][3][4][5][6][7]. Lacking electric, color, or hypercharges, they would not participate in electromagnetic, strong or weak interactions, but could thorough their gravitational interactions have consequences for astrophysics.…”