Light Sterile Neutrinos and Short Baseline Neutrino Oscillation Anomalies

TL;DR

This paper explores two theoretical explanations involving sterile neutrinos for short baseline neutrino oscillation anomalies, analyzing their consistency with experimental data and implications for neutrino mass hierarchies and cosmology.

Contribution

It introduces and compares two models, the mini-seesaw with two eV-scale sterile neutrinos and a non-unitary mixing scenario, explaining anomalies and their theoretical and observational implications.

Findings

Mini-seesaw can account for anomalies and fits active neutrino data.

The minimal mini-seesaw favors an inverted hierarchy.

Non-unitary mixing scenario faces strong cosmological and experimental constraints.

Abstract

We study two possible explanations for short baseline neutrino oscillation anomalies, such as the LSND and MiniBooNE anti-neutrino data, and for the reactor anomaly. The first scenario is the mini-seesaw mechanism with two eV-scale sterile neutrinos. We present both analytic formulas and numerical results showing that this scenario could account for the short baseline and reactor anomalies and is consistent with the observed masses and mixings of the three active neutrinos. We also show that this scenario could arise naturally from an effective theory containing a TeV-scale VEV, which could be related to other TeV-scale physics. The minimal version of the mini-seesaw relates the active-sterile mixings to five real parameters and favors an inverted hierarchy. It has the interesting property that the effective Majorana mass for neutrinoless double beta decay vanishes, while the effective masses relevant to tritium beta decay and to cosmology are respectively around 0.2 and 2.4 eV. The second scenario contains only one eV-scale sterile neutrino but with an effective non-unitary mixing matrix between the light sterile and active neutrinos. We find that though this may explain the anomalies, if the non-unitarity originates from a heavy sterile neutrino with a large (fine-tuned) mixing angle, this scenario is highly constrained by cosmological and laboratory observations.