White Paper on Light Sterile Neutrino Searches and Related Phenomenology

We study the status of the reactor antineutrino anomaly in light of recent reactor flux models obtained with the conversion and summation methods. We present a new improved calculation of the IBD yields of the standard Huber-Mueller (HM) model and those of the new models. We show that the reactor rates and the fuel evolution data are consistent with the predictions of the Kurchatov Institute (KI) conversion model and with those of the Estienne-Fallot (EF) summation model, leading to a plausible robust demise of the reactor antineutrino anomaly. We show that the results of several goodness of fit tests favor the KI and EF models over other models that we considered. We also discuss the implications of the new reactor flux models for short-baseline neutrino oscillations due to active-sterile mixing. We show that reactor data give upper bounds on active-sterile neutrino mixing that are not very different for the reactor flux models under consideration and are in tension with the large mix …

In spite of the presence of a tension in the interpretation of the data, 3+1 neutrino mixing is attractive for its simplicity and for the natural correspondence of one new entity (a sterile neutrino) with a new effect (short-baseline oscillations). The allowed regions in the oscillation parameter space can be tested in near-future experiments. In the framework of 3+2 neutrino mixing there is less tension in the interpretation of the data, at the price of introducing a second sterile neutrino. Moreover, the improvement of the parameter goodness of fit is mainly a statistical effect due to an increase of the number of parameters.

Recently, the Baksan Experiment on Sterile Transitions (BEST) has presented results [1] confirming the gallium anomaly---a lack of electron neutrinos ${\ensuremath{\nu}}_{e}$ at calibrations of SAGE [2,3] and GALLEX [4]---at the statistical significance exceeding $5\ensuremath{\sigma}$. This result is consistent with explanation of the gallium anomaly as electron neutrino oscillations into a sterile neutrino, ${\ensuremath{\nu}}_{s}$. Within this explanation, the BEST experiment itself provides the strongest evidence for the sterile neutrino among all the previous anomalous results in the neutrino sector. We combine the results of gallium experiments with searches for sterile neutrinos in reactor antineutrino experiments (assuming $CPT$ conservation in the $3+1$ neutrino sector). While the ``gallium'' best-fit point in the model parameter space (sterile neutrino mass squared ${m}_{{\ensuremath{\nu}}_{s}}^{2}\ensuremath{\approx}1.25\text{ }\text{ }{\mathrm{eV}}^{2}$, sterile-electron ne …

Abstract We present the bounds on 3+1 active-sterile neutrino mixing obtained from the first results of the KATRIN experiment. We show that the KATRIN data extend the Mainz and Troitsk bound to smaller values of $$ \Delta {m}_{41}^2 $$ Δ m 41 2 for large mixing and improves the exclusion of the large- $$ \Delta {m}_{41}^2 $$ Δ m 41 2 solution of the Huber-Muller reactor antineutrino anomaly. We also show that the combined bound of the Mainz, Troitsk, and KATRIN tritium experiments and the Bugey-3, NEOS, PROSPECT, and DANSS reactor spectral ratio measurements exclude most of the region in the (sin22ϑ ee , $$ \Delta {m}_{41}^2 $$ Δ m 41 2 ) plane allowed by the Huber-Muller reactor antineutrino anomaly. Considering two new calculations of the reactor antineutrino fluxes, we show that one, that predicts a lower 235U antineutrino flux, is in agreement with the tritium and reactor spectral ratio measurements, whereas the other leads to a larger tension than the Huber-Muller prediction. We a …

Abstract The precision reactor oscillation and spectrum experiment, PROSPECT, is designed to make a precise measurement of the antineutrino spectrum from a highly-enriched uranium reactor and probe eV-scale sterile neutrinos by searching for neutrino oscillations over a distance of several meters. PROSPECT is conceived as a 2-phase experiment utilizing segmented 6 Li-doped liquid scintillator detectors for both efficient detection of reactor antineutrinos through the inverse beta decay reaction and excellent background discrimination. PROSPECT Phase I consists of a movable 3 ton antineutrino detector at distances of 7–12 m from the reactor core. It will probe the best-fit point of the ν e disappearance experiments at 4 σ in 1 year and the favored region of the sterile neutrino parameter space at > 3 σ in 3 years. With a second antineutrino detector at 15–19 m from the reactor, Phase II of PROSPECT can probe the entire allowed parameter space below 10 eV 2 at 5 σ in 3 additional years. …