Improved Sterile Neutrino Constraints from the STEREO Experiment with 179 Days of Reactor-On Data

TL;DR

The STEREO experiment analyzed 179 days of reactor data to improve constraints on sterile neutrinos, finding no evidence for oscillations and strongly rejecting the reactor antineutrino anomaly.

Contribution

This work provides enhanced sterile neutrino constraints through improved detector modeling, background treatment, and statistical analysis using reactor-on data.

Findings

Data compatible with null oscillation hypothesis

Rejection of the reactor antineutrino anomaly at >99.9% C.L.

Improved modeling of detector and backgrounds

Abstract

The STEREO experiment is a very short baseline reactor antineutrino experiment. It is designed to test the hypothesis of light sterile neutrinos being the cause of a deficit of the observed antineutrino interaction rate at short baselines with respect to the predicted rate, known as the reactor antineutrino anomaly. The STEREO experiment measures the antineutrino energy spectrum in six identical detector cells covering baselines between 9 and 11 m from the compact core of the ILL research reactor. In this article, results from 179 days of reactor turned on and 235 days of reactor turned off are reported at a high degree of detail. The current results include improvements in the modelling of detector optical properties and the gamma-cascade after neutron captures by gadolinium, the treatment of backgrounds, and the statistical method of the oscillation analysis. Using a direct comparison between antineutrino spectra of all cells, largely independent of any flux prediction, we find the data compatible with the null oscillation hypothesis. The best-fit point of the reactor antineutrino anomaly is rejected at more than 99.9% C.L.