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.
Highlights
Over the last two decades, measurements of neutrino oscillations in the three flavor framework have determined all mixing angles and mass splittings [1,2,3]
This observed discrepancy, known as reactor antineutrino anomaly (RAA), with a significance of 2.7 standard deviations triggered a new set of reactor antineutrino experiments at very short baselines of about 10 m including the STEREO experiment
Alongside the STEREO antineutrino detector, the ILL accommodates about 40 different instruments for neutron scattering, nuclear spectroscopy and fundamental physics with neutrons, taking advantage of the high neutron flux produced by its nuclear reactor RHF
Summary
Over the last two decades, measurements of neutrino oscillations in the three flavor framework have determined all mixing angles and mass splittings [1,2,3]. Recent global analyses and some of the recently reported experimental results suggest the existence of a sterile neutrino at different parameter regions with significances up to about 3 standard deviations [17,19,20,21,22]. Some of these proposed parameter regions are in conflict with other data including constraints from cosmology [23,24].
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