Improved sterile neutrino constraints from the STEREO experiment with 179 days of reactor-on data

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 γ-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.

Reactor Antineutrino Anomaly Reanalysis in Context of Inverse-Square Law Violation

“…The results of these experiments provide intriguing evidence in favor of the "3 + 1" scenario, although they appear to be in deep conflict with each other. In the two consecutive analyses of the Neutrino-4 data [88,89] (eprint version 7), the following allowed values for the masssquared splitting and mixing angle were obtained by using the so-called coherent data summation method: These results do not agree with the constraints based on the combined analysis of the MINOS, Daya Bay, and Bugey-3 data [94], and with the limits obtained in the recent SBL experiments DANSS [95][96][97], STEREO [51], NEOS [78] (see also [98], and PROSPECT [53]; this issue is now broadly discussed in the literature [99][100][101][102][103]. On the other hand, the 68% C.L.…”

“…The gallium neutrino anomaly (GAA) is a deficit in the number of events caused by electron neutrinos from intense artificial 51 Cr and 37 Ar sources and measured by the gallium-based solar neutrino detectors GALLEX [209][210][211] and SAGE [212][213][214]. The aims of these measurements were to calibrate the detection efficiency of the detectors and to test the experimental procedures, including chemical extraction, counting, and analysis techniques.…”

“…Ratios of measured and expected 71 Ge event rates in the four radioactive source experiments calculated with several models for the gallium cross sections for 51 Cr and 37 Ar neutrinos (borrowed from [222,224]). 51 Cr (G1) 51 Cr (G2) 51 Cr (S1) 37 Ar (S2) Isotope 51 Cr decays by electron capture to the ground state of 51 V (90.1%) and to the 320 keV excited level (9.9%), and 37 Ar decays 100% by electron capture to the ground state of 37 Cl. Taking into account the atomic levels to which the transitions can occur, the chromium (argon) neutrino energy spectrum consists of four (two) lines.…”

“…The newer high-precision measurements [49,50] are absolute (in Figure 1; they are placed at the effective flux-weighted baselines). In order not to complicate the figure, we do not show very recent relative SBL measurements of Neutrino-4 (SM-3 HEU research reactor, Dimitrovgrad), STEREO (ILL HEU research reactor, Grenoble) [51], PROSPECT (HFIR HEU research reactor, Oak Ridge) [52,53], and DANSS (a 3.1 GW th LEU industrial reactor, Kalinin NPP) [54,55], all of which, however, are critical for the present study and will be discussed in detail in Section 6. The reactor antineutrino anomaly.…”

Latest results of the STEREO search for a sterile neutrino at a research reactor

“…In particular the best fit point of the RAA is rejected at more than 99.9% C.L. [7] PoS(ICHEP2020)154…”