Abstract
The reactor antineutrino anomaly might be explained by the oscillation of reactor antineutrinos toward a sterile neutrino of eV mass. In order to explore this hypothesis, the STEREO experiment measures the antineutrino energy spectrum in six different detector cells covering baselines between 9 and 11m from the compact core of the ILL research reactor. In this Letter, results from 66days of reactor turned on and 138days of reactor turned off are reported. A novel method to extract the antineutrino rates has been developed based on the distribution of the pulse shape discrimination parameter. The test of a new oscillation toward a sterile neutrino is performed by comparing ratios of cells, independent of absolute normalization and of the prediction of the reactor spectrum. The results are found to be compatible with the null oscillation hypothesis and the best fit of the reactor antineutrino anomaly is excluded at 97.5%C.L.
Highlights
The reactor antineutrino anomaly might be explained by the oscillation of reactor antineutrinos toward a sterile neutrino of eV mass
There are basically two possible explanations for this observation known as the reactor antineutrino anomaly (RAA) [2]
One is a deficient prediction of the antineutrino flux and spectrum from reactors, due to underestimated systematics of the measurements of beta spectra emitted after fission [3,4,5] or of the conversion method [6,7], see [8,9] for recent reviews
Summary
The analysis of DANSS compares the antineutrino spectrum of the movable detector for two baselines. In STEREO, the antineutrino spectrum with energies up to about 10 MeV is measured in a segmented detector using six identical target cells of 37 cm length, whose centers are placed from 9.4 to 11.1 m from the reactor core. The analysis presented here uses spectra ratios with one cell as reference It does not require a reactor spectrum prediction and is independent from the absolute flux normalization, minimizing systematic uncertainties. A GEANT4 [22] (version 10.1) Monte Carlo model (MC) based on DCGLG4sim [23] describes detector geometry, shielding, position to the reactor core, particle interactions including neutron moderation and capture, light production, transport including cross talks between cells and detection, and signal conversion in the electronics. Beyond the basic cuts on energy and capture time (cuts 1–3), the detector segmentation is exploited to tag the topology of energy deposition of IBD events: compact prompt event only allowing for escaping
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