The discovery of neutrino oscillations is a major achievement in the recent history of elementary particles. It implies that the most abundant matter particles in the universe are massive and that the three neutrino states alternately change from one type to another as they travel. A large experimental program is ongoing to measure accurately the parameters of the neutrino-mixing matrix.

A recent work published by CEA-Irfu has triggered a worldwide renaissance in the search of sterile neutrinos. In this work 19 published neutrino measurements at short distance (10-100 m) from reactors have been reanalyzed after a re-evaluation of the predicted reactor neutrino flux had revealed a bias in the previous calculations. The result is a mean deficit of 7% of detected neutrinos with respect to predictions, with a statistical significance of 3 s.  This is called the reactor neutrino anomaly and it combines nicely with another (long-standing) anomaly in the detection of electronic neutrinos from intense beta-decay sources.

By analogy with the already measured deficits of reactor neutrinos induced by their oscillations in the solar and atmospheric sectors, this new deficit at short distance can be interpreted as the existence of a new neutrino state, a light sterile neutrino. Evidence of this new particle would be a major discovery, with deep impact in particle physics and cosmology.

This new neutrino with no ordinary weak interactions could only be ‘visible’ by its mixing with the three ordinary neutrinos. In a global study of reactor and source anomalies, the most probable mixing parameters are sin2(2qnew)=0.17±0.04 and Dm2new= 2.3±0.1 eV2. These parameters correspond to an oscillation length in the range of a few meters for the few MeV antineutrinos emitted by reactors. Therefore the associated new oscillation pattern is easily smeared out by extended core size or by energy resolution effects at too long baseline. It explains why only a global rate deficit has been observed so far.

 

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The aim of the Stereo proposal is to confirm the existence of a sterile neutrino state by searching for an oscillation pattern at short distance with a high sensitive, segmented detection assembly placed few meters away from the core of the ILL research reactor (Grenoble, France). The originality of the project is to provide a clear signature of the possible new oscillation pattern by looking for the distortion of the energy spectrum and the dependency of that distortion along the detector axis. No reactor input is needed to first order, reducing systematic uncertainties.

The detection concept is based on the interaction of the electronic antineutrinos in a liquid scintillator (LS) via the inverse beta decay process . The target volume consists in 5 cells of 1.1 x 0.9 x 0.4 m3, stacked along the direction of the core. They are filled with Gd-doped LS in order to tag the radiative neutron capture on Gd in coincidence with the annihilation of the positron. An outer crown, filled with LS without Gd, recovers part of the escaping gammas to improve the detection efficiency and the energy resolution.

The long reactor shutdown of the ILL reactor scheduled from mid-2013 to mid-2014 will allow a dedicated arrangement of the area “GAMS5” where the installation of Stereo is foreseen. This site combines the assets of a very compact core (<1m), a very short baseline (8 m from core to detector centers) and a nuclear fuel highly enriched in 235U, suppressing all effects of evolution of the fuel composition in the determination of the spectrum shape. A large overburden of concrete and water reduces the flux of cosmic rays and a set of active and passive shielding suppresses the g and neutron background. The required heavy structure, estimated to be 70 tons, is well within the floor load specifications of the GAMS5 area, the strongest in the reactor building.

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The Stereo collaboration gathers a large experience in the field of reactor neutrino physics covering all crucial aspects of the experiments. The presented schedule of installation and the sensitivity of the measurement (the exclusion contour of Stereo fully covers the domain of existence of the sterile neutrino at 99% confidence level) provide a high discovery potential to Stereo.