Search for sterile neutrino at the ILL reactor. – Stereo

The Stereo project benefits from the recent developments on large neutrino detectors that have demonstrated the production of large quantities of Gadolinium-doped liquid scintillator (LS), very stable on the scale of several years. The signature of the neutrino interaction is established by the so-called inverse-beta-decay reaction that produces a positron and a neutron in the final state. The energy deposition of the positron provides a first light pulse in the LS. Then the neutron is captured by the gadolinium added in the liquid. This capture produces a second light pulse few tens of microseconds later, signing the neutrino.
The analysis is based on the relative changes in form and count rate between different the identical cells of the detector, thereby decreasing the impact of most normalization uncertainties.
The collection of light is performed by photomultipliers positioned at the top of each cell. The homogeneity of the response in the volume of a cell is provided by the high reflectivity of the inner walls, based on an air gap enclosed between two acrylic plates. Radioactive sources can be circulated around and inside the detector to precisely calibrate the detector throughout the relevant energy range.
The rejection of background induced by the reactor operation involves heavy lead and polyethylene shielding all around the detector. The residual background is then further reduced by an online rejection relying on a muon veto and the pulse shape discrimination of the light signals.

Several series of measurements allowed to fully characterize the background sources on site.
The design of the experiment is now essentially finalized and validated by seismic studies of the entire device .
The composition of the liquid has been optimized to provide both a good light output and good pulse shape discrimination between background events induced by cosmic rays and neutrino events.
Several subsets of the equipment are validated by prototypes: detector cell , muon veto, electronic cards for the acquisition and photomultipliers bases.

The installation of external shielding is in progress in the casemate of the experience. The manufacture of the different parts of the detector will be spread over this 2015 in order to start data taking in 2016.

The objectives and project development have been presented at several international conferences.

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. If proven, the existence of this 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(2?new)=0.17±0.04 and ?m2new= 2.3±0.1 eV2. 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.
The Stereo proposal provides a high sensitivity and a short time-scale measurement few meters away from the compact core of the ILL research reactor (Grenoble, France). Its originality lies in a clear signature of the possible new oscillation pattern by looking for the distortion of the energy spectrum and by using the phase shift of this 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 1.0 x 1.0 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 foreseen area “GAMS5”. 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 ? and neutron background. The required heavy structure, estimated at 70 tons, is well within the floor load specifications of the GAMS5 area, the strongest in the reactor building.
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.