Sterile Neutrinos at Reactors

NAntonin Vacheret (right) gave a review of the study of sterile neutrinos that is possible with reactor experiments.

In 2014 there were some surprises, coming from Double-Chooz, Reno and Daya Bay. It is difficult to understand if the energy spectrum distortions observed there is coming from a theoretical shortcoming in the prediction or if there is more to it. The NEOS experiment is an intermediate short baseline experiment in Korea. It uses a very high-power commercial reactor (2.8 GWth). The detector is relatively simple, a liquid scintillator container. They took data from August 2015 and May 2016. Their signal to noise and energy resolutions are very good, and they measured a very clear distortion at 5 MeV.

If one uses the NEOS data to do a sterile neutrino oscillation analysis, one cannot really tell very well if the pattern observed in the neutrino energy is meaningful or not. No significant effect is foundso the question is if the signficicance can be improved with a subtraction from the RENO data.

If one looks at previous data t oinfer which isotope could be causing the reactor anomaly, one sees deviations in cross section per fission on 235U at 2.2 sigma. A combined analysis of NEOS and DB was able to reject Plutonium isotopes to be the sole responsibles for the 5 MeV bump. It appears that only experiments at 10 meters or so from the reactor can really put strong constraints in the 1 eV^2 region. The possibility that the 5 MeV anomaly is due to 235U further motivates measurements at reactors that use highly enriched 235U fuel.

There are several VSBL experiments around. Apart from NEOS there are two in Russia, two in Europe, and two in the US. Vacheret showed a table comparing the parameters of these detectors. Most of them use liquid scintillator and Gd doping. Baselines are in the 10m range, and the volumes are constrained to be in the 1-3 ton range, due to the constraints of being so close to the detector.

The detector segmentation allows a relative measurement along the oscillation length, that can be combined with the energy measurement. Finer segmentation provides additional capability to rejeect background and select the positron energy. E.g., Prospect has 120x2D segments in a 3-ton system of 6Li loaded liquid scintillator.