STEREO experiment

[2] To be able to detect the neutrinos radiated from the reactor, the detector is filled up with 1800 litres of organic liquid scintillator which is doped with gadolinium.

When the positron moves through the scintillator a light signal is produced, which is detected by the 48 photomultiplier tubes (PMTs) placed at the top of the detector cells.

[2] To achieve this high sensitivity the 6 inner detector cells are surrounded by a liquid scintillator (without gadolinium) which acts as a "Gamma-Catcher" detecting in- and outgoing gamma radiation.

Although neutrino oscillation is a phenomenon that is quite well understood today, there are still some experimental observations that question the completeness of our understanding.

A number of short baseline reactor-neutrino experiments have measured a significantly lower anti-electron neutrino (νe) flux compared to the theoretical predictions (a 2,7 σ deviation).

However measurements of the decay width of the Z boson at the Large Electron–Positron Collider (LEP) exclude the existence of a light 4th "active" (i.e. interacting via the weak force) neutrino.

[10] Hence the oscillation into additional light "sterile" neutrinos is considered as a possible explanation of the observed anomalies.

In addition sterile neutrinos appear in many prominent extensions of the Standard Model of particle physics, e.g. in the seesaw type 1 mechanism.

Figure 2: Comparison of the different spectra at 10 m and 12 m distance to the reactor. The black line shows the case without oscillation into sterile neutrinos while the blue and red show the case including the oscillation into a light sterile neutrino
Figure 3: The reactor-antineutrino-anomaly (RAA)
Figure 4: Final results of the STEREO experiment. The 95% confidence limit parameter space for a sterile neutrino explanation of the RAA is shown in grey. Values right of the exclusion curves in red and blue are rejected.