today, there are still some experimental observations that question the completeness of our understanding. The most prominent of these observations is the so-called reactor antineutrino anomaly (RAA) (see Figure 3). A number of short baseline reactor-neutrino experiments have measured a significantly lower anti-electron neutrino () flux compared to the theoretical predictions (a 2,7 deviation). Further experimental anomalies are the unexpected appearance of in a short-baseline beam (LSND anomaly) as well as the disappearance of at short distances during the calibration phase of the GALLEX and SAGE experiments known as the gallium neutrino anomaly. These anomalies could signify that our understanding of neutrino oscillations is not yet complete and that neutrinos oscillate into another 4th neutrino species. 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. 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. Results as of December 2019 Initial results were released in 2018 exploiting a dataset of 66 days of reactor turned on. Most of the parameter space that could account for the RAA was excluded at a 90% confidence level. Updated results in December 2019 are based on ≈ 65,500 detected neutrinos (combined phase 1 and 2, 179 days of reactor-on data). Using the current data the region of exclusion is further extended (see figure 4). External links Website of the STEREO experiment STEREO experiment record on INSPIRE-HEP References Neutrino observatories 2016 establishments in France Particle experiments