Search for eV Sterile Neutrinos -- The STEREO Experiment [TAUP 2017]

In the recent years, major milestones in neutrino physics were accomplished at nuclear reactors: the smallest neutrino mixing angle $\theta_{13}$ was determined with high precision and the emitted antineutrino spectrum was measured at unprecedented resolution. However, two anomalies, the first one related to the absolute flux and the second one to the spectral shape, have yet to be solved. The flux anomaly is known as the Reactor Antineutrino Anomaly and could be caused by the existence of a light sterile neutrino eigenstate participating in the neutrino oscillation phenomenon. Introducing a sterile state implies the presence of a fourth mass eigenstate, while global fits favour oscillation parameters around $\sin^{2}(2\theta)=0.09$ and $\Delta m^{2}=1.8\textrm{eV}^{2}$. The STEREO experiment was built to finally solve this puzzle. It is one of the first running experiments built to search for eV sterile neutrinos and takes data since end of 2016 at ILL Grenoble, France. At a short baseline of 10 metres, it measures the antineutrino flux and spectrum emitted by a compact research reactor. The segmentation of the detector in six target cells allows for independent measurements of the neutrino spectrum at multiple baselines. An active-sterile flavour oscillation could be unambiguously detected, as it distorts the spectral shape of each cell's measurement differently. This contribution gives an overview on the STEREO experiment, along with details on the detector design, detection principle and the current status of data analysis.Comment: 5 pages, 4 figures, contribution to the proceedings of the TAUP 2017 conferenc

Production and Properties of the Liquid Scintillators used in the Stereo Reactor Neutrino Experiment

The electron antineutrino spectrum in the Stereo reactor experiment (ILL Grenoble) is measured via the inverse beta decay signals in an organic liquid scintillator. The six target cells of the Stereo detector are filled with about 1800 litres of Gd-loaded liquid scintillator optimised for the requirements of the experiment. These target cells are surrounded by similar cells containing liquid scintillator without the Gd-loading. The development and characteristics of these scintillators are reported. In particular, the transparency, light production and pulse shape discrimination capabilities of the organic liquids are discussed.Comment: 10 pages, 4 figure

Novel Opaque Scintillator for Neutrino Detection

There is rising interest in organic scintillators with low scattering length for future neutrino detectors. Therefore, a new scintillator system was developed based on admixtures of paraffin wax in linear alkyl benzene. The transparency and viscosity of this gel-like material can be tuned by temperature adjustment. Whereas it is a colorless transparent liquid at temperatures around 40C it has a milky wax structure below 20C. The production and properties of such a scintillator as well as its advantages compared to transparent liquids are described.Comment: 11 pages, 6 figure

Indication for the disappearance of reactor electron antineutrinos in the Double Chooz experiment

The Double Chooz Experiment presents an indication of reactor electron antineutrino disappearance consistent with neutrino oscillations. A ratio of 0.944 $\pm$ 0.016 (stat) $\pm$ 0.040 (syst) observed to predicted events was obtained in 101 days of running at the Chooz Nuclear Power Plant in France, with two 4.25 GW$_{th}$ reactors. The results were obtained from a single 10 m$^3$ fiducial volume detector located 1050 m from the two reactor cores. The reactor antineutrino flux prediction used the Bugey4 measurement as an anchor point. The deficit can be interpreted as an indication of a non-zero value of the still unmeasured neutrino mixing parameter \sang. Analyzing both the rate of the prompt positrons and their energy spectrum we find \sang = 0.086 $\pm$ 0.041 (stat) $\pm$ 0.030 (syst), or, at 90% CL, 0.015 $<$ \sang $\ <$ 0.16.Comment: 7 pages, 4 figures, (new version after PRL referee's comments

First Measurement of θ13\theta_{13} with the final two detector setup of the double chooz experiment

The Double Chooz experiment is a reactor antineutrino disappearance experiment located at the nuclear power plant CHOOZ-B near the village of Chooz, France. The aim of the experiment is a high precision measurement of the neutrino oscillation amplitude $\sin^{2}(2\theta_{13})$. The experiment is built of two identical liquid scintillator detectors. They measure the neutrino oscillations on two distinct baselines using the flux of electron-antineutrinos originating from two nuclear reactors. After a single detector phase starting in 2011, the second detector has been commissioned in early 2015. In its first part, this work describes the design, implementation and commissioning of the data handling system responsible for the collection and transfer of the majority of the recorded experiment data. The system replaces the former single detector setup and provides several new fail-safe and diagnostic features. It is a multi-server MySQL-based setup interleaved with the laboratory systems in Chooz on the one end and the central computing facility of the in2p3 institute on the other end. The system is shown to have excellent performance allowing for both, unimpaired data acquisition in the laboratories and data processing in the computing centre. Additionally, the system is basis for an experiment-wide general purpose monitoring tool. In the second part of this work, a newly developed oscillation analysis approach for the Double Chooz data is presented. It utilises rate and spectral shape information. For the first time, it offers the possibility to include a full three flavour oscillation model into the oscillation analysis. Furthermore, a detailed treatment of energy scale non-linearities in the oscillation analysis is developed. Moreover, the novel adaptive modelling of spectral shape uncertainties reveals better insights to the treatment of the reactor flux uncertainties within the oscillation analysis. With the new approach, the first two detector data of the Double Chooz experiment is analysed in addition to the formerly recorded one-detector data. Due to the extended dataset, the Double Chooz experiment successfully confirms the existence of reactor antineutrino oscillations at the 6 σ confidence level. With the improved precision, this analysis presents a value for the oscillation amplitude of $\sin^{2}(2\theta_{13})=0.117\pm0.019$ assuming $\Delta m_{ee}^{2}=(2.44 \pm 0.09) \cdot 10^{-3}\text{eV}^{2}$ and normal mass hierarchy

Status of Anomalies and Sterile Neutrino Searches at Nuclear Reactors

Two anomalies at nuclear reactors, one related to the absolute antineutrino flux, one related to the antineutrino spectral shape, have drawn special attention to the field of reactor neutrino physics during the past decade. Numerous experimental efforts have been launched to investigate the reliability of flux models and to explore whether sterile neutrino oscillations are at the base of the experimental findings. This review aims to provide an overview on the status of experimental searches at reactors for sterile neutrino oscillations and measurements of the antineutrino spectral shape in mid-2021. The individual experimental approaches and results are reviewed. Moreover, global and joint oscillation and spectral shape analyses are discussed. Many experiments allow setting of constraints on sterile oscillation parameters, but cannot yet cover the entire relevant parameter space. Others find evidence in favour of certain parameter space regions. In contrast, findings on the spectral shape appear to give an overall consistent picture across experiments and allow narrowing down of contributions of certain isotopes

Review of Novel Approaches to Organic Liquid Scintillators in Neutrino Physics

Organic liquid scintillators have been used for decades in many neutrino physics experiments. They are particularly suited for the detection of low-energy neutrinos where energy and timing information is required. Organic liquid scintillators exhibit advantages such as high light yield, cost effectiveness, radio purity, and more. However, they also entail disadvantages, most prominently a lack of vertex resolution and particle identification. In recent years, various novel ideas have emerged to improve the performance of organic liquid scintillators. In this review, novel approaches to organic liquid scintillators in neutrino experiments as of 2022 are reviewed and their prospects and applications compared