Search for eV Sterile Neutrinos - The Stereo Experiment

TL;DR

The Stereo experiment aims to resolve the Reactor Antineutrino Anomaly by searching for eV-scale sterile neutrinos through short-baseline measurements of reactor antineutrinos, utilizing a segmented detector to detect spectral distortions indicative of sterile neutrino oscillations.

Contribution

This paper presents the design, methodology, and current status of the Stereo experiment, a novel short-baseline neutrino experiment specifically built to detect eV-scale sterile neutrinos.

Findings

Detector successfully deployed at ILL Grenoble

Data collection ongoing since late 2016

Initial analysis shows potential spectral distortions

Abstract

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 participating in the neutrino oscillation phenomenon. Introducing a sterile state implies the presence of a fourth mass eigenstate, global fits favour oscillation parameters around $\sin^2({2\theta}) \approx 0.09$ and $\Delta m^2 \approx 1\,\mathrm{eV}^2$. 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 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.