[Paper Review] Search for a sterile neutrino with the STEREO detector at ILL

S. Zsoldos|arXiv (Cornell University)|Feb 1, 2016

Neutrino Physics Research5 references3 citations

TL;DR

This paper presents the STEREO experiment at the Institut Laue-Langevin (ILL), designed to test the reactor antineutrino anomaly by measuring the energy spectrum of antineutrinos from a compact reactor at distances of 9–11 meters. By detecting inverse beta decay events via delayed coincidence between positron and neutron signals in a gadolinium-doped liquid scintillator, STEREO aims to confirm or rule out the existence of a short-baseline sterile neutrino with Δm² ≈ 0.1–1 eV², achieving 95% CL sensitivity to the anomaly within two years of data-taking.

ABSTRACT

In 2011, a re-evaluation of the antineutrino spectrum emitted by nuclear reactors revealed a 6% deficit between the observed flux and the expected one. This anomaly is significant at 2.9$σ$ and can be explained by a new oscillation at short range due to a light sterile neutrino, with parameter $Δm^2=\mathcal{O}( ext{eV})$. The STEREO detector in construction at ILL will be the first ever to measure with precision its antineutrino spectrum and flux at very short distance (9-11m) and it will be able to confirm or reject the existence of this light sterile neutrino. In the following we introduce the relevant parameter to study the neutrino oscillation, then the STEREO detector and its discovery potential.

Motivation & Objective

• Address the reactor antineutrino anomaly, a 2.9σ deficit in observed flux compared to predictions, which may indicate sterile neutrino oscillations.
• Investigate the possibility of a light sterile neutrino state with Δm² ≈ 0.1–1 eV², as suggested by reactor and gallium experiments.
• Overcome uncertainties in antineutrino flux predictions by measuring the energy spectrum shape rather than just the total flux.
• Achieve high sensitivity to sterile neutrino oscillations through precise spectral distortion measurements at very short baselines.
• Provide a definitive test of the sterile neutrino hypothesis using a dedicated, high-precision detector at a compact research reactor.

Proposed method

• Use a segmented detector with six 40×40×90 cm³ liquid scintillator cells doped with gadolinium to detect antineutrinos via inverse beta decay (IBD): ν̄_e + p → e⁺ + n.
• Measure the prompt signal from positron annihilation (visible energy E_vis = E_ν̄_e − 0.782 MeV) and the delayed signal from neutron capture on gadolinium (8 MeV γ-cascade).
• Employ a γ-catcher layer between target cells to capture escaping gamma rays and improve energy reconstruction accuracy.
• Use a water-Cherenkov muon veto above the detector to suppress cosmic ray backgrounds.
• Apply heavy shielding (lead and polyethylene) to reduce accidental backgrounds from thermal neutrons and γ-rays.
• Use a thick acrylic buffer above photomultipliers to enhance light collection and suppress background.

Experimental results

Research questions

• RQ1 Can the reactor antineutrino anomaly be explained by oscillations into a light sterile neutrino state with Δm² ≈ 0.1–1 eV²?
• RQ2 Does the energy spectrum of reactor antineutrinos exhibit measurable distortions at short baselines (9–11 m) due to sterile neutrino mixing?
• RQ3 Can the STEREO detector distinguish spectral distortions from sterile neutrino oscillations from systematic uncertainties in the antineutrino spectrum?
• RQ4 What is the sensitivity of the STEREO detector to the sterile neutrino parameter space defined by the reactor anomaly?
• RQ5 How effectively can the detector suppress accidental and correlated backgrounds to achieve the required signal-to-noise ratio?

Key findings

• The STEREO detector is designed to achieve 95% confidence level sensitivity to the reactor antineutrino anomaly within two years of data-taking.
• The experiment is expected to reduce accidental background rates to less than 1 mHz through optimized shielding and timing resolution.
• Correlated background from fast neutrons is expected to be suppressed to ~1 mHz via polyethylene shielding and a muon veto system.
• The detector’s energy resolution and spectral measurement capability allow it to probe the sterile neutrino parameter space with Δm² ≈ 0.1–1 eV² and sin²(2θ_new) ≈ 0.17.
• The STEREO experiment will be the first to measure the antineutrino spectrum at very short baselines (9–11 m) with high precision, enabling direct testing of spectral distortions.
• The best-fit parameters for the sterile neutrino hypothesis (Δm² = 2.3 ± 0.1 eV², sin²(2θ_new) = 0.17 ± 0.04) lie within the region accessible to STEREO’s sensitivity.