Search for Very-Short-Baseline Oscillations of Reactor Antineutrinos with the SoLid Detector

TL;DR

This paper reports the first results from the SoLid detector measuring reactor antineutrinos at very short baselines, using a novel highly granular detector to search for sterile neutrino oscillations and constrain the Reactor Antineutrino Anomaly.

Contribution

It introduces a new highly granular detector technology and provides the first scientific results from reactor antineutrino measurements at very short baselines.

Findings

29479 antineutrino candidates detected

Signal-to-background ratio of 0.27 achieved

Constraints placed on sterile neutrino oscillation parameters

Abstract

In this letter we report the first scientific result based on antineutrinos emitted from the BR2 reactor at SCK CEN. The SoLid experiment uses a novel type of highly granular detector whose basic detection unit combines two scintillators, PVT and 6LiF:ZnS(Ag), to measure antineutrinos via their inverse-beta-decay products. An advantage of PVT is its highly linear response as a function of deposited particle energy. The full-scale detector comprises 12800 voxels and operates over a very short 6.3--8.9 m baseline from the reactor core. The detector segmentation and its 3D imaging capabilities facilitate the extraction of the positron energy from the rest of the visible energy, allowing the latter to be utilised for signal-background discrimination. We present a result based on 280 reactor-on days (55 MW mean power) and 172 reactor-off days, respectively, of live data-taking. A total of 29479 $\pm$ 603 (stat.) antineutrino candidates have been selected, corresponding to an average rate of 105 events per day and a signal-to-background ratio of 0.27. A search for disappearance of antineutrinos to a sterile state has been conducted using complementary model-dependent frequentist and Bayesian fits, providing constraints on the allowed region of the Reactor Antineutrino Anomaly.