First results from the search for an excess of $\barν_{e}$ events in JSNS$^2$

TL;DR

JSNS$^2$ provides a direct test of the LSND $ar{\nu}_e$ excess using muon-decay-at-rest neutrinos and a gadolinium-loaded liquid scintillator at a 24 m baseline. The analysis combines tight IBD event selection (prompt/delayed/paired), robust cosmic-ray suppression, and data-driven background control, complemented by a multivariate likelihood to separate signal from backgrounds. In the 2022 dataset, two events were observed, in line with the background expectation of $2.3\pm0.4$, yielding no definitive LSND-like signal; normalization- and oscillation-based predictions both imply modest expected signals ($\sim 1.1$–$1.2$ events) under the LSND hypothesis. The study establishes a 90\% CL exclusion region for short-baseline $\bar{\nu}_\mu \rightarrow \bar{\nu}_e$ oscillations and outlines JSNS$^2$-II with two detectors to achieve a relative baseline measurement and reduce systematic uncertainties in future measurements.

Abstract

The JSNS$^2$ (J-PARC Sterile Neutrino Search at the J-PARC Spallation Neutron Source) experiment at the Material and Life Science Facility (MLF) of J-PARC is designed to directly test an excess on $\barν_{e}$ events which was indicated by LSND (Liquid Scintillator Neutrino Detector). The combination of a short-pulsed proton beam and a gadolinium-loaded liquid scintillator provides an excellent signal-to-noise ratio. In this article, we report the first results of a direct test based on data collected in 2022. After applying all event selection criteria, two events are observed, consistent with the expected background of 2.3$\pm$0.4 events. No excess of $\barν_e$ events are seen in this report, however the expected number of events due to LSND anomaly is 1.1$\pm$0.5, thus this result is not yet conclusive. Data taking has been ongoing since 2021 and will continue in future runs. In addition, a new far detector has recently been constructed for the second phase experiment, JSNS$^2$-II, marking an important milestone toward forthcoming measurements.

Figures (7)

• Figure 1: (a) Expected timing distribution of IBD prompt candidates relative to the beam start, obtained from MC simulation. (b) PSD selection efficiency for IBD events as a function of energy. The vertical axes of (a) is in arbitrary units.
• Figure 2: (a) Expected and observed reconstructed energy spectra of IBD delayed candidates. (b) Expected $\Delta$VTX$_{OB-d}$ distribution (black: data + MC) and that from beam neutrons (blue: data). The red line indicates the applied selection threshold. All plots are area normalized.
• Figure 3: (a) $\Delta T_{p-d}$ distributions for the IBD (MC: black), cosmogenic neutron samples from data (red) and MC (orange). The plot is area normalized. (b) $\Delta$VTX$_{p-d}$. Black corresponds to the IBD signal, blue shows the accidental background and red shows the cosmogenic neutrons. For (b), all distributions are area normalized to one after applying the $\Delta$VTX$_{p-d} < 60$ cm selection.
• Figure 4: PSD score distribution after applying the selection criteria listed in Table \ref{['tab:EventSelection']}. The inset shows an expanded view of the same distribution. The vertical axis indicates the number of events.
• Figure 5: Distributions of (a) $\Delta T_{beam-p}$, (b) $\Delta T_{p-d}$, (c) $\Delta$VTX$_{p-d}$ and (d) z vertex (height direction) after the PSD application. The vertical axes indicate number of events.