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

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.

💡 Research Summary

The JSNS² (J-PARC Sterile Neutrino Search at the J-PARC Spallation Neutron Source) experiment is designed to provide a decisive test of the electron‑antineutrino ( ¯νₑ ) excess reported by the LSND experiment. Both experiments use muon‑decay‑at‑rest (μ⁺ → e⁺ + νₑ + ¯ν_μ) as the neutrino source, but JSNS² upgrades the detection technology with a gadolinium‑loaded liquid scintillator (Gd‑LS) and a short‑pulsed 3 GeV, 1 MW proton beam that delivers two 100 ns bunches at 25 Hz, separated by 600 ns. The timing structure allows a clean separation of the signal (inverse‑beta‑decay, IBD: ¯νₑ + p → e⁺ + n) from beam‑related and cosmogenic backgrounds.

The detector consists of a 50‑ton cylindrical volume: an inner target region (≈ 17 t of Gd‑LS with 0.1 % Gd by mass and 10 % di‑isopropylnaphthalene, DIN) surrounded by a gamma‑catcher and an outer veto region filled with pure LS. Light is collected by 96 ten‑inch PMTs (target + gamma‑catcher) and 24 veto PMTs; signals are digitized with 500 MHz, 8‑bit flash ADCs. Calibration is performed with ²⁵²Cf neutron sources (providing the 8 MeV γ‑cascade from n‑Gd capture) and cosmogenic Michel electrons (≈ 53 MeV endpoint). The energy scale uncertainty after calibration is 0.8 %; the fiducial‑volume uncertainty is currently about 20 %.

Event selection proceeds in several stages. Prompt candidates must have reconstructed energy 20–60 MeV (100 % efficiency by design) and occur 2–10 µs after the beam pulse, exploiting the 2.2 µs muon lifetime to suppress prompt beam‑related backgrounds. A pulse‑shape‑discrimination (PSD) algorithm, refined using a γ‑ray control sample, provides a neutron‑rejection efficiency of 87 % ± 9 % while retaining > 99.7 % of true IBD events. Delayed candidates are required to have 7–12 MeV energy, consistent with the ∼ 8 MeV γ‑cascade from Gd capture, and a capture time consistent with the ∼ 30 µs mean. Spatial correlation cuts (ΔVTX p‑d ≤ 60 cm) and timing cuts (ΔT p‑d ≤ 100 µs) link prompt and delayed signals. To further reject beam‑related neutrons that thermalize in the target, a distance cut ΔVTX OB‑d ≥ 110 cm between any activity during the beam window and the delayed vertex is applied, achieving a 93 % neutron‑rejection efficiency. Additional muon and Michel‑electron vetoes reduce cosmic‑ray backgrounds. The cumulative selection efficiency for a sterile‑neutrino‑induced IBD signal is 12.4 % (+2.1 / ‑2.2 %).

The data set analyzed here corresponds to the 2022 physics run, amounting to 0.82 × 10²² protons on target (POT), i.e., roughly 16 % of the total exposure planned through 2025 (5.14 × 10²² POT). After all cuts, two candidate events remain. The expected background, derived from a combination of Monte‑Carlo simulations and control samples, is 2.3 ± 0.4 events. This observation is fully compatible with background‑only expectations. Under the LSND‑derived sterile‑neutrino oscillation parameters, the expected signal would be 1.1 ± 0.5 events, which is not statistically distinguishable from the observed count given the current uncertainties. Consequently, the result is inconclusive regarding the LSND anomaly.

The paper also outlines the next phase, JSNS²‑II, which will add a far detector (≈ 30 t) at a baseline of ~48 m, complementing the existing near detector at 24 m. Simultaneous measurements at two baselines will dramatically improve sensitivity to short‑baseline oscillations, allowing a robust test of the sterile‑neutrino hypothesis and the ability to map out the (Δm², sin²2θ) parameter space with high confidence.

In summary, the first physics results from JSNS² show no evidence for an excess of electron‑antineutrino events beyond the predicted background, but the statistical power is still limited. Ongoing data taking (targeting > 5 × 10²² POT) and the deployment of the far detector in JSNS²‑II are expected to increase the sensitivity by an order of magnitude, potentially confirming or refuting the LSND anomaly and providing critical insight into the existence of sterile neutrinos.