The first observation of effect of oscillation in Neutrino-4 experiment on search for sterile neutrino (continuation)

TL;DR

The Neutrino-4 experiment observed oscillation effects in reactor antineutrino flux, providing evidence for sterile neutrinos with specific oscillation parameters, and combined results with other anomalies to estimate neutrino mass.

Contribution

This study presents the first direct observation of oscillation effects in the Neutrino-4 experiment, using a novel model-independent analysis method to constrain sterile neutrino parameters.

Findings

Observed oscillation effect at 3.5σ confidence level near Δm²₁₄ ≈ 7.26 eV²

Estimated mixing angle sin²2θ₁₄ ≈ 0.38, combined with gallium anomaly results to about 0.35 ± 0.07

Suggested neutrino mass m_β ≈ 0.8 eV based on oscillation parameters

Abstract

We present the results of the Neutrino-4 experiment on search for a sterile neutrino. The experiment has been carried out on the SM-3 reactor having a compact active zone of $42\times42\times35\textrm{cm}^3$ and operating on the highly enriched uranium-235 at 90 MW thermal power. We report the results of the Neutrino-4 experiment of measurements of reactor antineutrino flux and spectrum dependence on the distance in the range 6-12 meters from the center of the reactor core. Using the measured spectrum and the distance dependence of antineutrino flux, we performed the model independent analysis of restrictions on the oscillation parameters $\Delta m^2_{14}$ and $\sin^2 2\theta_{14}$. The method of coherent addition of results of measurements is proposed. It allows us to directly observe the effect of oscillations. We observed the oscillation effect at CL $3.5\sigma$ in the vicinity of $\Delta m^2_{14} \approx 7.26\textrm{eV}^2$ and $\sin^2 2\theta_{14} \approx 0.38$. Combining the result of the Neutrino-4 experiment and the result of the gallium anomaly effect we obtained value $\sin^2 2\theta_{14} \approx 0.35 \pm 0.07 (5\sigma)$. The analysis of systematics effects is presented. Comparison with results of other experiments is presented. Future prospect of the experiment is discussed. It is necessary to notice that obtained values $\sin^2 2\theta_{14} \approx 0.35 \pm 0.07 (5\sigma)$ and $\Delta m^2_{14} \approx (7.3 \pm 0.7)\textrm{eV}^2$ allow make assessment on the mass of a neutrino: $m_{\beta} \approx 0.8\textrm{eV}$.