First Observation of the Oscillation Effect in the Neutrino-4 Experiment on the Search for the Sterile Neutrino

Abstract

We report Neutrino-4 experiment results of measurements of reactor antineutrinos flux and spectrum dependence on the distance in range 6-12 meters from the center of the reactor core. The fit of experimental dependence with the law 1 L 2 ⁄ , where L is the distance from the reactor center, gave satisfactory result with goodness of fit 81%. However, we discovered that the experimental neutrino spectrum is different from the calculated one. Using experimental spectrum, we performed the model independent analysis of restrictions on oscillation parameters ∆m 14 2 and sin 2 2 14 . The results of this analysis exclude area of reactor and gallium anomaly at CL more than 99.7% (> 3 ) for values ∆m 14 2 < 3eV 2 and sin 2 2θ 14 > 0.1 However, we observed an oscillation effect at CL 2.8 in vicinity of ∆m 14 2 ≈ 7.34eV 2 and sin 2 2θ 14 ≈ 0.39. The method of coherent addition of results of measurements, which allows us to directly observe the effect of oscillations, is proposed. The analysis of that effect is presented. In general, it seems that the effect predicted in gallium and reactor experiments is confirmed but at sufficiently large value of ∆m 14 2 . Future prospects of the experiment are discussed.

Status of light sterile neutrino searches

“…The Neutrino-4 experiment [122] is running since 2016 at the compact (35 × 42 × 42 cm 3 core size) 100 MW SM-3 HEU reactor in Dimitrovgrad, Russia. The neutrinos interact via the IBD reaction inside a 0.1% Gd-LS detector with a total volume of 1.8 m 3 divided in 10 × 5 sections.…”

“…However, the quoted result is in tension with the limits obtained by the other measurements. Figure 16: The plot shows the L/E dependence for the Neutrino-4 data points (blue triangles) compared to the expected oscillation signal for the best fit values (red dots) [122].…”

Light Sterile Neutrinos: Oscillations and Cosmology

“…Considering also the new set of data by the DANSS experiment shown for the first time in the EPS-HEP conference [14], together with NEOS [10], PROSPECT [15] and the previous DANSS [11] observations, the global model-independent preference in favor of the light sterile neutrino decreases to a ∼ 2.5σ significance and the new best-fit point, nearly degenerate with the one mentioned above, corresponds to ∆m 2 41 ≃ 0.4 eV 2 and |U e4 | 2 ≃ 0.01 [16]. Finally, the Neutrino-4 experiment [17] also provided results that indicate a strong preference in favor of a light sterile neutrino, with a mass defined by a large ∆m 2 41 ≃ 7 eV 2 . The best-fit of Neutrino-4, however, is in direct tension with the constraints obtained by PROSPECT [15] with the first 33 days of observations.…”

BEST potential in testing the eV-scale sterile neutrino explanation of reactor antineutrino anomalies

“…Both experiments, DANSS and Neutrino-4 claim exclusion of the joint anomaly at 2σ level [9,10], but their sensitivity to each of the two anomalies differ. The reactor antineutrino anomaly itself favors smaller mixing angle, than that of the joint anomaly.…”

“…while the Neutrino-4 collaboration claims 2.8σ evidence for oscillations of electron into sterile antineutrinos with parameters [10] ∆m 2 = 7.34 eV 2 , sin 2 2θ = 0.39 .…”