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New results of measurements of the dependences of the flux of reactor antineutrinos and their spectrum on the distance from the center of the core of the SM-3 reactor (Dimitrovgrad, Russia) in the range from 6 to 12 m are presented. Additional measurements were conducted. The volume of accumulated experimental data was increased nearly by a factor of two. A model-independent analysis was performed on the basis of all data in order to determine the oscillation parameters $$\Delta m_{14}^{2}$$ and $$\sin^{2}2\theta_{14}$$ . The method of a coherent summation of measurement results makes it possible to demonstrate directly the oscillation effect. We observed the oscillation effect at a level of three standard deviations ( $$3.0\sigma$$ ) in the vicinities of parameters values of $$\Delta m_{14}^{2}\approx 7.25$$ eV $${}^{2}$$ and $$\sin^{2}2\theta_{14}\approx 0.26\pm 008\left({3.0}\right)\sigma$$ . Combining the results of the Neutrino-4 experiment with results for the gallium anomaly and for the reactor anomaly, we obtained the following value: $$\sin^{2}2\theta_{14}\approx 0.19\pm 0.04(4.6\sigma)$$ .

The experiment Neutrino-4 started in 2014 with a detector model and continued with a full-scale detector in 2016–2021. In this article, we describe all the steps of the preparatory work on this experiment. We present all results of the Neutrino-4 experiment with increased statistical accuracy provided to date. The experimental setup is constructed to measure the flux and spectrum of the reactor antineutrinos as a function of distance to the center of the active zone of the SM-3 reactor (Dimitrovgrad, Russia) in the range of 6–12 meters. Using all the collected data, we performed a model-independent analysis to determine the oscillation parameters Δm142 and sin22θ14. The method of coherent summation of measurement results allows us to directly demonstrate the oscillation effect. We present the analysis of possible systematic errors and the MC model of the experiment, which considers the possibility of the effect manifestation at the present precision level. As a result of the analysis, we can conclude that at currently available statistical accuracy, we observe the oscillations at the 2.9σ level with the parameters Δm142=(7.3±0.13st±1.16syst) eV2=(7.3±1.17) eV2 and sin22θ=0.36±0.12stat(2.9σ). Monte Carlo based statistical analysis gave an estimation of the confidence level at 2.7σ. We plan to improve the currently working experimental setup and create a completely new setup in order to increase the accuracy of the experiment by 3 times. We also provide a brief analysis of the general experimental situation in the search for sterile neutrinos.

We report Neutrino-4 experiment results of measurements of reactor antineutrinos flux and spectrum dependence on the distance in range 6–12 m from the center of the reactor core. The fit of experimental dependence with the law 1/L2, 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 Δm 14 2 and sin2 2θ14. The results of this analysis exclude area of reactor and gallium anomaly at CL more than 99.7% (>3σ) for values Δm 14 2 0.1. However, we observed an oscillation effect at CL 2.8σ in vicinity of Δm 14 2 Δm 14 2 ≈ 7.3 eV2 and sin2 2θ14 ≈ 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 being confirmed but at sufficiently large value of Δm 14 2 . Future prospects of the experiment are discussed.

First results on the energy spectrum and flux of reactor antineutrinos as a function of the distance L from the reactor core, measured in the Neutrino-4 experiment for 6 < L < 12 m, are presented. The flux L‑dependence fits to the 1/L2 form with a statistical significance of ~10% corresponding to 1.64σ, so that the measured spectrum deviates from the 1/L2 law at a confidence level ~90%. The measured energy spectrum deviates from the predicted one at a similar confidence level ~90%. That both deviations are best described with oscillation parameters in the same regions of $$\Delta m_{{14}}^{2} \approx 0.7{\text{-}}0.8$$ eV2and $${{\sin }^{2}}2{{\Theta }_{{14}}} \approx 0.10{\text{-}}0.15$$ boosts the overall confidence level to ~95% (2σ). These results suggest that antineutrinos oscillate to a sterile state, but still fall short of reliably establishing the oscillation phenomenon. At present, the possibility of mimicking the oscillation effect by systematic errors cannot be completely excluded.

An experimental search for sterile neutrinos has been carried out at a neutrino facility based on the SM-3 nuclear reactor in Dimitrovgrad, Russia. The movable detector with passive shielding against the external radiation may be positioned at a distance varying between 6 and 12 m from the center of the reactor. The antineutrino flux has for the first time been measured using a movable detector placed close to the antineutrino source. The accuracy of the measurements is largely restricted by the cosmic background. The results of the measurements performed at small and large distances are analyzed in terms of the sterile-neutrino model parameters Δm 14 2 and sin22θ14.

A laboratory for searching for oscillations of reactor antineutrinos has been created based on the SM-3 reactor in order to approach the problem of the possible existence of a sterile neutrino. The multisection detector prototype with a liquid scintillator volume of 350 L was installed in mid-2015. This detector can move inside the passive shield in a range of 6–11 m from the active core of the reactor. The antineutrino flux was measured for the first time at these short distances from the active core of the reactor by the movable detector. The measurements with the multisection detector prototype demonstrated that it is possible to measure the antineutrino flux from the reactor in the complicated conditions of cosmic background on the Earth’s surface.

In connection with the question of possible existence of sterile neutrino the laboratory on the basis of SM-3 reactor was created to search for oscillations of reactor antineutrino. A prototype of a neutrino detector with scintillator volume of 400 l can be moved at the distance of 6–11 m from the reactor core. The measurements of background conditions have been made. It is shown that the main experimental problem is associated with cosmic radiation background. Test measurements of dependence of a reactor antineutrino flux on the distance from a reactor core have been made. The prospects of search for oscillations of reactor antineutrino at short distances are discussed.

To check the existence of a sterile neutrino, a neutrino laboratory aimed at searching reactor antineutrino oscillations is created at the SM-3 reactor. A prototype of a neutrino detector with a scintillator volume of 400 L is moved at distances 6–11 m from the core of the reactor. Background conditions are measured. It is shown that the cosmic rays background is the main problem in the experiment. The prospects of the search for reactor antineutrino oscillations at short distances are discussed.

In view of the possibility of the existence of a sterile neutrino, test measurements of the dependence of the reactor antineutrino flux on the distance from the reactor core has been performed on SM-2 reactor with the Neutrino-2 detector model in the range of 6–11 m. Prospects of the search for reactor antineutrinos at short distances have been discussed.

At present, the possible existence of a sterile neutrino having a significantly smaller cross section of interaction with a substance than, for example, electronic antineutrinos from a reactor is being widely discussed. It has been suggested that, due to reactor antineutrino transition into a sterile state, one can observe both the oscillation effect at short distances (5–15 m) from the reactor and a deficiency of the reactor antineutrino flux at large distances. We have investigated the possibility of performing experiments in search for reactor antineutrino oscillations into a sterile state on research reactors. A model experiment has been carried out on a 16-MW WWR-M reactor at the Petersburg Nuclear Physics Institute with a view to implementing a full-scale experiment using a 100-MW SM-3 reactor at the Research Institute of Atomic Reactors (RIAR). Background conditions of these experiments have been studied for both reactors. It is concluded that the implementation of a full-scale Neutrino-4 experiment on the SM-3 reactor at the RIAR is possible.

The “Neutrino-4” experiment for the 100-MW SM-3 reactor has been developed with the aim of testing the reactor antineutrino anomaly at Petersburg Nuclear Physics Institute. The advantages of this reactor for studying the antineutrino anomaly are (i) a low background level and (ii) small dimensions (35 × 42 × 42 cm) of the active zone. Operation of a position-sensitive antineutrino detector comprising five working sections and moving so as to cover a region of distances within 6–13 m from the active zone has been simulated by the Monte-Carlo method. The range of experimental sensitivity with respect to the oscillation parameters Δm 2 and sin22θ is determined, which will make it possible to confirm the hypothesis of antineutrino oscillations into a sterile state.

In order to carry out research in the field of possible existence of a sterile neutrino the laboratory based on SM-3 reactor (Dimitrovgrad, Russia) was created to search for oscillations of reactor antineutrino. The prototype of a multi-section neutrino detector with liquid scintillator volume of 350 l was installed in the middle of 2015. It is a moveable inside the passive shielding detector, which can be set at distance range from 6 to 11 meters from the reactor core. Measurements of antineutrino flux at such short distances from the reactor core are carried out with moveable detector for the first time. The measurements with full-scale detector with liquid scintillator volume of 3 m^3 (5x10 sections) was started only in June, 2016. The today available data is presented in the article., 4 pages, 4 figures

In order to carry out research in the field of possible existence of sterile neutrino the laboratory based on SM-3 reactor (Dimitrovgrad, Russia) was created to search for oscillations of reactor antineutrino. The prototype of a multi-section neutrino detector with liquid scintillator volume of 350 l was installed in the middle of 2015. It is a moveable inside the passive shielding detector, which can be set at distance range from 6 to 11 meters from the reactor core. Measurements of antineutrino flux at such small distances from the reactor core carried out with movable detector for the first time. The measurements carried out with detector prototype demonstrated a possibility of measuring a reactor antineutrino flux in difficult conditions of cosmic background at Earth surface., Comment: 15 pages, 6 figures

The main goal of experiment "Neutrino-4" is to search for the oscillation of reactor antineutrino to a sterile state. Experiment is conducted on SM-3 research reactor (Dimitrovgrad, Russia). Data collection with full-scale detector with liquid scintillator volume of 3m3 was started in June 2016. We present the results of measurements of reactor antineutrino flux dependence on the distance in range 6– 12 meters from the center of the reactor. At that distance range, the fit of experimental dependence has good agreement with the law 1/L2. Which means, at achieved during the data collecting accuracy level oscillations to sterile state are not observed. In addition, the spectrum of prompt signals of neutrino-like events at different distances have been presented.