First observation of the oscillation effect in the neutrino-4 experiment on the search for the sterile neutrino

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 $1/L^2$, 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 $\Delta m_{14}^2$ and $\sin^22\theta_{14}$. The results of this analysis exclude area of reactor and gallium anomaly at CL more than $99.7\,\%$ (${>}\,3\sigma$) for values $\Delta m_{14}^2<3\,$ eV$^2$ and $\sin^22\theta_{14}>0.1$. However, we observed an oscillation effect at CL $2.8\sigma$ in vicinity of $\Delta m_{14}^2\approx7.3\,$ eV$^2$ and $\sin^22\theta_{14}\approx0.39$. 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 $\Delta m_{14}^2$. Future prospects of the experiment are discussed.