The first observation of oscillation effect in Neutrino-4 experiment and analysis of measurement results

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. Using experimental spectrum, we performed the model independent analysis of restrictions on oscillation parameters <?CDATA ${\rm{\Delta }}{{\rm{m}}}_{14}^{2}$?> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi mathvariant="normal">Δ</mml:mi> <mml:msubsup> <mml:mi mathvariant="normal">m</mml:mi> <mml:mrow> <mml:mn>14</mml:mn> </mml:mrow> <mml:mn>2</mml:mn> </mml:msubsup> </mml:mrow> </mml:math> and sin 2 2 θ 14 . The results of this analysis exclude area of reactor and gallium anomaly at C.L more than 99.7% (&gt; 3 σ ) for values <?CDATA $\Delta {{\rm{m}}}_{14}^{2}\lt 3\,{{\rm{eV}}}^{2}$?> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>Δ</mml:mi> <mml:msubsup> <mml:mi mathvariant="normal">m</mml:mi> <mml:mrow> <mml:mn>14</mml:mn> </mml:mrow> <mml:mn>2</mml:mn> </mml:msubsup> <mml:mo>&lt;</mml:mo> <mml:mn>3</mml:mn> <mml:mspace width="0.25em" /> <mml:msup> <mml:mrow> <mml:mi mathvariant="normal">eV</mml:mi> </mml:mrow> <mml:mn>2</mml:mn> </mml:msup> </mml:mrow> </mml:math> and sin 2 2θ 14 &gt; 0.1. However, we observed an oscillation effect at C.L 2.8 σ in vicinity of <?CDATA $\Delta {{\rm{m}}}_{14}^{2}\approx 7.3\,{{\rm{eV}}}^{2}$?> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>Δ</mml:mi> <mml:msubsup> <mml:mi mathvariant="normal">m</mml:mi> <mml:mrow> <mml:mn>14</mml:mn> </mml:mrow> <mml:mn>2</mml:mn> </mml:msubsup> <mml:mo>≈</mml:mo> <mml:mn>7.3</mml:mn> <mml:mspace width="0.25em" /> <mml:msup> <mml:mrow> <mml:mi mathvariant="normal">eV</mml:mi> </mml:mrow> <mml:mn>2</mml:mn> </mml:msup> </mml:mrow> </mml:math> and sin 2 2 θ 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 being confirmed but at sufficiently large value of <?CDATA ${\rm{\Delta }}{{\rm{m}}}_{14}^{2}$?> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi mathvariant="normal">Δ</mml:mi> <mml:msubsup> <mml:mi mathvariant="normal">m</mml:mi> <mml:mrow> <mml:mn>14</mml:mn> </mml:mrow> <mml:mn>2</mml:mn> </mml:msubsup> </mml:mrow> </mml:math> . An additional analysis of the measurements was performed taking into account inhomogenei ty of the detector and background instability.