The Reactor Antineutrino Anomaly

TL;DR

This work re-evaluates reactor antineutrino flux predictions using updated spectra, finding a≈3.5% increase in predicted flux per fission. When applied to a compendium of short-baseline reactor experiments, the observed-to-predicted rate shows a consistent deficit, termed the reactor antineutrino anomaly, with a combined rate significance around 98–99% C.L. The authors interpret this deficit within a 3+1 sterile-neutrino framework, deriving $|oldsymbol{ abla}m_{ m new}^2|>1.5$ eV$^2$ and $ ext{sin}^2(2 heta_{ m new})=0.14\, ilde{\pm}\,0.08$ (95%), and demonstrate that allowing for a sterile state can elevate or suppress constraints on the standard mixing angle $ heta_{13}$ depending on normalization. They propose near-detector measurements and very-short-baseline source experiments as decisive tests of the anomaly, with implications for interpreting current and upcoming multi-detector reactor experiments. The results motivate targeted experimental tests to confirm or refute a new neutrino state while highlighting the interplay between flux modeling, reactor physics, and oscillation phenomenology.

Abstract

Recently new reactor antineutrino spectra have been provided for 235U, 239Pu, 241Pu and 238U, increasing the mean flux by about 3 percent. To good approximation, this reevaluation applies to all reactor neutrino experiments. The synthesis of published experiments at reactor-detector distances <100 m leads to a ratio of observed event rate to predicted rate of 0.976(0.024). With our new flux evaluation, this ratio shifts to 0.943(0.023), leading to a deviation from unity at 98.6% C.L. which we call the reactor antineutrino anomaly. The compatibility of our results with the existence of a fourth non-standard neutrino state driving neutrino oscillations at short distances is discussed. The combined analysis of reactor data, gallium solar neutrino calibration experiments, and MiniBooNE-neutrino data disfavors the no-oscillation hypothesis at 99.8% C.L. The oscillation parameters are such that |Delta m_{new}^2|>1.5 eV^2 (95%) and sin^2(2θ_{new})=0.14(0.08) (95%). Constraints on the theta13 neutrino mixing angle are revised.

Figures (16)

• Figure 1: Correlation matrix of 19 measurements of reactor neutrino experiments operating at short baselines. Experiment labels are given in Table \ref{['tab:other']}.
• Figure 2: 90% C.L. exclusion domains obtained in the $\Delta m^2$-$\sin^2(2\theta)$ plane from a raster scan of Bugey-3's data. Our result (continuous line) is in good agreement with the original result from Bugey3 (dashed line), excluding oscillations such that $0.06<\Delta m^2<1$ eV$^2$ for $\sin^2(2\theta)>0.05$.
• Figure 3: Ratio of measured to expected positron energy spectra of the ILL neutrino experiment (data points extracted from ILL95). We show the best fit line with oscillations, along with the no-oscillation line, from our shape-only fit. The short error bars are statistical and the longer ones include the 11% systematic error described in the text.
• Figure 4: Allowed regions in the $\sin^2(2\theta_{\rm new})-\Delta m_{\rm new}^2$ plane obtained from the fit of the reactor neutrino data, without ILL-shape information, but with the stringent oscillation constraint of Bugey-3 based on the 40 m/15 m ratios to the 3+1 neutrino hypothesis, with $\sin^2(2\theta_{13})=0$. The best-fit point is indicated by a star.
• Figure 5: Illustration of the short baseline reactor antineutrino anomaly. The experimental results are compared to the prediction without oscillation, taking into account the new antineutrino spectra, the corrections of the neutron mean lifetime, and the off-equilibrium effects. Published experimental errors and antineutrino spectra errors are added in quadrature. The mean averaged ratio including possible correlations is $0.943\pm 0.023$. The red line shows a possible 3 active neutrino mixing solution, with $\sin^2(2\theta_{13})=0.06$. The blue line displays a solution including a new neutrino mass state, such as $|\Delta m_{\rm new,R}^2 | \gg 1$ eV$^2$ and $\sin^2(2\theta_{\rm new,R})=0.12$ (for illustration purpose only).