Abstract
Anomalies in past neutrino measurements have led to the discovery that these particles have non-zero mass and oscillate between their three flavours when they propagate. In the 2010s, similar anomalies observed in the antineutrino spectra emitted by nuclear reactors have triggered the hypothesis of the existence of a supplementary neutrino state that would be sterile, that is, not interacting by means of the weak interaction1. The STEREO experiment2,3,4,5,6 was designed to investigate this conjecture, which would potentially extend the standard model of particle physics. Here we present an analysis of the full set of data generated by STEREO, confirming observed anomalies while rejecting the hypothesis of a light sterile neutrino. Installed at the Institut Laue–Langevin (ILL) research reactor, STEREO accurately measures the antineutrino energy spectrum associated to the fission of 235U. The segmentation of the detector and its very short distance to the compact core are crucial properties of STEREO for our analysis. The measured antineutrino energy spectrum suggests that anomalies originate from biases in the nuclear experimental data used for the predictions7,8. Our result supports the neutrino content of the standard model and establishes a new reference for the 235U antineutrino energy spectrum. We anticipate that this result will allow progress towards finer tests of the fundamental properties of neutrinos but also to benchmark models and nuclear data of interest for reactor physics9,10 and for observations of astrophysical or geoneutrinos11,12.
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Data availability
All the results (sterile neutrino search and spectrum analysis) and the elements necessary to reproduce them are provided in the supplementary materials available in ref. 38. In accordance with the ILL data policy, all raw data are available in refs. 50,51,52,53,54,55,56,57,58. Further information is available on request by contacting the STEREO Collaboration.
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Acknowledgements
This work is financed by the French National Research Agency (ANR) within the project ANR-13-BS05-0007 and the ‘Investments for the Future’ programme ENIGMASS LabEx (ANR-11-LABX-0012). The authors are grateful for the technical and administrative support of the ILL for the installation and operation of the STEREO detector. We further acknowledge the support of the CEA, the CNRS/IN2P3 and the Max Planck Society.
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All listed authors have contributed to the present publication. The different contributions span from the design and construction of the STEREO detector and its installation on the reactor site to the acquisition of data and the development of the simulation and analysis software. The manuscript was reviewed by the whole collaboration (https://www.stereo-experiment.org/) and all authors approved its final version; the authors’ names are listed alphabetically.
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Extended data figures and tables
Extended Data Fig. 1 STEREO data taking.
The left-hand axis refers to the reactor power graph (orange), whereas the associated cumulative number of detected antineutrinos (dark green) can be read on the right-hand axis. Three reactor cycles occurred during phase II and four other cycles in phase III. The alternation of reactor-on (‘ON’) and reactor-off (‘OFF’) periods is a key aspect of the experiment to accurately control the subtraction of cosmogenic background.
Extended Data Fig. 2 Accurate control of the energy scale.
a, Experimental and simulated reconstructed energy spectra of the 54Mn source, with the source being 45 cm above the bottom of cell 4. b, Experimental and simulated 12B beta spectra. Only statistical uncertainties are shown in these two first plots. c, Relative difference of the experimental and simulated position of γ-peaks from various radioactive sources (red points). For multi-γ sources, all photons are reconstructed in the same event. n-H and n-Gd peaks originate from the Am-Be source. The reconstructed energies differ from the physical ones because of the quenching effect. The statistical uncertainties are negligible for both the simulation and the data. The dominant contribution comes from the systematics in the determination of the peak positions (including time stability, choice of fit function and fit range). d, Ratio of measured and simulated 12B spectrum (red points). The error bars include the systematic uncertainties of the simulated 12B spectrum (dominant contribution) and the statistical uncertainty of the measured spectrum. In c and d, the black line and blue band correspond to the global fit of all residuals and its associated uncertainty, respectively. Here a third-order polynomial is used as a model of the relative distortions between the experimental and simulated energy scales. This function applies directly to the residuals in c, whereas it is converted into spectrum shape distortions to fit the points in d following the formalism described in ref. 61. All uncertainties are taken at a 68% CL (noted 1σ hereafter). Only phase III results are shown here; similar results are obtained for phase II (ref. 4).
Extended Data Fig. 3 Time evolution of the detector response.
a, Evolution of the CCs for the six target cells of the STEREO detector. The decrease over time can mostly be explained by a reduction of the attenuation length of the liquid scintillator. The two dashed lines represent the fine-tuning dates of the simulation. The solid line marks the transition between phases II and III and the corresponding change of parameter set for the simulation fine-tuning. The very small discontinuity of the CCs at this point validates the fact that two fine-tunings of the simulation are sufficient for a precise control of the detector response over the whole experiment. b, Residual fluctuations over time of the position of the 2.2-MeV peak from capture of cosmic neutrons on hydrogen (n-H peak), obtained after application of the energy reconstruction with the evolving CCs. The 1σ uncertainty of each point is taken from the fit of the experimental peaks with a crystal-ball function. The root mean square of the relative deviations to the mean value (0.25%) is used as an estimate of the systematic uncertainty on the time stability of the energy reconstruction. c, Time evolution of the energy resolution of the n-H peak, setting the scale for the average resolution in the full target volume and its time evolution owing to the slow decrease of collected light. The fine-tuning of the MC accounts for the different mean resolutions in phases II and III.
Extended Data Fig. 4 Stability of the PSD variable.
a, Illustration of the PSD method, which separates electron recoils and proton recoils based on the shape of the collected light pulse. The PSD variable is defined as Qtail/Qtot. b, PSD distributions of Am-Be events from a run in May 2019 (reference date for the correction) and another in November 2020 (end of the data taking), showing the distortion over time (mostly a shift of the distribution). All plots are for cell 4 and reconstructed energies between 1.625 and 4.125 MeV. c, Evolution of the shift required to align the PSD distribution of any given date to the reference of May 2019. The time–temperature polynomial fit is shown. d1, Evolution of the shift with temperature: once the time dependence is corrected, the evolution follows a first-order polynomial. d2, Evolution of the shift over time: once the temperature dependence is corrected, the evolution follows a second-order polynomial. All uncertainties shown are statistical at 1σ CL.
Extended Data Fig. 5 IBD signal and backgrounds.
a, Selection cuts for IBD candidates (see text for details). The topology cut on \({E}_{{\rm{prompt}}}^{{\rm{cell}}}\) was loosened for phase III because of detector ageing. b, Illustration of the extraction of the IBD signal using event distributions based on the PSD variable Qtail/Qtot (after application of the selection cuts) for phase III. The two populations (electron recoils and proton recoils) are well separated. The pair rate in ON (blue) is the sum of: (1) correlated background pairs (red, rescaled from OFF data), (2) accidental pairs (grey squares), (3) the IBD signal (green, modelled by a Gaussian whose integral gives the IBD rate in this bin). The scaling factor a on the OFF distribution depends on environmental parameters (such as atmospheric pressure) and ON/OFF relative running time; it is treated as a free parameter in the fit. c, Signal-to-background ratios obtained at the target level (combining the six cells), for phase II and phase III. Higher reactor power in phase III led to more IBD signal. Only events with PSD values in the signal range, defined as lower than the ‘mean position + 2.5 sigmas’ of the electronic recoils, are used to produce this plot. d, Search for reactor-related background events in the proton-recoil region from the ON − a × OFF distributions at the target level. A low-energy excess is found and fitted by a power law \(f(E)={p}_{0}{E}^{-{p}_{1}}\). Here the a parameter is not fitted but fixed from the measured sensitivity of the cosmic background to atmospheric pressure and the mean pressure difference between the ON and OFF periods; the consistency with zero at high energy is a good validation of the extraction method (fitted and computed values of a are in agreement). All uncertainties shown are statistical at 1σ CL.
Extended Data Fig. 6 Time stability of the extracted antineutrino spectra.
IBD spectra are extracted from each of the seven acquired reactor cycles using adjacent OFF data for background subtraction. The coloured bands illustrate the 1σ statistical uncertainties. Inset, for each individual ON spectrum, the residuals with respect to the average of all other spectra are computed. The distribution of all these residuals is found to be compatible with a normal distribution.
Extended Data Fig. 7 Neutron efficiency.
Top panels, neutron efficiency εn obtained with the Am-Be source as a function of horizontal (averaged over all measured heights) (a) and vertical (averaged over all measured cells) (b) position for phase II (red) and phase III (blue). Measurements (circles) are performed in cells 1, 2, 4, 5 and 6 at heights Z = 10, 30, 45, 60 and 80 cm. Simulated efficiencies obtained for the same positions are shown as triangles. A 3D spatial model is fitted and gives a continuous description of the efficiency in the detector: \({\varepsilon }_{{\rm{n}}}^{{\rm{data}}}(X,Y,Z)\) (solid lines) and \({\varepsilon }_{{\rm{n}}}^{{\rm{MC}}}(X,Y,Z)\) (dashed lines). Bottom panels, the coefficient \({c}_{{\rm{n}}}={\varepsilon }_{{\rm{n}}}^{{\rm{data}}}/{\varepsilon }_{{\rm{n}}}^{{\rm{MC}}}\) is used to correct for efficiency biases owing to neutron simulation imperfections. It is consistent with a constant function of X and a second-order polynomial in Z; the difference in the integrated \(\overline{{c}_{{\rm{n}}}}\) coming from the choice of a (X, Y, Z) model for cn is considered as systematic uncertainty. All uncertainties shown are statistical at 1σ CL.
Extended Data Fig. 8 Antineutrino spectra in each cell relative to the no-oscillation model.
Antineutrino spectra are shown as a ratio to the adjusted no-oscillation prediction \({\hat{\varphi }}_{i}{M}_{p,l,i}(0,0;{\hat{\alpha }}^{j})\) for cell l ∈ {1,…, 6} and phase p ∈ {II, III}. The best-fit \({\hat{\varphi }}_{i}\) parameters, common to all cells and phases, absorb an overall spectrum shape so that only relative distortions between cells remain. No marked deviation is found between data and the no-oscillation prediction. For illustration, we also show the spectra induced in each cell by the best-fit sterile oscillation parameters (dashed red line). All uncertainties shown are statistical at 1σ CL.
Extended Data Fig. 9 Antineutrino yield of 235U fission.
Overview of the measured antineutrino flux from pure fission of 235U (highly enriched nuclear fuel) relative to the HM model. For a direct comparison of data from different detectors with different thresholds and resolutions, the quantity of interest is the ratio of the measured to expected cross-sections per fission, σf, defined as the integral of antineutrino spectrum multiplied by the IBD cross-section. The measurement by STEREO (0.945 ± 0.021) is the most accurate so far and found to be in excellent agreement with the previous world average of 0.954 ± 0.014 taken from ref. 66. For comparison, we also show the measurement from Daya Bay and RENO with commercial reactors (low enriched nuclear fuel, green), although it relies on reactor evolution simulations to separate the contribution of 235U from other isotopes. The size of the error bars corresponds to the total uncertainty of the respective measurement. The uncertainty of the HM model, common to all measurements and not included in the error bars, is illustrated by the width of the grey band, whereas the purple point and band represent the central value and the uncertainty of the new world average, after including our result. All uncertainties shown are at 1σ CL.
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The STEREO Collaboration. STEREO neutrino spectrum of 235U fission rejects sterile neutrino hypothesis. Nature 613, 257–261 (2023). https://doi.org/10.1038/s41586-022-05568-2
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DOI: https://doi.org/10.1038/s41586-022-05568-2
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