Nuclear Physics Division

Reactor antineutrino anomalies are a decade-long puzzle in neutrino physics. They are manifested by deviations of the order of a few percent between measurements and predictions. These deviations have been observed in the number of antineutrinos measured by more than a dozen experiments at nuclear reactors, and in the shape of the kinetic energy distributions by the seven most recent ones. They could have been the way to a new physics beyond the standard model, but the recent experiments, including the STEREO experiment carried by IRFU, have closed this door.

In a work just published in Physical Review Letter [1], a team of physicists from IRFU and the Laboratoire National Henri Becquerel of DRT have shown that these anomalies could come from biases in the measurements of fission electrons used as a reference for the prediction. They have developed a beta strength function  model to reduce the biases in the calculation of the energy spectra of electrons from fission of fissile reactor nuclei. The two "anomalies" on the antineutrino flux and the "bump" at 5 MeV in the antineutrino energy spectrum are now reproduced by their model. This allows to propose an explanation to solve an enigma of more than 10 years.

The final results of the Stereo experiment have just been published in the journal Nature. A record of precision is established for the spectrum of neutrinos emitted by the fission of 235U, measured between 9 and 11m distance from the ILL reactor core in Grenoble. The hypothesis of a sterile neutrino to explain the reactor neutrino anomaly is rejected. The quality of these direct neutrino measurements now surpasses that of the underlying nuclear data describing the beta decays of fission products. Stereo provides the community with a fission neutrino spectrum corrected for all detection effects, which will serve as a reference for future reactor experiments and points out residual biases in the nuclear databases.

The Stereo experiment has just completed a beautiful scientific adventure that began in 2011 with the revelation by the IRFU group of the "reactor antineutrino anomaly". Physicists were left with a significant deficit of 6% between the flux of neutrinos measured at the reactors and the predicted flux. The history of science has taught us enough about the potential richness of new phenomena that can hide behind an anomaly. What was going on here was the possible existence of a new type of neutrino that would open up a new area of physics beyond the standard model. Without any direct interaction with matter, this neutrino, described as "sterile", could however mix with "standard" neutrinos and thus leave an imprint of its existence through ... a deficit of count rate in our detectors.

The final piece of a longstanding adventure started in 2014 with an experiment led by Irfu and RIKEN Nishina Center has been set with the publication of a paper containing the comparative study of dineutron correlation in ¹¹Li, ¹⁴Be and ¹⁷B [1]. This paper published in Phys. Lett. B completes a series of publications on the topic [2,3] issued from the same experiment using the MINOS device designed and built at Irfu. It suggests that such kind of correlations is a universal feature of Borromean halo nuclei.

Prediction of nuclear properties based on a realistic description of the strong interaction is the main quest of low-energy nuclear theory. One of the issues lies is the cost of solving Schrödinger’s equation that scales exponentially with the number of nucleons. Consequently, theoretical predictions based on first principles have long been limited to very light nuclei, to nuclei with specific proton and neutron numbers offering simplifications or to their ground states. While releasing each of these three constraints separately is ongoing, overtaking them at the same time constitutes the current forefront in the field. Employing a novel many-body formalism [1] recently developed by the PAN@CEA (“Problème à N corps au CEA”) collaboration (DRF-DAM-DES), the calculation of collective excitations, i.e. nuclear rotations and vibrations, in all medium-mass nuclei  is envisioned for the first time [2,3]. This work paves the way to large-scale predictions of nuclear properties from first principles.