Particle 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.

To reveal the influence of the dark components of the Universe, over the next six years Euclid will be observing the shapes, distances and movements of billions of galaxies. This mapping will cover periods going back to the last 10 billion years of cosmic history, in order to gain a better understanding of where, when and how dark energy and matter - two key components of the universe that are still a mystery - act.

As a prelude to the mission, it was decided to illustrate the satellite's scientific and instrumental capabilities through a series of images of the nearby Universe. Jean-Charles Cuillandre, an astronomer in CEA/IRFU's astrophysics department, led this 'ERO' (Early Release Observations) campaign over several months, with a group of scientists from the Euclid collaboration and ESA, from the choice of the five astrophysical sources to image processing, from raw data to analysed images, combining the responses of the VIS (visible light) and NISP (near infrared) instruments.
 

The Universe is immensely big, and getting bigger all the time. To study dark energy, the mysterious force behind the accelerating expansion of our Universe, scientists are using the large Dark Energy Spectroscopic Instrument (DESI) survey to map over 40 million galaxies, quasars and stars. Today, the collaboration has released its first batch of data, with nearly 2 million astrophysical objects for researchers to study. It is also publishing 15 papers on the scientific study of these data, and on the instrument, operations and validation of the survey observation strategy.

The data set (80 terabytes) comes from 2,480 exposures taken over six months during the so-called "survey validation" phase in 2020 and 2021 and processed in Python language on the supercomputer at the National Energy Research Scientific Computing Center (NERSC, Berkeley, USA). During this period, between the instrument's commissioning and the start of the official scientific campaign, researchers ensured that the instrument's performance would meet their scientific objectives - for example, by checking the time needed to observe galaxies of different luminosities, and validating the selection of astrophysical objects such as galaxies, quasars and stars to be observed.

Read the Berkeley press release : https://newscenter.lbl.gov/2023/06/13/desi-early-data-release-holds-nearly-two-million-objects/

Neutrino oscillations have confirmed that these mysterious particles have mass, contradicting the predictions of the Standard Model. The DPhP group at CEA/IRFU is seeking to solve this mystery by observing the very rare double-beta decay without neutrino emission of the Mo-100 nucleus using scintillating bolometers. Following the CUPID-Mo demonstration experiment at the Modane underground laboratory, the group has finalised a detailed background model that offers high precision for studying the 2v2β decay. The model thus achieves the lowest background index ever obtained by the scientific community for a 0ν2β bolometric experiment.

To achieve the target of 10^-4 counts/keV/kg/year needed to detect this extremely rare decay, the CUPID experiment has also adopted a new detector technology: Nefanov-Trofimov-Luke (NTL) light detectors to improve background rejection. A measurement, using 10 identical light detectors coupled to Li₂MoO₄ and TeO₂ crystals, was carried out at the Canfranc underground laboratory and demonstrated the applicability of this technology to CUPID detectors. Given the combination of low background, particle discrimination capability, high efficiency and high energy resolution, CUPID is recognised as one of the most promising next-generation 0ν2β search experiments. After a validation review, the experiment will begin its production and construction phase to obtain a complete detector from 2029.

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.