AurélIe Bonhomme

L’anomalie des antineutrinos de réacteurs (RAA) a été révélée en 2011 suite à une réévaluation de la prédiction des spectres associés. Elle indique un déficit de 6% dans les taux mesurés auprès de réacteurs. Une explication consiste en l’introduction d’un neutrino stérile à l’eV, ne se manifestant que par le mécanisme d’oscillations. L’expérience STEREO a été conçue pour explorer l’espace des paramètres d’oscillations indiqués par la RAA, en plaçant une cible segmentée à 10 m du cœur quasiment pur en uranium 235 de l’ILL, à Grenoble. Les antineutrinos sont détectés par désintégration bêta inverse (IBD) dans du liquide scintillant dopé au gadolinium. Si elle existe, une oscillation se développerait alors le long des six cellules identiques de la cible. La comparaison relative de leurs spectres d’antineutrinos en énergie permet de tester l’hypothèse d’une oscillation en s’affranchissant des prédictions et avec des incertitudes systématiques réduites. Les forts rendements lumineux des liquides permettent d’atteindre de bonnes résolutions en énergie, nécessaires pour la sensibilité de la mesure. Les cellules sont séparées optiquement par des plaques hautement réfléchissantes; cette technologie a été validée sur cellule prototype dans un premier temps lors de cette thèse. Un modèle effectif pour ces parois optiques a été inclus à la simulation, et les détails de la géométrie ont été implémentés. La deuxième partie de ce travail est orientée vers l’analyse des données. La signature de l’IBD est un signal en deux temps, requérant un algorithme de recherche de paires pour extraire les candidats antineutrino. Le programme dédié est optimisé pour l’application de sélections d’isolations temporelles, permettant de se débarrasser de la majeure partie du bruit de fond d’origine cosmique. Le bruit de fond corrélé résiduel dans la région d’intérêt provient de désintégrations de muons et de réactions induites par des neutrons. Une modélisation des figures de discrimination en forme des signaux (PSD) de ce bruit de fond est employée pour extraire le signal antineutrino. Basée sur les spectres qui en résultent, les premières analyses d’oscillations de STEREO rejettent le meilleur ajustement de la RAA à 98% de niveau de confiance.

Context. The nuclear rates for reactions involving ¹²C and ¹⁶O are key to computing the energy release and nucleosynthesis evolution of massive stars during their advanced burning phases. Ultimately, these burning rates shape the stellar structure and evolution and influence the nature of the compact objects produced at the end of the stellar life. Aims. We explore the implications of new nuclear reaction rates from both experimental and theoretical studies for ¹²C(α, γ)¹⁶​O, ¹²C+¹²​C, ¹²C+¹⁶​O, and ¹⁶O+¹⁶​O reactions for massive stars. Our goal is to investigate how the chemical structure and nucleosynthesis evolve from the He-exhaustion stage to the O-burning phase and how these processes influence the ultimate stellar fate. Methods. We computed rotating and non-rotating models for stars of different masses at solar metallicity. We used the stellar evolution code GENEC, which includes a large network of nuclear reactions and isotopes involved in advanced phases, as well as updated rates for ¹²C(α,γ)¹⁶O. For the three fusion reactions involving ¹²C and ¹⁶O, we considered new rates following a data-driven fusion suppression scenario (hereafter HIN(RES)) and new theoretical rates obtained with time-dependent Hartree-Fock (TDHF) calculations. Results. The updated ¹²C(α, γ)¹⁶​O rates mainly impact the chemical structure evolution changing the ¹²C/¹⁶O ratio at He-exhaustion and have little effect on the CO core mass. This variation in the ¹²C/¹⁶O ratio is in some cases critical for predicting the final fate of the model, which is very sensitive to ¹²C abundance, and in particular the 20 M_⊙ remnant may change from a black hole to a neutron star. The He-burning (C-burning) lifetime is also decreased (increased) by about −2% (+15%). The combined new rates for ¹²C+¹²​C and ¹⁶O+¹⁶​O fusion reactions according to the HIN(RES) model lead to shorter C- and O-burning lifetimes by ≈ − 10%, and −50%, respectively, and shift the ignition conditions to higher temperatures and densities. In contrast, the theoretical TDHF rates primarily affect C-burning, increasing its duration by about 30% and lowering the ignition temperature. These changes modify the chemical structure of the core, the size and duration of C-burning shells, and hence their compactness. They also impact the central and shell nucleosynthesis (by ±1 dex and by factors of ±2–10, respectively), while ¹²C+¹⁶​O reaction rates variations remain the least important. Conclusions. The present work shows that accurate reaction rates for key processes in massive star evolution and nucleosynthesis drive significant changes in stellar burning lifetimes, chemical evolution, and stellar fate. The multiple and cumulative consequences of these changes are significant and should not be neglected. In addition, discrepancies between experimental and theoretical rates introduce uncertainties in model predictions, influencing both the internal structure and the composition of the supernova ejecta.

Neutrinos are elementary particles that interact only very weakly with matter. Neutrino experiments are, therefore, usually big, with masses in the multi-tonne range. The thresholdless interaction of coherent elastic scattering of neutrinos on atomic nuclei leads to greatly enhanced interaction rates, which allows for much smaller detectors. The study of this process gives insights into physics beyond the Standard Model of particle physics. The CONUS+ experiment¹ was designed to first detect elastic neutrino–nucleus scattering in the fully coherent regime with low-energy neutrinos produced in nuclear reactors. For this purpose, semiconductor detectors based on high-purity germanium crystals with extremely low-energy thresholds were developed². Here we report the first observation of a neutrino signal with a statistical significance of 3.7σ from the CONUS+ experiment, operated at the nuclear power plant in Leibstadt, Switzerland. In 119 days of reactor operation (395 ± 106) neutrinos were measured compared with a predicted number from calculations assuming Standard Model physics of (347 ± 59) events. With increased precision, there is potential for fundamental discoveries in the future. The CONUS+ results in combination with other measurements of this interaction channel might therefore mark a starting point for a new era in neutrino physics.

The CONUS experiment studies coherent elastic neutrino-nucleus scattering in four 1 kg germanium spectrometers. Low ionization energy thresholds of 210 eV were achieved. The detectors were operated inside an optimized shield at the Brokdorf nuclear power plant which provided a reactor antineutrino flux of up to 2.3 × 10 13 cm − 2 s − 1 . In the final phase of data collection at this site, the constraints on the neutrino interaction rate were improved by an order of magnitude as compared to the previous CONUS analysis. The new limit of less than 0.34 signal events kg − 1 d − 1 is within a factor 2 of the rate predicted by the standard model. This constraint is discussed in the context of conflicting measurements and results from another reactor neutrino experiment using similar technology. Published by the American Physical Society 2024

Context. Nuclear reactions drive stellar evolution and contribute to stellar and galactic chemical abundances. New determinations of the nuclear reaction rates in key fusion reactions of stellar evolution are now available, paving the way for improved stellar model predictions. Aims. We explore the impact of new ¹²C + ¹²C reaction rates in massive star evolution, structure, and nucleosynthesis at carbon ignition and during the core carbon-burning phase. We analyse the consequences for stars of different masses including rotation-induced mixing. Methods. We computed a grid of massive stars from 8 to 30 M_⊙ at solar metallicity using the stellar evolution code GENEC, and including the new reaction rates. We explored the results using three different references for the rates, with or without rotation. We studied the effect in terms of evolution, structure, and the critical mass limit between intermediate and massive stars. We explored the consequences for heavy-element nucleosynthesis during the core carbon-burning phase by means of a one-zone nucleosynthesis code. Results. We confirm the significant impact of using the recent nuclear reaction rates following the fusion suppression hypothesis at deep sub-barrier energies (hindrance hypothesis) as well as the mass-dependent effect of a resonance at 2.14 MeV with dominant feeding of the α exit channel of ¹²C + ¹²C fusion reaction. This impacts the characteristics of the core of stars from the C-ignition and during the entire core C-burning phase (temperature and density, lifetime, size, convective or radiative core). The change in nuclear reaction rates modifies the central nucleosynthesis of the stars during the core-carbon burning phase, resulting in an underproduction of s-process elements, especially when including the rotation-induced mixing that amplifies the effects. Conclusions. The correct and accurate determination of the nuclear reaction rates, especially with the existence and location of resonances, impacts stellar evolution in many respects, affecting models’ predictions. The choice of the nuclear reaction rates reference for the ¹²C + ¹²C fusion reaction significantly changes the behaviour of the core during the carbon-burning phase, and consequently drives changes in the nucleosynthesis and end-of-life of stars. This choice needs, then, to be made carefully in order to interpret stellar evolution from the super asymptotic giant branch phase and its massive white dwarf remnants to the core-collapse supernovae of massive stars.