Nuclear Physics Division

The FIFRELIN code simulates nuclear fission and de-excitation of the nuclei produced therein. STEREO is a compact neutrino detector that looks for a hypothetical sterile neutrino. Two a priori separate topics developed at CEA, the first at DEN, the second at DRF/Irfu, which have however recently met to achieve unprecedented precision on a crucial ingredient in the detection of neutrinos: the de-excitation of a gadolinium nucleus after the capture of a neutron. The results of this meeting have just been published in the journal The European Physical Journal A [1].

The STEREO experiment releases new results based on the detection of about 65000 neutrinos at short distance from the research reactor of the ILL-Grenoble. The improved accuracy is rejecting the hypothesis of a 4^th neutrino in a large fraction of the domain predicted from the reactor neutrino anomaly. Profiting from a good control of the detector response, STEREO now also releases its first absolute measurements of the neutrino rate and the spectrum shape.

Predicting properties of, e.g., molecules or atomic nuclei from first principles requires to solve the Schrödinger equation with high accuracy. The computing cost to find exact solutions of the Schrödinger equation scales exponentially with the number of particles constituting the system. Thus, with nuclei composed of tens or hundreds of nucleons, it necessitates accurate approximate methods of lower computing cost. However, such methods can be applied to a limited number of systems: the weakly correlated ones. Consequently, a universally applicable method is still missing. Employing a novel formalism recently developed at Irfu/DPhN [1], highly accurate solutions of the Schrödinger equation – in the context of the exactly solvable Richardson model - have been obtained, independently of the weakly- to strongly-correlated character of the system. This work has been performed in collaboration with ab initio quantum chemists from Rice University. This exciting new achievement, paving the way for precise ab initio computations of molecular or nuclear properties of a large number of systems, was recently published in Physical Review C [2] and highlighted as the Editor’s suggestion.

Pairing is ubiquitous in physics. From superconductivity to quantum shell structure, coupling particles into pairs is one of nature's preferred ways to lower the energy of a system. New results obtained at the Radioactive Isotope Beam Factory (RIBF, Japan) with the MINOS device, which was conceived and constructed at Irfu, show for the first time that pairing also plays an important role in single-proton removal reactions from neutron-rich nuclei. These results show that proton-removal cross sections can be used as a tool to investigate pairing correlations for very neutron rich nuclei not accessible via spectroscopy. Indeed, the latter are produced in too small quantities to consider spectroscopy, studying the gammas emitted during de-excitation for example. This study was recently published in Physical Review Letters [1].

An international collaboration led by the institutes of CEA-IRFU and of RIKEN (Japan) demonstrates, for the first time, the exceptional stability of the very-neutron rich nickel-78 nucleus and its doubly-magic character. The experiment at RIKEN was made possible by the unique combination of the MINOS device developed at CEA-Irfu and the very exotic beams produced by the RIBF facility of the Japanese accelerator.These results are published in Nature [Nat19].

Prediction of nuclear properties based on a realistic description of the strong interaction is at the heart of the ab initio endeavor in low-energy nuclear theory. Ab initio calculations have long been limited to light nuclei or to nuclei with specific proton and neutron numbers. Theoreticians from Irfu/DPhN have developed a new ab initio method from which properties of many more nuclei than before can be predicted while drastically decreasing the computational cost. This has been made possible by allowing symmetries of the nuclear Hamiltonian to spontaneously break in the calculation. This exciting new development, paving the way for precise computations of heavier nuclei within a reasonable time-frame, has recently been published in Physics Letter B [1].

It is possible to trace the shape of a drum from its vibration modes. Similarly, it is possible to measure the 3D structure of the proton and access its elementary components, quarks and gluons, from observables obtained using deeply virtual Compton scattering experiments off the proton.  By studying this scattering process, we can access this geometric information. This research topic is very active and mobilizes a large international theoretical and experimental community. As part of the PARTONS (PARtonic Tomography Of Nucleon Software) project, physicists from Irfu and NCBJ in Warsaw successively performed two detailed analyses using all the measurements associated with this process published since the early 2000s. This represents nearly 2600 measurement points and 30 observables from 6 different experiments. This work, published in the European Physical Journal C [1, 2], is now the most advanced analysis of these experimental data. New data, combined with new analysis methods, will enrich the PARTONS library in the future; these observables (Compton form factors) will make it possible to go one step further in the reconstruction of the proton structure in 3D.

The series of Jefferson Laboratory (USA) experiments dedicated to the measurement of electron-proton elastic scattering showed that the extracted information on the proton structure did not agree when extracted from two kinds of experiments. To reconcile these results, it was suggested that, beyond the exchange of one photon, that is the dominant mechanism, the exchange of a second photon could become important. The presence of a second photon would bring serious consequences, casting doubts on the results of number of experiences.  That’s why the search of such mechanism motivated three independent experiments, recently realized. We have analyzed all the obtained results and shown that two photon exchange is not enhanced. Other explanations are favored, as a precise calculation of radiative corrections. This study, performed by two researchers of IRFU and JINR-BLTP, Dubna (Russia) has been published in the Physical Review C[1].