Nuclear interactions underpin the stability of atomic nuclei and also play a fundamental role in processes taking place inside stars. Understanding them is therefore essential for describing the behaviour of matter at the microscopic level and for understanding what happens in the universe. A new study, recently published in Nature Communications, on the structure of the helium-4 (⁴He) nucleus – an isotope of helium consisting of two protons and two neutrons – provides an important contribution to our understanding of nuclear interactions. The study addresses several open questions concerning the description of a particular state of helium-4 and provides new data with which to test and improve models of nuclear physics. The research is the result of a collaboration between INFN, the Universities of Catania, Trento, Mainz and São Paulo, and several other Italian and international institutes.
Over the past few decades, increasingly accurate models have been developed that have proved highly successful in reproducing the properties of the simplest nuclei, composed of just a few protons and neutrons, as well as experimental measurements. Despite this progress, some recent results have revealed unexpected features of the first excited state of helium-4, a particular configuration in which the nucleus temporarily occupies a higher energy level than its ground state. These observations have raised questions about the ability of current theoretical models to fully describe the behaviour of this system.
The new study addresses this issue through measurements of the inelastic scattering of alpha particles, that is, helium-4 nuclei, off helium-4 nuclei. The experimental data were obtained at INFN National Laboratories of the South using the MAGNEX magnetic spectrometer and the OSCAR detector, and rely on unprecedented measurement sensitivity and advanced analyses of the spectral line shape, which made it possible to characterise the excited state with greater precision.
The experimental results were compared with predictions from the most advanced ab initio theories of helium-4 structure, which seek to describe the nucleus starting from the fundamental interactions between its constituent particles. Overall, the analysis of the data shows that the observations can be consistently described within the current theoretical framework of nuclear interaction physics. At the same time, however, the study highlights the need to further refine the models used to describe few-body open quantum systems, in which particles can interact and transform during the process being observed.
In addition to its scientific significance, the study also represents an important outcome of the collaborative work carried out by different INFN structures within a national and international research effort, and confirms the role of INFN National Laboratories of the South and the importance of their superconducting cyclotron as a key facility for experimental research in nuclear physics.
