Helium-4 Atom: An Anomaly Explained by... a Famous Astronomy Problem

Published by Adrien,
Source: CNRS IN2P3
Other Languages: FR, DE, ES, PT

A recent experimental measurement has identified a significant discrepancy between theoretical predictions and the observed behavior of the helium-4 nucleus.

The work of an international collaboration of theorists has led to a new perspective on the configurations of nucleons and their coupling in this nucleus. In particular, a shift in the concept of the resonant state of this light nucleus, which may enlighten scientists about major astrophysical phenomena, has reconciled theory and experimentation.


Image Wikimedia

Mainz, 2021: a team of researchers bombarded helium-4 nuclei with electrons, aiming to observe the transition of this nucleus, also known as the alpha particle, from its ground state to its first excited (resonant) state. Among other things, this experiment aimed to compare a measured value related to this transition to the one predicted by a leading model of the atomic nucleus, based on effective theories, the "Chiral Effective Field Theories" (ChEFTs).

Shockingly, the value measured at the University of Mainz Microtron (MaMi) was half that of the theoretical predictions, marking a painful setback for some nuclear models. The quest was then launched: How to reconcile this experimental observation with models based on ChEFT theory?

This puzzle was recently solved by an international collaboration of theorists, including Nicolas Michel (Chinese Academy of Sciences), Witek Nazarewicz (Michigan State University), and Marek Płoszajczak, a researcher at GANIL. Marek explains: "Using a ChEFT to calculate certain properties of the atomic nucleus initially requires modeling the behavior of nucleons within the nucleus. We talk about solving the 'N-body problem'.

For the helium-4 nucleus, theorists had until recently perceived the alpha particle as an autonomous and independent system within which nucleons simply move away from each other when the nucleus absorbs energy. In this scenario, described as a closed quantum system, one can imagine the nucleus expanding like a balloon when it receives energy
".


The alpha particle is not a closed quantum system, but rather an open system in which several reaction channel systems coexist.
Image: Witek Nazarewicz
The innovative idea from the three researchers involved proposing a different view of the excited nucleus: instead of seeing it as a closed quantum system, the trio of researchers preferred to consider the helium-4 nucleus as an open system within which various systems of reaction channels coexist (a nucleus of hydrogen-3 paired with a proton, a nucleus of helium-3 paired with a neutron, or two helium-2 nuclei). "It is indeed the resolution of the N-body problem within this paradigm that allowed us to match the experimental results obtained at MaMi", confirms Marek.

All's well that ends well? This theoretical maneuver undoubtedly lends credibility to existing models and can also be applied to other light nuclei to better understand their excitation process. However, the episode also highlights the difficulty of accounting for the most subtle details in studying atomic nuclei and the need for such depth of analysis to grasp simple nuclear phenomena.

The effort is well worth it: transitions between fundamental and resonant states of light nuclei, as well as the microscopic structure of these resonant states, play a role in astrophysical phenomena such as the formation of neutron stars or stellar nucleosynthesis, bringing us closer to a global understanding of the Universe's chemistry.
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