New Calculations Solve an Alpha Particle Physics Puzzle

The Science

In physics, predictions from theory and the results of experiments don’t always agree. Understanding why is an important part of how physicists improve their understanding of the universe. But these disagreements can also occur because of the difficulty of analyzing the results of experiments or theoretical calculations. In early 2023, scientists published a new measurement testing the strong nuclear force that binds protons and neutrons. The experiment involved the way the nucleus of a helium atom, also called an alpha particle, gains energy to become excited. 

The Impact

The earlier experimental paper suggested a puzzle that could not be solved with existing theoretical methods. This means the finding might have indicated physics not accounted for by the contemporary nuclear models. Findings this big need careful evaluation. The new theoretical calculations described here used a novel quantum mechanics computing code to explain the experimental findings. This new result closes the apparent gap between theory and experiment. 

Summary

In a recent study, theorists from the Chinese Academy of Sciences in Lanzhou, Grand Accelerateur National d’Ions Lourds in France, and the Facility for Rare Isotope Beams (FRIB) employed a novel quantum mechanics computing code to explain the results of a recent precise experiment used to determine how a helium-4 nucleus (an alpha particle) gains energy to become excited in what is known as a “monopole transition.” According to the study’s results, the excited state at energy 20.21 MeV, which is unstable and decays by emitting a proton, is an intricate combination of binary clusters: hydrogen-3 and a proton, helium-3 and a neutron, and two hydrogen-2 nuclei.

The no-core Gamow shell model used in this new analysis can describe how an alpha particle undergoes a transition to its first excited state. By considering the excitation process, and the structure of the excited state as a combination of multiple cluster configurations, the new result explains the previous experimental findings and shows that the mystery surrounding the excited state has been solved.

Contact

Witold Nazarewicz
FRIB/Michigan State University
[email protected]

Funding

This material is based on work supported by the Department of Energy Office of Science, Office of Nuclear Physics.

Publications

Michel, N., Nazarewicz, W., and Płoszajczak, M., Description of the proton-decaying 0⁺ resonance of the α particle. Physical Review Letters 131, 242502 (2023). [DOI: 10.1103/PhysRevLett.131.242502]

Related Links

Physics Today: Theory and experiment disagree on alpha particles
Quanta Magazine: A new experiment casts doubt on the leading theory of the nucleus
Life Science: Scientists tried to solve the mystery of the helium nucleus — and ended up more confused than ever
Physics Magazine: Closing a gap in nuclear theory