Exciting the alpha particle

The Science

An important part of physics research is examining why theoretical calculations and experimental results sometimes don’t match. A recent experiment in Germany studied the helium-4 nucleus, also known as an alpha particle. Helium-4 consists of only two protons and two neutrons, making it relatively easy to describe using theoretical calculations. This makes it useful for precise tests of nuclear physics. The experiment used the scattering of electrons to examine how helium-4 transitions from its basic energy state to its first excited state. The experiment found evidence of a disagreement between theory and experiment and sparked concern about the accuracy of current theoretical calculations. Now, an international team has performed new calculations of the observed transition. Their results agree well with the new experimental results.

The Impact

The first excited state of the helium-4  nucleus has an energy just above the threshold for the nucleus to separate into a proton and a hydrogen-3 nucleus. The properties of this state depend sensitively on the small gap between its energy and the threshold for separation. The new calculations accurately reproduce this energy difference, and this may explain why they also reproduce the new experimental data from Germany. The findings will help physicists make accurate theoretical predictions of nuclear structure in the future.

Summary

Researchers from University of Bonn (Germany), Forschungszentrum Jülich (Germany), Gaziantep Islam Science and Technology University (Turkey), and the Facility for Rare Isotope Beams at Michigan State University performed ab initio lattice calculations using an interaction that reproduces the overall properties of light and medium-mass nuclei. The calculations of the transition from the ⁴He ground state to its first excited state are in good agreement with recent experimental results from Mainz.

A key feature of the new lattice calculations is that they accurately reproduce the difference between the excited state energy and the energy threshold for separation into a proton and a helium-3 nucleus. Other recent theoretical work has explored the importance of accurately reproducing this energy difference. The new results give confidence that the nuclear force is well understood, but researchers must carefully consider sensitivities to nearby energy thresholds in future theoretical calculations.

Funding

This research was funded in part by the Department of Energy SciDAC-5 NUCLEI Collaboration, by the European Research Council, Deutsche Forschungsgemeinschaft, the Chinese Academy of Sciences, and Volkswagen Stiftung. One researcher was supported in part by the Scientific and Technological Research Council of Turkey. Computational resources provided by the Gauss Centre for Supercomputing e.V. for computing time on the GCS Supercomputer JUWELS at the Julich Supercomputing Centre.