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Mirror Nuclei For Precision Theory Test Studied
June 26, 2020

It’s not often in nuclear physics that you can clearly get both sides of the story, but a recent experiment allowed researchers to do just that. They compared very similar nuclei to each other to get a clearer view of how the components of nuclei are arranged and found that there’s still more to learn about the heart of matter. The research, carried out at the Department of Energy’s (DOE) Thomas Jefferson National Accelerator Facility, was recently published as an editors’ suggested read in Physical Review Letters.

“We want to study nuclear structure, which is basically how protons and neutrons behave inside a nucleus,” explains Reynier Cruz-Torres, a postdoctoral researcher at DOE’s Lawrence Berkeley National Lab who worked on the experiment as a graduate student at the Massachusetts Institute of Technology (MIT). “To do that, we can measure any nucleus that we want. But to do a high-precision test of nuclear theory, we are limited to light nuclei that have precision calculations. Measuring these light nuclei is a benchmark for understanding nuclear structure in general.”

For this measurement, the researchers focused on two of the simplest and lightest nuclei in the universe: helium and hydrogen. They focused on an isotope of helium called helium-3, so called because it contains just three major components: two protons and one neutron. The isotope of hydrogen that they tested, tritium, is also composed of three components: one proton and two neutrons.

“They are mirror nuclei. So, you can assume that the protons in helium-3 are basically the same as the neutrons in tritium and vice versa,” says Florian Hauenstein, a joint postdoctoral researcher at Old Dominion University and MIT.

According to the researchers, by comparing these relatively simple nuclei, they can get a window into the strong nuclear interactions that can’t be duplicated elsewhere. That’s because as some of the lightest and least complicated of the nuclei in the universe, these nuclei are excellent examples for comparing with the state-of-the-art theories that describe the basic structures of different nuclei.

To make the comparison, the researchers measured both nuclei in high-precision experiments in the Continuous Electronic Beam Accelerator Facility (CEBAF), a DOE User Facility based at Jefferson Lab.

Electrons from CEBAF were aimed at the nuclei of tritium and helium-3, where some interacted with the nuclei’s protons. The struck protons and the interacting electrons were then captured and measured in large detectors called spectrometers in Jefferson Lab’s Experimental Hall A.

This experiment was challenging and groundbreaking in that it was conducted at a wider range of energies with unprecedented precision. In addition, the tritium data are the very first ever for these studying these reactions.

According to information, the researchers then compared the full range of data from the experiments to theory calculations on the structures of the nuclei of helium-3 and tritium. They found that the data generally matched theory well for both nuclei to the precision allowed by experiment, a feat that was described by one researcher as a triumph of modern-day nuclear physics. However, differences were also observed relative to some of the calculations, indicating that further refinements in the theoretical treatments are required.

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