Nucleons select pair partners differently in small nuclei

Two of tritium’s three nucleons can form short-range correlations that include a proton and one of its neutrons or two neutrons. Credit: DOE’s Jefferson Lab

When the odds are equal, particles pair up with others of the same type more often than expected.

Protons and neutrons, which make up the atom‘s core, often pair up. Now, a new high-precision experiment has discovered that these particles can choose different partners depending on the density of the nucleus. The work was carried out at the US Department of Energy’s Thomas Jefferson National Accelerator Facility.

The findings also reveal new details about short-range interactions between protons and neutrons in nuclei and could impact the results of experiments aimed at unraveling deeper details of nuclear structure. The data is an order of magnitude more precise than in previous studies, and the research will be published today (August 31, 2022) in the journal Nature.

Shujie Li is the lead author of the article. She is a postdoctoral researcher in nuclear physics at the DOE’s Lawrence Berkeley National Laboratory in Berkeley, Calif., and began working on the experiment as a graduate student at the University of New Hampshire. Li said the experiment was designed to compare short-lived partnerships between protons and neutrons, called short-range correlations, in small nuclei.

Protons and neutrons are collectively called nucleons. When involved in short-range correlations, nucleons overlap briefly before separating with high momentum. Correlations can form between a proton and a neutron, between two protons or between two neutrons.

This experiment compared the prevalence of each type of short-range correlation in the so-called mirror nuclei of helium-3 and tritium, an isotope of hydrogen. These nuclei each contain three nucleons. They are considered “mirror nuclei” because the proton content of each reflects the neutron content of the other.

“Tritium is composed of one proton and two neutrons, and helium-3 is composed of two protons and one neutron. By comparing tritium and helium-3, we can assume that the neutron- proton in tritium are the same as the neutron-proton pairs in helium-3. And tritium can produce an extra neutron-neutron pair, and helium-3 can produce an extra proton-proton pair,” Li explained.

Taken together, the data from the two nuclei reveals how often nucleons pair up with others like them versus those that are dissimilar.

“The simple idea is just to compare how many pairs the two cores have in each configuration,” she said.

Physicists expected to see a result similar to previous studies, which found that nucleons prefer to pair more than 20 to 1 with a different type (for example, protons paired with neutrons 20 times for each time that they pair with another proton). These studies were conducted in heavier nuclei with many more protons and neutrons available for pairing, such as carbon, iron, and lead.

“The ratio we extracted in this experiment is four neutron-proton pairs for every proton-proton or neutron-neutron pair,” Li revealed.

This surprising result gives new insight into the interactions between protons and neutrons in nuclei, according to John Arrington, spokesperson for the experiment and Berkeley Lab scientist.

“So in this case, we see that the proton-proton contribution is much, much larger than expected. So that raises questions about what’s different here,” he said.

One idea is that interactions between nucleons are a driver of this difference, and these interactions are somewhat modified by the distance between nucleons in tritium versus helium-3 versus very large nuclei.

“In the nucleon-nucleon interaction, there is the ‘tensor’ part, which generates neutron-proton pairs. And there’s a shorter-range “nucleus” that can generate proton-proton pairs. When the nucleons are further apart, like in these very light nuclei, you can get a different balance between these interactions.

Differences in the average distances between potentially correlated nucleons can have a strong influence on which particles they choose to pair up with in an overlapping short-range correlation. For reference, a proton is just under a femtometer, or fermi, wide. The longer-range tensor part of the short-range interaction dominates when the particles overlap on the order of a half-fermi, or about a half-particle overlap. The shorter-range central part of the interaction dominates because the particles mostly overlap at one fermi.

He says further research on this topic will help test this idea. In the meantime, scientists are studying whether the result will impact other measures. For example, in deep inelastic scattering experiments, nuclear physicists use hard short-range collisions to explore the structure of nucleons.

“We are pushing precision in nuclear structure experiments, and so these seemingly small effects can become very significant as we continue to produce high-precision results at Jefferson Lab,” said Douglas Higinbotham, spokesperson for the Jefferson Lab experiment and scientist. . “So if nuclear effects are not only persistent but unexpected in light nuclei, that means you may have unexpected things in your deep inelastic scattering results.”

Arrington agreed.

“We are always making new measurements in familiar nuclei that are relevant to nuclear structure and finding surprises. So the fact that we’re still finding surprises on a simple kernel is very interesting,” Arrington commented. “We really want to understand where it’s coming from, because it has to tell us something about how nucleons interact at close range, which is hard to measure outside of Jefferson Lab.”

This experiment was conducted in Jefferson Lab’s Continuous Electron Beam Accelerator Facility (CEBAF), a Science Bureau User Facility, in its Experimental Hall A. It featured a single tritium target designed for a series of rare experiments, and it used another tactic to capture a data set that is a factor of 10 more accurate than previous experiments: measuring only electrons that have bounced off a correlated nucleon inside mirror nuclei.

“Through looking at tritium and helium-3, we were able to use inclusive scattering, which gives us much higher stats than other metrics. This is a very unique chance, and a excellent design, and a lot of effort from the tritium project to achieve this result,” Li added.

Nuclear physicists want to follow up this intriguing result with additional measurements in heavier nuclei. The first experiments in these nuclei used high-energy electrons generated in CEBAF. Electrons bounced off protons or neutrons engaged in short-range correlation and the “triple coincidence” of the outgoing electron, the knocked out proton and the correlated partner was measured.

A challenge for this type of two-nucleon short-range correlation measurement is to capture all three particles. Still, it’s hoped that future measurements can capture the short-range correlations of three nucleons for an even more detailed view of what’s going on inside the nucleus.

In the near term, Arrington is a co-spokesperson for another experiment that is gearing up for additional measurements of short-range correlations at CEBAF. The experiment will measure the correlations in a range of light nuclei, including the isotopes of helium, lithium, beryllium and boron, as well as a number of heavier targets that vary in neutron-proton ratio.

Reference: “Revealing the short-range structure of the mirror kernels 3H and 3He” August 31, 2022, Nature.
DOI: 10.1038/s41586-022-05007-2

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