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High-Precision Experiment Offers Insight into Interactions Between Protons and Neutrons in Nuclei

The protons and neutrons that build the nucleus of the atom frequently pair up. Now, a new high-precision experiment conducted at the U.S. Department of Energy's Thomas Jefferson National Accelerator Facility has found that these particles may pick different partners depending on how packed the nucleus is.

The data also reveal new details about short-distance interactions between protons and neutrons in nuclei and may impact results from experiments seeking to tease out further details of nuclear structure. The data are an order of magnitude more precise than previous studies, and the research has just been published in the journal Nature.

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

Protons and neutrons are collectively called nucleons. When nucleons are involved in short-range correlations, they briefly overlap before they fly apart with high momentum. Correlations may 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 each one's proton content mirrors the other's neutron content.

"Tritium is one proton and two neutrons, and helium-3 is two protons and one neutron. By comparing tritium and helium-3, we can assume that neutron-proton pairs in tritium are the same as neutron-proton pairs in helium- 3. And tritium can make one additional neutron-neutron pair, and helium-3 can make one additional proton-proton pair," Li explained.

Taken together, the data from both nuclei reveal how often nucleons pair up with others like themselves versus those that are different.

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

The researchers expected to see a result similar to earlier studies, which found that nucleons prefer pairing up by more than 20 to 1 with a different type (e.g. protons paired up with neutrons 20 times for every one time they paired up with another proton). These studies were conducted in heavier nuclei with far 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 per each proton-proton or neutron-neutron pair," Li revealed.

According to John Arrington, a spokesperson for the experiment and staff scientist at Berkeley Lab, this surprising result is providing new insight into the interactions between protons and neutrons in nuclei.

"So in this case, we find that the proton-proton contribution is much, much bigger than expected. So it raises some questions about what's different here," he said.

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

"In the nucleon-nucleon interaction, there's the "tensor" piece, which generates neutron-proton pairs. And there's a shorter-range "core" that can generate proton-proton pairs. When the nucleons are further apart, as in these very light nuclei, you may get a different balance between these interactions."

Differences in the average distances between would-be correlated nucleons can have a strong influence on which particles they pick to pair with in an overlapping short-range correlation. For reference, a proton measures a little less than a femtometer, or fermi, wide. The longer-distance, tensor piece of the short-range interaction dominates as the particles overlap on the order of one-half fermi, or about a half-particle overlap. The shorter-range core part of the interaction dominates as the particles mostly overlap at one fermi.

He says further research on this topic will help test this idea. In the meantime, the researchers are exploring whether the result will impact other measurements. For instance, in deep inelastic scattering experiments, nuclear physicists use short distance, hard collisions to explore nucleons' structure.

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

Arrington agreed.

"We're still making new measurements in familiar nuclei that are relevant to the nuclear structure and finding surprises. So the fact that we're still finding surprises on a simple nucleus is very interesting," Arrington commented. "We really want to understand where it comes from, because it has to tell us something about the way that the nucleons interact at short distance, which is hard to measure anywhere other than Jefferson Lab."

This experiment was conducted in Jefferson Lab's Continuous Electron Beam Accelerator Facility (CEBAF), an Office of Science user facility, in its Experimental Hall A. It featured a unique tritium target that was designed for a series of rare experiments, and it used a different tactic to capture a dataset that is a factor of 10 more precise than earlier experiments: measuring just the electrons that bounced off of a correlated nucleon inside the mirror nuclei.

"Because of looking at tritium and helium-3, we were able to use inclusive scattering, and that gives us much higher statistics than other measurements. It's a very unique chance, and a great design, and a lot of effort from the tritium project to get this result," Li added.

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

One challenge for this type of two-nucleon short-range correlation measurement is catching all three particles. Yet, it's hoped that future measurements will be able to capture three-nucleon short-range correlations for an even more detailed view of what is happening inside the nucleus.

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

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