Particle Physics Surprise: Nucleons Pick Pair Partners Differently in Small Nuclei

The atom's nucleus, which is composed of protons and neutrons, frequently pairs up. According to a recent high-precision experiment, these particles may choose different partners based on how densely packed the nucleus is. The work was done at the Thomas Jefferson National Accelerator Facility of the US Department of Energy.

The results provide fresh information regarding the short-range interactions between protons and neutrons in nuclei and could have an impact on the outcomes of studies meant to elucidate more intricate aspects of nuclear structure. The research will be released today, August 31, 2022, in the magazine Nature. The statistics are an order of magnitude more exact than in prior studies.

The paper's principal author is Shujie Li. She started working on the experiment while a graduate student at the University of New Hampshire and is currently a postdoctoral researcher in nuclear physics at the DOE's Lawrence Berkeley National Laboratory in Berkeley, California. According to Li, the experiment was created to contrast short-range correlations, which are transient alliances between protons and neutrons in tiny nuclei.

Nucleons are the aggregate name for protons and neutrons. Nucleons momentarily overlap when they are involved in short-range correlations before breaking apart quickly. There can be correlations between two protons, two neutrons, or between a proton and a neutron.

In this experiment, the frequency of each kind of short-range correlation was compared in the so-called mirror nuclei of tritium, an isotope of hydrogen, and helium-3. Each of these nuclei has three nucleons. Because the proton content of each mirrors the neutron concentration of the other, they are referred to as "mirror nuclei".

"Helium-3 has two protons and one neutron, while tritium has one proton and two neutrons. We can infer from a comparison of tritium and helium-3 that the neutron-proton pairs in tritium are equivalent to those in helium-3. Additionally, tritium and helium-3 both have the ability to produce an extra proton-proton pair and a neutron-neutron pair, according to Li.

When combined, the data from the two nuclei show how frequently nucleons pair up with individuals who are similar to them and those who are not.

The straightforward plan, she explained, is to compare the number of pairings that each configuration of the two nuclei has.

The results of prior investigations, which discovered that nucleons prefer partnering up by more than 20 to 1 with a different type, were similar to what the physicists anticipated seeing (e.g. protons paired up with neutrons 20 times for every one time they paired up with another proton). These tests were carried out on heavier nuclei, like lead, iron, and carbon, which have significantly more protons and neutrons available for pairing.

Four neutron-proton pairs were obtained from this experiment for every proton-proton or neutron-neutron pair, according to Li.

According to John Arrington, a staff scientist at Berkeley Lab and a spokesperson for the project, this unexpected outcome is shedding fresh light on the interactions between protons and neutrons in nuclei.

Therefore, we discover that the proton-proton contribution in this instance is far larger than anticipated. Therefore, it begs the issue of what is different in this situation, he stated.

One theory holds that this difference is caused by the interactions between nucleons, and that these interactions are somewhat altered by the distance between the nucleons in tritium compared to helium-3 compared to very big nuclei.

"Neutron-proton couples are produced by the "tensor" component of the nucleon-nucleon interaction. Additionally, a "core" with a shorter range can produce proton-proton pairs. You might have a different balance between these interactions when the nucleons are farther apart, such in these very light nuclei.

The particles that would-be correlated nucleons pair with in an overlapping short-range correlation can be strongly influenced by differences in the average distances between them. For comparison, a proton is slightly smaller than a femtometer, or fermi, in width. As the particles overlap on the order of one-half fermi, or around a half-particle overlap, the longer-distance, tensor component of the short-range interaction takes center stage. Since the particles mainly overlap at one fermi, the shorter-range core portion of the interaction predominates.

He claims that additional study will assist test this theory. The scientists are investigating if the outcome will affect other measurements in the interim. Nuclear physicists, for instance, study the structure of nucleons via short-range, strong collisions in deep inelastic scattering studies.

According to Douglas Higinbotham, a Jefferson Lab staff scientist and the experiment's spokesperson, "We are pushing the accuracy in research on nuclear structure, so these seemingly tiny effects can become very important as we continue to achieve high-precision results at Jefferson Lab." The results of deep inelastic scattering may reveal surprising phenomena if the nuclear effects in light nuclei are not only persistent but also unexpected.

We continue to discover surprises when performing novel measurements in well-known nuclei that are important to the nuclear structure. So it's quite intriguing that we're still discovering surprises on a straightforward nucleus, said Arrington. Because it must reveal something about the way that nucleons interact at close range, which is difficult to detect anywhere other than Jefferson Lab, we are quite interested in finding out where it originates from.

This experiment was carried out in Experimental Hall A of Jefferson Lab's Continuous Electron Beam Accelerator Facility (CEBAF), a facility used by the Office of Science. It used a different approach to collect data that is ten times more exact than previous experiments, detecting only the electrons that bounced off coupled nucleons inside the mirror nucleus. It also contained a special tritium target created for a series of uncommon tests.

"We were able to employ inclusive scattering because we looked at tritium and helium-3, and this provides us significantly higher statistics than other measurements. To get this result, the tritium project put a lot of work, great design, and chance into it, according to Li.

The exciting finding will be followed up by additional observations in heavier nuclei, according to nuclear physicists. High-energy electrons produced by CEBAF were used in the early research in these nuclei. The "triple coincidence" of the outgoing electron, knocked-out proton, and correlated partner was recorded as the electrons bounced from protons or neutrons engaged in a short-range correlation.

Catching all three particles is a problem for this kind of two-nucleon short-range correlation experiment. For a more in-depth understanding of what is happening inside the nucleus, it is envisaged that future measurements will be able to record three-nucleon short-range correlations.

Arrington is a co-spokesperson for a different experiment that is getting ready for more short-range correlation observations at CEBAF in the near future. Correlations in a variety of light nuclei, such as lithium, beryllium, boron, helium, and lithium isotopes, as well as in a number of heavier targets with various neutron-to-proton ratios, will be measured by the experiment.

By THOMAS JEFFERSON NATIONAL LABORATORY