NEWPORT NEWS, VA – Though atomic nuclei are often depicted as static clusters of protons and neutrons (nucleons), the particles are actually bustling with movement. Thus, the nucleons carry a range of momenta. Sometimes, these nucleons may even briefly interact through the strong interaction. This interaction between two nucleons can boost the momentum of both and form high-momentum nucleon pairs. This effect yields two-nucleon short-range correlations.
Experiments at the U.S. Department of Energy’s Thomas Jefferson National Accelerator Facility have studied these pairs to learn how protons and neutrons preferentially pair up at short distances. However, short-range correlations involving three or more nucleons haven’t been detected yet. Now, in Physics Letters B, researchers used data from a 2018 experiment in Jefferson Lab’s Hall A to measure the signature of three-nucleon short-range correlations for the first time.
Physicists are pursuing these trios because they would explain the extremely high-momentum component in the nucleus. Regular nucleons, with their typical, uncorrelated momenta, make up most of the nucleon momentum distribution in the nucleus. Short-range correlated pairs produce a noticeable fraction of high-momentum nucleons but some of the higher momentum is still unaccounted for.
“We’re unraveling the nucleus to find what’s missing in our understanding,” said John Arrington, a senior scientist and Relativistic Nuclear Collisions group head at the DOE’s Lawrence Berkeley National Laboratory. “We know that the three-nucleon interaction is important in the description of nuclear properties, even though it’s a very small contribution. Until now, there’s never really been any indication that we’d observed them at all. This work provides a first glimpse at them.”
Mirrored nuclei simplify the search
The experiment was carried out in Jefferson Lab’s Continuous Electron Beam Accelerator Facility (CEBAF), a DOE Office of Science user facility dedicated to nuclear physics research. To access short-range-correlated nucleons, researchers aimed CEBAF’s electron beam at nuclei. The high-energy electrons interacted with the nucleons inside these nuclei. Detecting the properties of the electrons after these interactions revealed how fast the nucleon they hit were moving. This allowed physicists to pick out events in which the electron scattered off high-momentum, short-range-correlated nucleons.
The experiment was carried out in Jefferson Lab’s Experimental Hall A, shown here.
Protons and neutrons involved in three-nucleon short-range correlations are moving even faster than those in correlated pairs. This makes it more difficult to access in experiments. Originally, theoretical predictions proposed that accessing three-nucleon short-range correlations would require a beam energy beyond that at CEBAF. However, the researchers designed an experiment that works around this limitation by taking advantage of light nuclei.
The team used two light nuclear targets: helium-3 and tritium. Helium-3 has two protons and one neutron; tritium has two neutrons and one proton. They are known as mirror nuclei for their similar-but-opposite composition.
Because these light nuclei each only have three nucleons, researchers know exactly which particles are involved when an electron scatters off a three-nucleon interaction. That’s because there are no other nucleons to create other possible combinations.
This is not the case for heavy nuclei, in which the three nucleons could be many different combinations of protons and neutrons (not to mention the short-range-correlated pairs happening in the background). The lack of other possible combinations in this experiment simplified the analysis.
“We’re trying to show that it’s possible to study three-nucleon correlations at Jefferson Lab even though we can’t get the energies necessary to do these studies in heavy nuclei,” said Shujie Li, a research scientist at Lawrence Berkeley and a principal investigator on this experiment. “These light systems give us a clean picture — that’s the reason we put in the effort of getting a radioactive target material.”
Tritium is a radioactive isotope of hydrogen. Jefferson Lab had to implement rigorous safety precautions, including a redesign of the ventilation system in Hall A. The container that holds the radioactive tritium gas was filled at the DOE’s Savannah River National Laboratory, sealed, and shipped back to Jefferson Lab. Fortunately, the special instrumentation at Jefferson Lab allows the team to use a minimal amount of tritium, reducing potential safety concerns.
“This is a testament to what Jefferson Lab can do,” Arrington said. “CEBAF’s high intensity beam combined with the good detectors allowed us to use less tritium.”
From atomic nuclei to neutron stars
The results hint at the detection of three-nucleon short-range correlations. However, the researchers need more data before they feel comfortable claiming certainty.
“We want to do a similar experiment at Jefferson Lab to get more data at higher energy so that we can confirm what we observed already is a sign of three-nucleon short-range correlations,” Li said. “Eventually we want to understand how those extreme, high-momentum nucleons are generated in the nuclear system.”
Theory predicts these three-nucleon systems are generated in two ways. In one, three particles interact simultaneously. In the other, two nucleons interact and then one of those goes on to interact with another nucleon.
In addition to figuring out the mechanism of three-nucleon short-range correlations, the researchers would like to pin down exactly how fast they move. Ultimately, understanding short-range-correlated pairs and trios shows physicists how different particles and interactions contribute to the overall properties of the atomic nucleus, which is important for interpreting other kinds of nuclear experiments.
And this exploration brings an added bonus. Neutron stars are the remnants of exploded giant stars. Their inner workings are mysterious, but we know they are incredibly dense — just like the atomic nucleus. Physicists think that the way matter behaves inside a neutron star could be similar to the mechanisms of these short-distance nucleon interactions, meaning these experiments on matter at its tiniest scales may help interpret phenomena lightyears away.
After all, according to Arrington, “It’s much easier to study a three-nucleon correlation in the lab than in a neutron star.”
Further Reading
nuclei”>Particles Pick Pair Partners Differently in Small Nuclei
SRTE Support Enables Tritium Experiments at Jefferson Lab
nuclei”>Different Particles Get Different Treatment Inside Nuclei
Correlated Nucleons May Solve 35-Year-Old Mystery