Science Magazine Reports Experimental Results of Jefferson Lab Research Team Including ODU's Weinstein
A 1985 planning document for the atom-smasher facility known now as the Jefferson Lab in Newport News identifies five key areas for experiments, one of which involves two-body correlations in nuclei. Findings reported in recent weeks by researchers at the accelerator facility show that the correlation studies are still very much a priority, says Old Dominion University physicist Lawrence Weinstein.
The latest findings from Jefferson Lab are reported in the paper "Probing Cold Dense Nuclear Matter" published June 13 in Science magazine. They show that fast-moving nucleons (protons and neutrons) in the nucleus of an atom form short-lived, short-range correlated pairs and that the nature of the correlation produces nearly 20 times more proton-neutron pairs than proton-proton or neutron-neutron pairs.
The researchers conducted an experiment in Jefferson Lab's Experimental Hall A to directly and simultaneously measure what happens when an accelerated electron knocks a paired-up proton out of the nucleus of a carbon atom. Sophisticated detectors could measure the recoiling electrons, the knockout protons, as well as the correlated nucleons, which also recoil and are ejected in a different direction.
Scientists involved in the experiment say their results show for the first time in an unambiguous way that large momentum nucleons in nuclei do come in pairs, and come mainly in proton-neutron pairs.
Why is this significant? For one reason, the findings add to our understanding of cold dense nuclear systems, such as neutron stars, which contain relatively few protons. "We therefore speculate that the small concentration of protons inside neutron stars might have a disproportionately large effect that needs to be addressed in realistic descriptions of neutron stars," the research paper states.
What we can learn about ultra-dense neutron stars is important for many reasons. These stellar balls-of a diameter of a dozen miles or so-are what remain from exploded stars. Matter in them is so compressed that protons and electrons fuse into neutrons, according to the calculations of physicists. In fact, the material at the center of one of these stars is so dense it cannot be reproduced on Earth. If we could bring back a sample of this material, a teaspoon of it would weigh about a billion tons. So physicists have long believed that they could learn things about the universe from neutron stars that otherwise would remain a mystery.
For Weinstein, all of this boils down to steady progress the nuclear physicists at Jefferson Lab are making toward revealing fundamental structures and properties of matter. "One more brick in the building of science," is how he describes the latest research paper.
He led an earlier two-body correlations study at Jefferson Lab, the results for which were published in 2003, and he is a co-author of the paper published in June by Science. The first author of the latest paper is Ramesh Subedi, whose research at Jefferson Lab as a Kent State University doctoral student is the basis for the paper. More than 2,000 scientists from around the world perform experiments at the U.S. Department of Energy lab, the full name for which is the Thomas Jefferson National Accelerator Facility. ODU has a 12-member experimental nuclear research group, as well as graduate students, who conduct research at the lab.
Weinstein, who was chosen a Fellow in the American Physical Society for his "original contributions to the study of nucleon-nucleon correlations in nuclei," has won university awards for his teaching as well as his research. He is known for his ability to make science interesting to the layman. (The recently published book, "Guesstimation: Solving the World's Problems on the Back of a Cocktail Napkin," that he wrote with ODU mathematics professor John Adam, is entertaining as well as instructional and has enjoyed popular success.)
When Weinstein begins to explain developments in his research field, he speaks of two opposing views of the nucleus. The older view, known as the nuclear shell model, might be described by marbles being vigorously shaken in a bag. If we shoot a projectile at the bag, much like scientists at Jefferson Lab shoot electrons at targets, we can knock out one marble without much affecting the other marbles. This implies a nucleus composed of independent particles. Another view might be described by way of a water balloon. If we shoot a projectile at the balloon, we get "splat" and all of the strongly correlated nucleons would be affected.
Recent research, including the findings published in Science, shows that the nucleus is composed of a majority of slower-moving independent nucleons and a minority of faster-moving nucleons in correlated pairs. In other words, the nucleus is part bag of marbles and part balloon full of water.
Jefferson Lab researchers in recent years have measured momentum distribution in nucleon pairs and otherwise sought to characterize the correlations. When nucleons are paired, they overlap, which is intriguing because this someday may provide the answer to what Weinstein calls a big question: "Why do protons and neutrons remain packages of three quarks? Why not a quark soup rather than three-quark packages?"
The latest paper points to a dominant "tensor correlation," which, according to quantum mechanics, can exist between a proton and nucleon, but not between proton-proton and neutron-neutron pairs. That explains the nearly 20 to 1 prevalence among high-momentum pairs of heterogeneous pairs versus homogeneous pairs.