In the mid-20th century, scientists discovered that protons have the ability to resonate, like the vibrations of a clock. Over the next three decades, three-dimensional images of the proton continued to advance, and a deeper understanding of the structure of the proton in its ground state was gained. However, understanding of the three-dimensional structure of resonant protons is still limited.
A recent experiment conducted at the U.S. Department of Energy's Thomas Jefferson National Accelerator Facility delves into the three-dimensional structure of proton and neutron resonances. This research provides another piece of the puzzle for the picture of the universe where chaos first emerged after the Big Bang.
Studying the basic properties and behavior of nucleons provides important clues to our understanding of the basic building blocks of matter. Nucleons are the protons and neutrons that make up the nucleus of an atom. Each nucleon is made up of three quarks, held together by gluons under the strong interaction - the strongest force in nature.
The most stable and lowest energy state of a nucleus is called the ground state. But when a nucleon is forcibly excited to a high-energy state, its quarks rotate and vibrate with each other, exhibiting what's called a nucleon resonance.
A team of physicists from Justus Liebig University (JLU) in Giessen, Germany, and the University of Connecticut, leading the CLAS collaboration, conducted an experiment to explore these nucleon resonances. The experiments were conducted at Jefferson Laboratory's world-class Continuous Electron Beam Accelerator Facility (CEBAF). CEBAF is a user facility of the Department of Energy's Office of Science that supports the research of more than 1,800 nuclear physicists around the world. The research results were recently published in the prestigious peer-reviewed journal Physical Review Letters.
Stefan Diehl, leader of the analysis team, said the team's work revealed the fundamental properties of nuclear resonance. Diehl is a postdoctoral researcher and project leader at the Institute of Physics II of the Union University of Giessen and a research professor at the University of Connecticut. This work also stimulates new research into the three-dimensional structure and excitation processes of resonant protons.
"This is the first time we have made measurements and observations that are sensitive to the three-dimensional characteristics of this excited state," says Diehl. "In principle, this is just the beginning, and this kind of measurement is opening up a new field of research."
The experiment was conducted in Experimental Hall B in 2018-2019, using the CLAS12 detector at Jefferson Laboratory. A beam of high-energy electrons is fed into a chamber of cooled hydrogen gas. The electron strikes the target's proton, exciting the quarks within it and combining with quark-antiquark states (so-called mesons) to create a nucleon resonance.
Such excitations are fleeting, but they leave evidence of their existence in the form of new particles that are created by the energy fission of the excited particles. These new particles are long enough for detectors to capture them, so the team can reconstruct the resonances.
Diehl et al. recently discussed their results at the joint workshop "Exploring Resonant Structures with Transitional GPDs" in Trento, Italy. This research has inspired two theoretical groups to publish related papers.
The team also plans to conduct additional experiments at Jefferson Lab using different targets and polarizations. By electron scattering from polarized protons, they can obtain different characteristics of the scattering process. In addition, the study of similar processes, such as combining high-energy photons to create resonances, can also provide more important information.
Diehl said that through these experiments, physicists can figure out the characteristics of the early universe after the Big Bang: "In the beginning, the early universe only had some plasma composed of quarks and gluons, and these plasmas were spinning because the energy was too high. Then, at a certain point, matter began to form, and the first ones to form were excited nucleon states. When the universe expanded further, it cooled down and the ground state nucleons emerged."
"Through these studies, we can understand the characteristics of these resonances. This will tell us how matter in the universe formed and why the universe exists in the form it does."