high protons provide new insights
New research reveals the 3D structure of nuclear resonance.
In the mid-20th century, scientists discovered that protons have the ability to resonate, similar to the vibrations of a clock. The next three decades were marked by advances in 3D imaging of the proton and a deeper understanding of its ground-state structure. However, the understanding of the 3D structure of resonant protons is still limited.
A recent experiment at the US Department of Energy’s Thomas Jefferson National Accelerator Facility took a closer look at the three-dimensional structure of proton-neutron resonances. The research adds another piece of the puzzle to a big picture of the chaotic nascent universe that existed after the Big Bang.
Studying the fundamental properties and behavior of nucleons provides important insights into the fundamental elements of matter. Nucleons are the protons and neutrons that make up the nucleus of an atom. Each nucleon consists of three quarks tightly bound together by gluons due to the strong interaction, the strongest force in nature. The most stable and lowest energy state in the nucleus is called the ground state. But when a nucleon is forced into a higher energy state, its quarks spin and vibrate against each other, exhibiting what is known as nucleon resonance. A team of physicists from Justus Liebig-University (JLU) in Giessen, Germany, and the University of Connecticut, who led the CLAS collaboration, conducted an experiment to study these nuclear resonances. The experiment was conducted at Jefferson Laboratory’s world-class Continuous Electron Beam Accelerator Facility (CEBAF). CEBAF is a user facility in the US Department of Energy’s Office of Science that supports the research of more than 1,800 nuclear physicists worldwide. The results were recently published in the prestigious peer-reviewed journal Physical Review Letters. Stefan Diehl, head of the analysis, said the team’s work revealed the fundamental properties of nuclear resonance. Diehl is a postdoctoral researcher and program leader at the Institute of Physics II at the University of Giessen and a researcher at the University of Connecticut. The work also inspires new research into the 3D structure and excitation processes of resonant protons, he said.
“This is the first time we’ve made some measurements and observations that are sensitive to the 3D properties of this excited state,” Diehl said. “In principle, this is just the beginning, this kind of measurement opens up a new area of research.”
The mystery of how matter came to be.
The experiment was conducted in Experiment Hall B from 2018 to 2019 using the CLAS12 detector at Jefferson Laboratory. A beam of high-energy electrons is sent into a cooled hydrogen chamber. An electron hits a target proton, excites the quarks inside, and combines with quark-antiquark states (so-called mesons) to form a nucleon resonance.
Excitations are short-lived, but they leave evidence of their existence in the form of new particles formed by the energy that excited the particles as they dissipate. These new particles lived long enough for the detectors to pick them up, allowing the team to reconstruct the resonances.
Diehl and others recently discussed their findings at a joint workshop “Probing Resonance Structures with Transient GPD” in Trento, Italy. The research has inspired two theoretical groups to publish papers on the work.
The team also plans to perform more experiments at Jefferson Lab using different targets and polarizations. By causing polarized protons to scatter electrons, they can achieve different properties of the scattering process. In addition, the study of similar processes, such as resonance with high-energy photons, could provide additional important information. With such experiments, Diehl said, physicists can tease apart the properties of the early universe after the Big Bang.
“In the beginning, the early universe was just a bunch of plasma made up of quarks and gluons, and because the energy was so high, they were all spinning,” Diehl said. “Then at some point matter started to form, and the first states to form were excited nucleons. As the universe expanded further, it cooled and ground-state nucleons emerged. “Through these studies, we can understand the properties of these resonances. This will tell us how the matter in the universe came to be and why the universe exists in the form it does.”
Reference: “The first measurement of the beam and spin asymmetry of the solid exclusive π−Δ++ electroproduction from the proton” Author: S. Diehl et al. (CLAS collaboration), 11 July 2023, Physical Review Letters. DOI: 10.1103/PhysRevLett.131.021901
Born in Lich, Germany, Diehl pursued physics as a means of understanding natural phenomena and the nature of the world. He received his bachelor’s, master’s and doctoral degrees at the University of Giessen in Giessen. He is a collaborative member of CLAS, PANDA, ePIC and COMPASS and has co-authored more than 70 peer-reviewed publications.
The research was funded by the US Department of Energy.