New Experiment Reveals Complex Structure of Proton States

The structure of the proton, a fundamental particle at the heart of atomic nuclei, has long intrigued scientists. Recent experiments at the U.S. Department of Energy’s Thomas Jefferson National Accelerator Facility have provided new insights into the complex makeup of protons and their excited states. This research, conducted using the CLAS12 detector, marks a significant advancement in understanding how protons behave under various energy conditions.

Understanding Proton Structure

The proton constitutes a major portion of visible matter in the universe. According to Kyungseon Joo, a physics professor at the University of Connecticut, “A large part of the visible matter in the universe is made of protons. If you want to understand the universe, it is important to understand the proton.” Despite its significance, the internal structure of the proton remains complex and not fully understood, particularly in lower energy interactions where protons can be excited into higher energy states known as resonances.

Current knowledge about protons is primarily derived from high-energy processes that probe their internal components—quarks and gluons—through significant momentum transfer. In contrast, lower-energy interactions have been less explored, leaving a gap in our understanding of proton resonances. These excited states, which possess higher mass than the ground-state proton, are crucial for nuclear physicists aiming to grasp the proton’s structure more comprehensively.

Breakthroughs with CLAS12

Results from experiments conducted with the CLAS12 apparatus have opened new pathways for probing proton structure. The findings were recently published in Physical Review C. The CLAS12 detector is designed to study the interactions of electrons with protons, allowing researchers to observe how protons behave across a wide range of energy levels.

During the experiments, scientists focused on inclusive electron scattering, where only the scattered electron is detected. This method has provided a new avenue for examining the ground-state proton’s structure within the resonance region. Daniel Carman, a staff scientist at Jefferson Lab, emphasized that the new data challenge previous expectations that resonance contributions would diminish at higher momentum transfers. Instead, the results show clear resonance signatures across all studied energies.

This breakthrough is significant, as it allows researchers to investigate the transition from the strong coupling regime of quarks and gluons to the perturbative regime, where interactions are weaker. According to Carman, “New experimental results will not only shed more light on the proton but also on the strong interaction that underlies the generation of protons from quarks and gluons.”

The analysis of the first cross-section data from CLAS12 was spearheaded by Valerii Klimenko as part of his doctoral research at the University of Connecticut. Klimenko’s work revealed that contributions from excited proton states remain relevant even at high momentum transfers, providing further insight into the strong interaction’s nature.

This research highlights the importance of Quantum Chromodynamics (QCD) in understanding the strong force that binds quarks and gluons together. By stress-testing the theoretical frameworks of QCD, scientists hope to unlock further mysteries surrounding the formation of matter in the universe.

The Jefferson Lab, managed by Jefferson Science Associates, LLC, operates as a leading research facility for nuclear physics and supports over 1,650 physicists globally. The facility’s advanced capabilities enable researchers to probe the fundamental components of matter and address pressing scientific challenges.

As the field of nuclear physics continues to evolve, the insights gained from these experiments may significantly enhance our understanding of protons and the fundamental forces that govern the matter we observe in the universe.