麻豆淫院

The small but ubiquitous proton serves as a foundation for the bulk of the visible matter in the universe. It abides at the very heart of matter, giving rise to everything we see around us as it anchors the nuclei of atoms. Yet, its structure is amazingly complex, and the quest to understand these details has occupied theorists and experimenters alike since its discovery over a century ago.

"A large part of the visible matter in the universe is made of protons," said Kyungseon Joo, a physics professor at the University of Connecticut. "And so, if you want to understand the universe, it is important to understand the proton."

Currently, proton structure is only well understood in processes where they are probed at high energy and where a lot of momentum is transferred to the proton. In such cases, the probes interact with the quarks and gluons (together called "partons") that form the proton so quickly that they react like a tightly set rack of billiard balls hit by a well-struck cue ball.

However, where protons are probed with lower energies or a lower momentum transfer occurs, our knowledge is much more limited. In this regime, the interaction can transform the proton into one of its excited states鈥攌nown as a resonance. These excited states possess higher energy, or equivalently, greater mass than the ground-state proton.

Nuclear physicists are interested in studying excited-state protons in this resonance region to learn more about their detailed makeup. Furthermore, to learn about the structure of the ground state proton within the resonance region, the contributions from its resonance states must be disentangled.

Now, results from an experiment conducted at the U.S. Department of Energy's Thomas Jefferson National Accelerator Facility with an apparatus called CLAS12 are offering new pathways for gaining insight into the structure of the proton from measurements within this resonance region.

These measurements allow nuclear physicists, for the first time, to search for the manifestation of signals from excited proton states at still unexplored distance scales and address the long-term open question of whether excited proton states remain relevant as momentum transfer increases. The results were recently in 麻豆淫院ical Review C.

Looking into the proton within the resonance region

The proton underlies all the matter we can see, existing in every nucleus of every atom of the universe. As already mentioned, it's made of quarks and gluons, and these partons are bound together by the strong interaction鈥攐ne of the four forces in nature.

Daniel Carman, a Jefferson Lab staff scientist, explained that while complicating our understanding of the internal structure of the ground-state proton, the proton's excited states offer unique information on many facets of the strong interaction seen in their generation.

According to Carman, recent measurements with the CLAS12 detector of electron scattering off protons when only the scattered electron is detected, known as inclusive electron scattering, open a new avenue for exploring the structure of the ground-state proton in the resonance region.

For the first time, these measurements provide information on the evolution of the structure of the ground-state proton within the resonance region over a broad range of distance scales, spanning the transition from the regime of strongly coupled quarks and gluons to the domain where their interactions become weaker, known as the perturbative regime.

"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鈥攖his is a fundamental goal of nuclear physics," he said.

In this experiment, nuclear physicists compared results they get from experiments at higher energy and momentum transfer鈥攃alled deep inelastic scattering鈥攚ith those at lower energy transfer in the resonance region.

Before measurements with CLAS12, it was expected that the resonance contributions would disappear as the momentum transfer to the proton increases. However, the new CLAS12 data conclusively demonstrate for the first time that this is not the case. The resonance signatures are clearly seen in the data over the full range of energies studied, from low to very high momentum transfer.

First observation of proton excited states at high momentum transfer

CLAS12 is the CEBAF Large Acceptance Spectrometer at 12 GeV. It was designed, built, and installed as part of the 12 GeV Upgrade of the Continuous Electron Beam Accelerator Facility (CEBAF). This apparatus is being used by a collaboration of nuclear physicists to study the structure of the subatomic particles and forces that build the visible universe. As a DOE Office of Science user facility, CEBAF supports the research of more than 1,650 nuclear physicists worldwide.

In experiments, CEBAF fires a beam of electrons at hydrogen atoms inside a target. The target is nestled deep within the CLAS12 apparatus.

"When we use an electron beam," Carman explained, "it's actually a virtual photon that's radiated by the incoming electron that interacts with our proton target."

He noted that the virtual photon is not like a real photon, but rather it serves as a probe that allows researchers to tune its wavelength independently of its energy.

"By decreasing the wavelength of the virtual photon, we can see deeper and deeper into the proton to probe its structure from the deep inelastic region of high energy transfers into the resonance region of smaller energy transfers," he said.

Because CLAS12 has a large acceptance, it allows coverage over a broad range of energies over the entire resonance region at any given momentum transfer to the proton鈥攁nd this can all be done in a single experiment for the first time. This offers a unique opportunity to unravel the proton structure and account for the resonant contributions.

The data analysis was led by Valerii Klimenko as part of his Ph.D. thesis project at the University of Connecticut under the direction of Kyungseon Joo. Klimenko is now a postdoctoral researcher at DOE's Argonne National Lab.

The first cross section data from CLAS12 have become available from Klimenko's analysis. These data revealed that the contributions from proton excited states to inclusive electron scattering remain relevant to very high momentum transfer to the proton.

These new data also provide a means for further applications in "stress testing" nuclear physicists' understanding of the evolution of the strong interaction with distance scale that emerges from the theory of the strong interaction known as Quantum Chromodynamics. QCD encompasses nuclear physicists' theoretical understanding of how the strong force influences quarks and gluons in building matter.

Thus, stress testing QCD opens the door to further discoveries about how matter in the universe is created from the most elementary objects known so far, quarks and gluons.