An unexplained bump in the data has been confirmed by a new precision measurement of the proton’s electric polarizability. The electric polarizability of a proton indicates how susceptible it is to deformation, or stretching, in an electric field. Electric polarizability, like size and charge, is a fundamental property of proton structure. When the data bump was first observed, it was widely assumed to be a fluke; however, this new, more precise measurement confirms the presence of the anomaly and indicates that an unknown facet of the strong force is at work.
Nuclear physicists have confirmed that the current model of proton structure isn’t perfect. A new precision measurement of the proton’s electric polarizability at the US Department of Energy’s Thomas Jefferson National Accelerator Facility revealed a data bump in probes of the proton’s structure. Though previously thought to be a fluke, this new, more precise measurement has confirmed the presence of the anomaly and raises questions about its origin. The findings were recently published in the journal Nature.
According to Ruonan Li, first author on the new paper and a graduate student at Temple University, measurements of the proton’s electric polarizability reveal how susceptible the proton is to deformation, or stretching, in an electric field. Like size or charge, the electric polarizability is a fundamental property of proton structure.
We want to understand the proton’s substructure. And we can visualize it as a model with three balanced quarks in the center. Place the proton in the electric field now. Quarks can be positively or negatively charged. They’ll be moving in opposite directions. As a result, the electric polarizability of a proton reflects how easily it will be distorted by an electric field.
Ruonan Li
Furthermore, a precise determination of the proton’s electric polarizability can help bridge the gap between the various proton descriptions. A proton can appear as an opaque single particle or as a composite particle made of three quarks held together by a strong force, depending on how it is probed.
“We want to understand the proton’s substructure. And we can visualize it as a model with three balanced quarks in the center” Li elaborated. “Place the proton in the electric field now. Quarks can be positively or negatively charged. They’ll be moving in opposite directions. As a result, the electric polarizability of a proton reflects how easily it will be distorted by an electric field.”
To probe this distortion, nuclear physicists used a process called virtual Compton scattering. It starts with a carefully controlled beam of energetic electrons from Jefferson Lab’s Continuous Electron Beam Accelerator Facility, a DOE Office of Science user facility. The electrons are sent crashing into protons.
Electrons interact with other particles in virtual Compton scattering by emitting an energetic photon, or light particle. The electron’s energy determines the energy of the photon it emits, which in turn determines how the photon interacts with other particles.
Lower energy photons may bounce off the proton’s surface, whereas higher energy photons will blast inside the proton and interact with one of its quarks. When these photon-quark interactions are plotted from lower to higher energies, theory predicts that they will form a smooth curve.
Nikos Sparveris, an associate professor of physics at Temple University and spokesperson for the experiment, said this simple picture didn’t hold up to scrutiny. The measurements instead revealed an as-yet-unexplained bump.
“What we see is that there is some local enhancement to the magnitude of the polarizability,” he explained. “The polarizability decreases as the energy increases as expected, and at some point, it appears to be temporarily rising before falling again.”
“Based on our current theoretical understanding, it should follow a very simple behavior; however, we see something that deviates from this simple behavior, and this is what is currently perplexing us.” According to the theory, the more energetic electrons probe the strong force more directly as it binds the quarks together to form the proton.
This weird spike in the stiffness that nuclear physicists have now confirmed in the proton’s quarks signals that an unknown facet of the strong force may be at work.
“There is something that we’re clearly missing at this point. The proton is the only composite building block in nature that is stable. So, if we are missing something fundamental there, it has implications or consequences for all of physics,” Sparveris confirmed.
The next step, according to the physicists, is to tease out the details of this anomaly and conduct precision probes to look for other points of deviation and provide more information about the anomaly’s source.
“We want to measure more points at different energies to present a clearer picture and see if there is any additional structure,” Li explained. Sparveris concurred. “We also need to precisely measure the shape of this enhancement, because the shape is important for further elucidating the theory,” he added.
