Nuclear scientists have confirmed that the present description of the proton structure isn’t perfect. There has been an uptick in the data in probes of the proton’s structure, according to new precision measurement of the proton’s electric polarizability at the U.S. Department of Energy’s Thomas Jefferson National Accelerator Facility.
Precision measurement of how a proton’s structure deforms in an electric field has revealed new details about an unexplained spike in proton data. The sizes of the proton’s electric polarizability reveal how susceptible the proton is to deformation, or stretching, in an electric field. It also confirmed the presence of the anomaly and raised questions about its origin.
Additionally, a precise assessment of the proton’s electric polarizability can assist in bridging the gap between the various explanations of the proton. A proton may seem like a single opaque particle or a composite particle consisting of three quarks held together by a strong force, depending on how it is probed.
Ruonan Li, the first author of the new paper and a graduate student at Temple University, said, “We want to understand the substructure of the proton. And we can imagine it like a model with three balanced quarks in the middle. Now, put the proton in the electric field. The quarks have positive or negative charges. They will move in opposite directions. So, the electric polarizability reflects how easily the electric field will distort the proton.”
Nuclear scientists utilized a technique known as virtual Compton scattering to examine this distortion. It begins with a meticulously regulated beam of powerful electrons from Jefferson Lab’s Continuous Electron Beam Accelerator Facility. The electrons are sent crashing into protons.
In virtual Compton scattering, electrons interact with other particles by emitting an energetic photon or particle of light. The electron’s energy determines the energy of the photon it emits, which also determines how the photon interacts with other particles.
While more energetic photons will shoot inside the proton to engage with one of its quarks, lower energy photons may bounce off the proton’s surface. According to theory, a smooth curve will appear when these photon-quark interactions are plotted from lower to higher energies.
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.
“We see that there is some local enhancement to the magnitude of the polarizability. The polarizability decreases as the energy increases, as expected. And, at some point, it appears to be coming temporarily up again before it goes down. Based on our current theoretical understanding, it should follow a very simple behavior. We see something that deviates from this simple behavior. And this is the fact that is puzzling us at the moment.”
“The theory predicts that the more energetic electrons are more directly probing the strong force as it binds the quarks together to make 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 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.”
Physicists said, “The next step is to tease out further the details of this anomaly and conduct precision probes to check for other points of deviation and to provide more information about the anomaly’s source.”
Sparveris said, “We also need to measure precisely the shape of this enhancement. The shape is important to further elucidate the theory.”
Journal Reference:
- Li, R., Sparveris, N., Atac, H. et al. Measured proton electromagnetic structure deviates from theoretical predictions. Nature (2022). DOI: 10.1038/s41586-022-05248-1
