The physics of proton gravitational form factors and their understanding in quantum chromodynamics have advanced significantly over the past two decades through both theory and experiment.a new paper inside modern physics review We provide an overview of this progress, highlighting the physical insights revealed by studies of the gravitational form factor and reviewing its interpretation in terms of the mechanical properties of protons.
A 2D representation of the quark contribution to the force distribution within the proton as a function of distance from the proton center. Light gray shading and long arrows indicate areas of stronger force, while dark gray shading and short arrows indicate areas of weaker force. Left panel: Normal force as a function of distance from center. The arrows change size and always point radially outward. Right panel: tangential force as a function of distance from center. The force changes direction and magnitude as indicated by the direction and length of the arrow. The sign of the force changes around 0.4 fm from the proton center. Image credit: Burkert other., doi: 10.1103/RevModPhys.95.041002.
“This measurement reveals insight into the environment experienced by the proton's components,” said Volker Burkert, principal investigator at the Jefferson Institute.
“A proton is made up of three quarks held together by a strong force.”
“At its peak, this amounts to more than four tons of force that would have to be applied to the quark to pull it out of the proton.”
“Of course, it is not possible in nature to separate just one quark from a proton because quarks have a property called color.”
“Protons have three colors mixed with quarks, and appear colorless from the outside. This is a requirement for them to exist in the universe.”
“When you try to extract a colored quark from a proton, the energy you invested in separating the quarks is used to create a meson, a pair of colorless quark and antiquark, leaving behind a colorless proton (or neutron).”
“In other words, the number four tons represents the strength of the force inherent in protons.”
The result is only the second of the mechanical properties of the protons to be measured.
Mechanical properties of protons include internal pressure (measured in 2018), mass distribution (physical size), angular momentum, and shear stress (shown here).
This result was made possible by predictions from half a century ago and data from 20 years ago.
In the mid-1960s, nuclear physicists realized that if they could observe how gravity interacted with subatomic particles like protons, such experiments could directly reveal the mechanical properties of protons. It was theorized that
“But at the time, we had no choice. For example, if you compare gravity to electromagnetic forces, there's a difference of 39 orders of magnitude. So it's pretty hopeless, right?” said Latifa El-Adhriri, a staff scientist at the Jefferson Institute. .
This data comes from experiments conducted at the Continuous Electron Beam Accelerator Facility (CEBAF) at the Jefferson Research Institute.
A typical CEBAF experiment involves a high-energy electron interacting with another particle by exchanging a packet of energy and a unit of angular momentum called a virtual photon with the particle. The energy of an electron determines which particles it interacts with in this way and how it reacts.
In the experiment, a high-energy beam of electrons interacting with protons inside a target of liquefied hydrogen gas exerted a much greater force on the protons than the four tons needed to pull out the quark/antiquark pair.
“We have developed a program to study deep virtual Compton scattering,” said Dr. El-Adrili.
“This is where electrons exchange virtual photons with protons.”
“And in the final state, the proton stays the same but recoils, and you actually produce one very high-energy photon, and you also get a scattered electron.”
“At the time we acquired the data, we did not know that beyond the intended 3D imaging with these data, we were also collecting the data needed to access the mechanical properties of the protons.”
“It turns out that this particular process, the highly virtual Compton scattering, may be related to how gravity interacts with matter.”
“A general version of this relationship is stated in Einstein's 1973 textbook on general relativity.gravityWritten by Charles W. Meisner, Kip S. Thorne, and John Archibald Wheeler. ”
“In it, they say, “A massless spin 2 field would give rise to a force indistinguishable from gravity, because a massless spin 2 field would couple with a stress-energy tensor in the same way as a gravitational interaction.'' It is written as 'It is from.'.'.
“Thirty years later, theorist Maxim Polyakov continued this idea and established a theoretical foundation linking deep virtual Compton scattering processes and gravitational interactions.”
“This theoretical breakthrough establishes a relationship between measurements of deep virtual Compton scattering and the gravitational shape factor.”
“And we were able to take advantage of that for the first time and bring out the pressure that we gave during the game.” Nature A paper was published in 2018 and now normal and shear forces are being studied,” Dr. Burkert said.
“A more detailed explanation of the relationship between deep virtual Compton scattering processes and gravitational interactions is provided in a new paper describing the first results obtained from this study.”
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V.D. Burkert other. 2023. Colloquium: Gravitational shape factor of protons. Rev.Mod. Physics 95(4):041002; doi: 10.1103/RevModPhys.95.041002
