The physics of the gravitational form factors of the proton, as well as their understanding within quantum chromodynamics, has advanced significantly in the past two decades through both theory and experiment. A new paper in the Reviews of Modern Physics provides an overview of this progress, highlights the physical insights unveiled by studies of gravitational form factors, and reviews their interpretation in terms of the mechanical properties of the proton.
2D display of the quark contribution to the distribution of forces in the proton as a function of the distance from the proton’s center. The light gray shading and longer arrows indicate areas of stronger forces, while the dark shading and shorter arrows indicate areas of weaker forces. Left panel: normal forces as a function of distance from the center; the arrows change magnitude and always point radially outward. Right panel: tangential forces as a function of distance from the center; the force changes direction and magnitude as indicated by the direction and length of the arrow; the forces change sign near 0.4 fm from the proton center. Image credit: Burkert et al., doi: 10.1103/RevModPhys.95.041002.
“The measurement reveals insight into the environment experienced by the proton’s building blocks,” said Jefferson Lab principal staff scientist Volker Burkert.
“Protons are built of three quarks that are bound together by the strong force.”
“At its peak, this is more than a four-ton force that one would have to apply to a quark to pull it out of the proton.”
“Nature, of course, does not allow us to separate just one quark from the proton because of a property of quarks called color.”
“There are three colors that mix quarks in the proton to make it appear colorless from the outside, a requirement for its existence in space.”
“Trying to pull a colored quark out of the proton will produce a colorless quark/anti-quark pair, a meson, using the energy you put in to attempt to separate the quark, leaving a colorless proton (or neutron) behind.”
“So, the 4-tons is an illustration of the strength of the force that is intrinsic in the proton.”
The result is only the second of the proton’s mechanical properties to be measured.
The proton’s mechanical properties include its internal pressure (measured in 2018), its mass distribution (physical size), its angular momentum, and its shear stress (shown here).
The result was made possible by a half-century-old prediction and two-decade-old data.
In the mid 1960s, it was theorized that if nuclear physicists could see how gravity interacts with subatomic particles, such as the proton, such experiments could reveal the proton’s mechanical properties directly.
“But at that time, there was no way. If you compare gravity with the electromagnetic force, for instance, there is 39 orders of magnitude of difference — so it’s completely hopeless, right?” said Jefferson Lab staff scientist Latifa Elouadhriri.
The data came from experiments conducted with Jefferson Lab’s Continuous Electron Beam Accelerator Facility (CEBAF).
A typical CEBAF experiment would entail an energetic 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 the electron dictates which particles it interacts with in this way and how they respond.
In the experiment, a force even much greater than the four tons needed to pull out a quark/antiquark pair was applied to the proton by the highly energetic electron beam interacting with the proton in a target of liquified hydrogen gas.
“We developed the program to study deeply virtual Compton scattering,” said Dr. Elouadhriri said.
“This is where you have an electron exchanging a virtual photon with the proton.”
“And at the final state, the proton remained the same but recoiled, and you have one real very highly energetic photon produced, plus the scattered electron.”
“At the time we took the data, we were not aware that beyond the 3D imaging we intended with these data, we were also collecting the data needed for accessing the mechanical properties of the proton.”
“It turns out that this specific process — deeply virtual Compton scattering — could be connected to how gravity interacts with matter.”
“The general version of this connection was stated in the 1973 textbook on Einstein’s general theory of relativity titled ‘Gravitation’ by Charles W. Misner, Kip S. Thorne and John Archibald Wheeler.”
“In it, they wrote, ‘Any mass-less spin-2 field would give rise to a force indistinguishable from gravitation, because a mass-less spin-2 field would couple to the stress-energy tensor in the same way that gravitational interactions do’.”
“Three decades later, theorist Maxim Polyakov followed up on this idea by establishing the theoretical foundation that connects the deeply virtual Compton scattering process and gravitational interaction.”
“This breakthrough in theory established the relationship between the measurement of deeply virtual Compton scattering to the gravitational form factor.”
“And we were able to use that for the first time and extract the pressure that we did in the Nature paper in 2018, and now the normal force and the shear force,” Dr. Burkert said.
“A more detailed description of the connections between the deeply virtual Compton scattering process and the gravitational interaction can be found in our new paper describing the first result obtained from this research.”
V.D. Burkert et al. 2023. Colloquium: Gravitational form factors of the proton. Rev. Mod. Phys 95 (4): 041002; doi: 10.1103/RevModPhys.95.041002
Source : Breaking Science News