Gravity helps show strong force in proton

The power of gravity extends across our visible universe. This can be seen in the lockstep of the moons as they orbit the planets; in wandering comets deflected by massive stars; and in the swirl of gigantic galaxies. These superb exhibitions highlight the influence of gravity on the largest scales of matter. Today, nuclear physicists are discovering that gravity also has much to offer at the smallest scales of matter.

New research by nuclear physicists at the U.S. Department of Energy’s Thomas Jefferson National Accelerator uses a method that links theories of gravitation to interactions between the smallest particles of matter to reveal new details at this larger scale. small. The research revealed, for the first time, a snapshot of the distribution of the strong force inside the proton. This snapshot details the shear stress that the force can exert on the quark particles that make up the proton. The result was recently published in Opinions on modern physics.

According to the study’s lead author, Jefferson Lab senior scientist Volker Burkert, the measurement reveals insight into the environment in which the proton’s building blocks evolve. Protons are made up of three quarks bound together by a strong force.

“At its peak, that’s more than four tons of force that would need to be applied to a quark to pull it from the proton,” Burkert explained. “Nature, of course, does not allow us to separate a single quark from the proton because of a property of quarks called “color.” There are three colors that mix the quarks in the proton to make it colorless from the outside , a necessary condition for its existence in space.

“Trying to extract a colored quark from the proton will produce a colorless quark/anti-quark pair, a meson, using the energy you put into it to try to separate the quark, leaving behind a colorless proton (or neutron). So, the 4 tons are an illustration of the intrinsic force of the proton.

The result is only the second of the proton’s mechanical properties to be measured. The mechanical properties of the proton include its internal pressure, mass distribution (physical size), angular momentum, and shear stress. The result was made possible thanks to a half-century-old prediction and two-decade-old data.

In the mid-1960s, it was hypothesized that if nuclear physicists could observe how gravity interacts with subatomic particles, such as the proton, such experiments could directly reveal the mechanical properties of the proton.

“But at that time there was no way. If you compare gravity with electromagnetic force, for example, there’s 39 orders of magnitude difference. So it’s completely hopeless, isn’t it ?” explained Latifa Elouadhriri, Jefferson Lab scientist and co-author of the study.

The decades-old data came from experiments conducted with Jefferson Lab’s Continuous Electron Beam Accelerator Facility (CEBAF), a DOE Office of Science user facility. A typical CEBAF experiment would involve an energetic electron interacting with another particle by exchanging with the particle a packet of energy and a unit of angular momentum called a virtual photon. The energy of the electron dictates which particles it interacts with in this way and how they react.

In the experiment, a force much greater than the four tons needed to extract a quark/antiquark pair was applied to the proton by the highly energetic electron beam interacting with the proton in a liquefied hydrogen target.

“We developed the program to study deeply virtual Compton scattering. This is where an electron exchanges a virtual photon with the proton. And in the final state, the proton stayed the same but moved backwards, and you have a very high-energy real photon produced, plus the scattered electron,” Elouadhriri said. “At the time we took the data, we didn’t know that beyond the three-dimensional imaging we wanted to use with this data, we were also collecting the data needed to access the mechanical properties of the proton.”

It turns out that this specific process – deeply virtual Compton scattering (DVCS) – could be linked to how gravity interacts with matter. The general version of this connection was stated in the 1973 textbook on Einstein’s theory of general relativity titled “Gravitation” by Charles W. Misner, Kip S. Thorne, and John Archibald Wheeler.

In it, they write: “Any massless spin 2 field would give rise to a force indistinguishable from gravitation, because a massless spin 2 field would couple to the stress-energy tensor in the same way as gravitational interactions. . “.

Three decades later, theorist Maxim Polyakov pursued this idea by establishing the theoretical foundations that link the DVCS process and gravitational interaction.

“This theoretical breakthrough established the relationship between the deeply virtual Compton scattering measurement and the gravitational form factor. And we were able to use it for the first time and extract the pressure we realized in the Nature paper in 2018, and now normal force and shear force,” Burkert explained.

A more detailed description of the links between the DVCS process and gravitational interaction can be found in this article describing the first result obtained from this research.

The researchers say their next step will be to extract the information they need from existing DVCS data to enable the first determination of the mechanical size of the proton. They also hope to take advantage of newer experiments, with higher statistics and energies, that continue DVCS research on the proton.

Meanwhile, the study’s co-authors have been amazed by the plethora of new theoretical efforts, detailed in hundreds of theoretical publications, that have begun to exploit this newly discovered avenue to explore the mechanical properties of the proton.

“And also, now that we are in this new era of discovery with the recently released 2023 Long-Term Plan for Nuclear Science. This will be a major pillar in the direction of science with new facilities and the development of new detectors. to see more of what can be done,” Burkert said.

Elouadhriri agrees.

“And in my opinion, this is just the start of something much bigger to come. It has already changed the way we think about the structure of the proton,” she said.

“Now we can express the structure of subnuclear particles in terms of forces, pressure and physical sizes that non-physicists can also relate to,” Burkert added.