Nuclear physicists at Jefferson Lab have mapped the distribution of the strong force within the proton by employing a structure that ties to gravity, opening a new avenue for exploration.
The influence of gravity is unambiguously evident throughout the observable universe. Its effects are observed in the synchronized orbits of moons around planets, in comets that deviate from their trajectories due to the gravitational attraction of large stars and in the majestic spirals of enormous galaxies. These magnificent phenomena highlight the role of gravity on the largest scales of matter. Meanwhile, nuclear physicists are discovering the significant contributions of gravity at the smallest scales of matter.
New research led by nuclear physicists at the U.S. Department of Energy’s Thomas Jefferson National Accelerator Facility is using a method that connects theories of gravitation to interactions between the smallest particles of matter to reveal new details on this smallest scale. The investigation has now revealed, for the first time, a snapshot of the distribution of the strong force inside the proton. This snapshot details the shear stress that force can exert on the quark particles that make up the proton. The result was recently published in Modern Physics Reviews.
Insights into the structure of the proton
According to the study’s lead author, Volker Burkert, principal scientist at Jefferson Lab, the measurement reveals information about the environment experienced by the proton’s building blocks. Protons are made up of three quarks held together by strong force.
“At its peak, this is more than the four-ton force that would need to be applied to a quark to remove it from the proton,” explained Burkert. “Nature, of course, does not allow us to separate just one quark from the proton due to 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 extract a colored quark from the proton will produce a colorless quark/antiquark pair, a meson, using the energy you invested in trying to separate the quark, leaving a colorless proton (or neutron) behind. Therefore, the 4 tons are an illustration of the intensity of the force intrinsic to the proton.”
The result is only the second mechanical property of the proton 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 possible thanks to half a century of prediction and two decades of 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 directly reveal the proton’s mechanical properties.
“But at that time there was no way. If you compare gravity to electromagnetic force, for example, there are 39 orders of magnitude difference – so it’s completely impossible, right?” explained Latifa Elouadhriri, staff scientist at Jefferson Lab and co-author of the study.
Theoretical Foundations and Experimental Advances
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, exchanging an energy packet and a so-called virtual unit of angular momentum.