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.
“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…
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