Quarks Under Pressure in the Proton
The first measurement of a subatomic particle’s mechanical property reveals the distribution of pressure inside the proton.
The determination of the pressure distribution inside the proton is the first measurement of a mechanical property of a subatomic particle. The measurement found that the proton’s building blocks, quarks, are subjected to a pressure of 100 decillion Pascal (1035) near the center of a proton, which is about 10 times greater than the pressure in the heart of a neutron star.
The result gives insight into the distribution of the strong force inside the proton, which is the force that holds the proton together. Additionally, the success of the experimental technique used to obtain this result now opens the door for exploring other mechanical properties of the proton.
Inside every proton in every atom in the universe is a pressure cooker environment that surpasses the atom-crushing heart of a neutron star. That’s according to the first measurement of a mechanical property of subatomic particles, the pressure distribution inside the proton, which was carried out by scientists at Jefferson Lab. Once thought impossible to obtain, this measurement is the result of a clever pairing of two theoretical frameworks with existing data. First, there are the generalized parton distributions. GPDs allow researchers to produce a 3D image of the proton’s structure as probed by the electromagnetic force. The second are the gravitational form factors of the proton. These form factors describe what the mechanical structure of the proton would be if researchers could probe the proton via the gravitational force. While the gravitational form factors of the proton are beyond current experimental reach, recent theoretical work has connected them to GPDs, allowing the results from electromagnetic probes of protons to substitute for gravitational probes. The researchers used existing data for GPDs from Jefferson Lab’s CEBAF Large Acceptance Spectrometer to determine that the proton’s building blocks, the quarks, are subjected to a pressure of 100 decillion Pascal (1035) near the center of a proton, which is about 10 times greater than the pressure in the heart of a neutron star. The next step is to apply this revolutionary technique to even more precise data from the Continuous Electron Beam Accelerator Facility at Jefferson Lab to reduce the uncertainties in the current analysis and begin working toward revealing other mechanical properties of the ubiquitous proton, such as the internal shear forces and the proton’s mechanical radius.
This material is based on work supported by the US Department of Energy, Office of Science, Office of Nuclear Physics under contract DE-AC05-06OR23177.
V. D. Burkert, L. Elouadrhiri & F. X. Girod. “The Pressure Distribution Inside the proton.” Nature 557, 396-399 (2018). doi: 10.1038/s41586-018-0060-z