Nuclear Physicists Use High-Energy Electron Beam To Hunt for Clues of Color Transparency in Protons

Protons Electron Probe

Nuclear physicists hoped to observe color transparency in protons (red) by spotting small-size protons with an electron probe (green). These small-size protons could more easily exit the nucleus of the atom, shown here, demonstrating color transparency. Credit: DOE’s Jefferson Lab

The Science

Protons populate the nucleus of every atom in the universe. Inside the nucleus, they cling tightly to neighboring protons and neutrons. However, it may be possible to knock out protons that are in a smaller size configuration, so that they interact less with nearby particles as they exit the nucleus. This phenomenon is called color transparency. Nuclear physicists hunting for signs of color transparency in protons recently came up empty handed.

The Impact

The theory that describes the behavior of particles made of quarks is called quantum chromodynamics (QCD). QCD includes many common subatomic particles, such as protons and neutrons. It also predicts the phenomenon of color transparency. Physicists have observed color transparency in simpler, two-quark particles called pions. If physicists can observe or rule out color transparency for protons, a more complicated three-quark system, they would gain important clues regarding the differences between two- and three-quark systems in QCD.


Protons are made of three quarks bound up by the strong force that is part of the Standard Model of Particle Physics. In an ordinary proton, the strong force leaks out, making the proton interact with nearby protons and neutrons in the nucleus. That’s according to QCD, the theory that describes how quarks and the strong force interact. In QCD, the strong force is also referred to as the color force. QCD predicts that the proton can fluctuate to a state where its constituent quarks become even more tightly knit and wrapped up so tightly that the color force no longer leaks out. When that happens, the proton can move more freely. This phenomenon is called “color transparency,” since the proton has become invisible to the color force of nearby particles.

An earlier experiment showed color transparency in simpler particles made of just two quarks called pions, and another experiment suggested that protons also exhibit color transparency. This newest experiment was conducted with the Continuous Electron Beam Accelerator Facility (CEBAF), an Office of Science user facility. CEBAF’s high-energy electrons crashed into the nuclei of carbon atoms, and physicists measured outgoing electrons and several thousand protons. The researchers observed no signs of color transparency. The next step is to conduct higher-precision experiments to both better observe the phenomenon in two-quark particles and to continue to hunt for it in three-quark particles. These further measurements may help physicists better elucidate the differences between two- and three-quark systems in QCD.

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