Scientists would use the Electron-Ion Collider to take three-dimensional precision snapshots of the internal structure of protons and atomic nuclei. As they pierce through the larger particles, the high-energy electrons will interact with the internal microcosm to reveal unprecedented details—zooming in beyond the simplistic structure of three valence quarks bound by a mysterious force. Recent experiments indicate that gluons—the glue-like carriers of the strong nuclear force that binds quarks together—multiply and appear to linger within particles accelerated close to the speed of light, and play a significant role in establishing key properties of protons and nuclear matter. By taking images at a range of energies, an EIC will reveal features of this “ocean” of gluons and the “sea” of quark-antiquark pairs that form when gluons split—allowing scientists to map out the particles’ distribution and movement within protons and nuclei, similar to the way medical imaging technologies construct 3D dynamic images of the brain. These studies may help reveal how the energy of the massless gluons is transformed through Einstein’s famous equation, E=mc2, to generate most of the mass of visible matter.

The Electron-Ion Collider would be the world’s first polarized electron-proton collider where both the electron and proton beams have their spins aligned in a controllable way. This polarization makes it possible to make precision measurements of how a proton’s constituent quarks and gluons and their interactions contribute to the proton’s intrinsic angular momentum, or spin. Spin influences the proton’s optical, electrical, and magnetic characteristics and makes technologies such as MRI scanning work, but its origin has eluded physicists ever since experiments in the 1980s revealed that quarks can account for only about a third of the total spin. More recent experiments show that gluons make a significant contribution, perhaps even more than the quarks. An Electron-Ion Collider would produce definitive measurements of the gluons’ contributions, including how their movements within the proton microcosm affect its overall spin structure—thus providing the final pieces needed to solve this longstanding puzzle.

Capturing the dynamic action of gluons within protons and nuclei will give scientists a way to test their understanding of these particles’ ephemeral properties. As gluons flit in and out of the vacuum, multiplying and recombining, scientists suspect they may reach a steady state of saturation called a “color glass condensate.” This unique form of nuclear matter gets its name from the “color” charges that mediate the interactions of the strong nuclear force, and the dense, glasslike walls these particles are thought to form in nuclei accelerated to nearly the speed of light, seemingly suspended by the effects of time dilation. Scientists will use the Electron-Ion Collider to search for definitive proof of whether this form of matter exists, and test the limits of gluons’ ability to expand beyond the bounds of a single proton/ neutron inside a nucleus. They’ll also explore the mechanism that keeps gluon growth in check, like a lid clamping down on an overflowing popcorn pot. Precisely measuring the strength of the gluon fields, which constitute the strongest fields found in nature, will tell us how gluons interact with each other and how they contribute to building the bulk of visible matter in the universe today.

Experiments at an EIC would offer novel insight into why quarks or gluons can never be observed in isolation, but must transform into and remain confined within protons and nuclei. The EIC—with its unique combinations of high beam energies and intensities—would cast fresh light into quark and gluon confinement, a key puzzle in the Standard Model of physics.