An experiment at CERN's Large Hadron Collider has produced the most detailed look yet at how gluons — the particles that bind quarks and generate nearly all the mass of visible matter — organize themselves inside atomic nuclei, reporting evidence for a long-predicted state called gluon saturation.

Using data from Run 2 of the LHC, the ALICE collaboration measured "incoherent J/ψ photonuclear production": fast-moving lead nuclei pass close to one another without directly colliding, and their intense electromagnetic fields act like beams of high-energy photons. When one of these photons strikes another nucleus, it can briefly produce a J/ψ particle, whose production acts as a sensitive probe of the underlying gluon structure.

By varying the momentum transfer, the team imaged gluon behavior at resolutions of 0.6, 0.3 and 0.2 femtometers — the finest resolution corresponding to structures only about one-quarter the size of a proton. "Our experiments using incoherent production is like switching from a blurry image to a high-resolution microscope," said Daniel Tapia Takaki, professor of physics and astronomy at the University of Kansas, who helped lead the work with researchers at the Czech Technical University in Prague.

At the smallest scales, J/ψ production was significantly suppressed, with a statistical significance of about three standard deviations. The pattern cannot be fully explained by "nuclear shadowing," the long-standing idea that gluons inside a nucleus partially overlap and obscure one another, similar to layers of clouds blocking sunlight. Instead, the observations are consistent with gluon saturation, predicted by the theory of quantum chromodynamics, in which gluons become so densely packed that they interact strongly with one another, limiting how many can exist in a given region.

The measurements, published in Physical Review Letters, provide the first multidimensional measurement of incoherent J/ψ photonuclear production across both interaction energy and momentum transfer. Physicists say understanding how gluons behave is essential to understanding how matter acquires its mass and structure, and the results could inform future experiments at planned facilities such as the Electron-Ion Collider.

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