New measurements obtained in the ALICE experiment at CERN have provided the most detailed view to date of gluon behavior inside atomic nuclei. The results question the long-established theoretical concept of nuclear shadowing and point to evidence in favor of a competing phenomenon—gluon saturation, Bioengineer writes.

Probing the nucleus at unprecedented scales

The study, published in the journal Physical Review Letters, presents the first multidimensional measurement of incoherent photoproduction of J/ψ mesons as a function of interaction energy and momentum transfer. Scientists used data collected during the second run of the Large Hadron Collider, studying ultraperipheral collisions in which lead nuclei passed close to each other without colliding directly. The powerful electromagnetic fields surrounding the fast-moving nuclei served as sources of high-energy photons, which could briefly generate J/ψ particles—the rate of their formation is sensitive to the gluon structure of the nucleus.

The experiment probed gluon fields with spatial resolutions of 0.6, 0.3, and 0.2 femtometers—the smallest of these values corresponds to structures roughly a quarter the size of a proton. Daniel Tapia Takaki, a nuclear physicist from the University of Kansas who played a key role in the study, compared the technique used to transitioning from a blurry photo to an image under a high-powered microscope.

Nuclear shadowing falls short as an explanation

At the smallest spatial scales studied, the production rate of J/ψ particles was suppressed with a statistical significance of about three standard deviations. Such a pattern is difficult to reconcile with nuclear shadowing—a concept in which gluons from different nucleons overlap and partially "shield" one another, reducing the likelihood of certain interactions. Although nuclear shadowing had successfully described previous measurements, it cannot fully account for the new data.

"In contrast, the observations align with a different phenomenon—gluon saturation, predicted by the theory of quantum chromodynamics," Takaki said. "In this regime, gluons become so densely packed that they begin to actively interact with one another, capping their possible number in a given area."

Why this matters

Gluons carry the strong nuclear force and provide almost all the mass of visible matter in the universe through the energy stored in the strong interaction. Takaki noted that understanding gluon behavior "is essential for understanding how matter acquires its mass and structure."

The results obtained do not yet reveal all the details of gluon saturation, and additional measurements will be required for that. Nevertheless, by combining energy-dependent measurements with momentum transfer data across a photon-nucleus energy range from 20 to 633 billion electronvolts, the ALICE collaboration has created the most compelling experimental basis to date for distinguishing between the two competing models.