CERN’s ALICE Experiment Captures the Sharpest View Yet of Gluons Inside Atomic Nuclei

For decades, physicists have debated how gluons — the force-carrying particles that bind quarks inside protons and neutrons — behave when crowded together inside atomic nuclei. Do they simply cast shadows on each other, reducing their apparent density? Or do they reach a limit where they become so densely packed that they begin to repel one another? A landmark measurement from CERN’s ALICE experiment has, for the first time, collected data precise enough to distinguish between these two pictures .

The Measurement: First Multidimensional View of Incoherent J/ψ Photonuclear Production

The new result is the first multidimensional measurement of incoherent J/ψ photonuclear production in ultra-peripheral lead-lead (Pb-Pb) collisions at the Large Hadron Collider . In these collisions, lead nuclei pass extremely close to one another without directly hitting — an arrangement that allows researchers to use the strong electromagnetic fields of one nucleus as a photon beam to probe the gluon structure of the other.

By measuring how J/ψ mesons (particles made of a charm quark and anti-charm quark) are produced in these photon-nucleus interactions, physicists can map the spatial distribution and density of gluons inside the nucleus at very small length scales. The study reports this measurement as a function of both interaction energy and momentum transfer, a level of multidimensional detail never before achieved .

Unprecedented Resolution: 0.2 Femtometers

The key advance is the resolution. By analyzing momentum transfers in the data, the ALICE team was able to probe structures as small as 0.2 femtometers — roughly one-tenth the size of a single proton . This is the finest resolution ever applied to gluon distributions inside an atomic nucleus.

“At resolutions of 0.6, 0.3 and 0.2 femtometers, ALICE progressively probed smaller regions inside the nucleus,” said University of Kansas nuclear physicist Daniel Tapia Takaki, who led the study . This stair-step approach allowed the team to watch how gluon behavior changes as you zoom in closer and closer.

What the Data Shows: Nuclear Shadowing vs. Gluon Saturation

To interpret the results, the team compared their measurements against two competing theoretical models:

• Nuclear shadowing: In this model, gluon density is reduced at low momentum fractions through destructive interference between scattering from different nucleons. This has been the standard explanation for decades .
• Gluon saturation: This more extreme picture predicts that at very high densities, gluons begin to interact nonlinearly with each other — essentially, they become so packed that they “repel” new gluons, setting a limit on how many can exist in a given volume .

The ALICE data favor gluon saturation over the conventional nuclear shadowing picture . This challenges the shadowing framework that has been the default explanation for nuclear gluon effects since the 1970s.

Published in Physical Review Letters

The study was published in Physical Review Letters in August 2026, with data collected during Run 2 of the Large Hadron Collider (2015–2018) . The ALICE collaboration includes researchers from dozens of institutions worldwide, with the analysis led in part by Tapia Takaki’s group at the University of Kansas .

Why It Matters

The result matters because gluons dominate the mass and binding energy of ordinary matter. Understanding how gluons behave at high densities is essential for building a complete theory of the strong nuclear force (quantum chromodynamics, or QCD) and for interpreting a wide range of nuclear and particle physics experiments, including future measurements at the Electron-Ion Collider planned in the United States .

If gluon saturation is confirmed, it would represent a new phase of nuclear matter — a dense, collective state of gluons unlike anything observed before. The ALICE result is the strongest experimental hint yet that this phase exists inside atomic nuclei at high energies .