"In these encounters, intense electromagnetic fields surrounding the nuclei behave like beams of high-energy photons," Tapia Takaki explained. "When one of these photons strikes another nucleus, it can briefly produce a particle called the J/ψ, whose production provides a sensitive probe of the underlying gluon structure."

The J/ψ (pronounced "JAY-sigh") serves as the probe. The trick lies in measuring incoherent production, which — unlike many techniques that average the gluon distribution across an entire nucleus — reveals local variations in gluon density. Adjusting the momentum transfer effectively adjusts the microscope's focus.

ALICE probed the nucleus at resolutions of 0.6, 0.3 and 0.2 femtometers. A femtometer is one quadrillionth of a meter. The finest resolution corresponds to structures roughly one-quarter the size of a proton.

To picture that precision: if a nucleus were enlarged to a football stadium, the experiment could distinguish features only a few yards wide on the field.

"Our experiments using incoherent production is like switching from a blurry image to a high-resolution microscope," Tapia Takaki said. "This process allows us to see how gluons fluctuate and organize themselves inside nuclei."

What did the researchers actually see?

The team measured incoherent J/ψ production across photon-nucleus energies from 20 to 633 billion electron volts, and studied how the process varied with momentum transfer.

"The results revealed a striking pattern," Tapia Takaki said. At the smallest spatial scales, J/ψ production was significantly suppressed.

That result is awkward for "nuclear shadowing," a framework that has successfully explained earlier measurements. In that picture, gluons inside a nucleus partially overlap and hide each other, reducing the probability of certain particle production — Tapia Takaki compares it to layers of clouds blocking sunlight. But the new measurements suggest conventional shadowing alone cannot account for the observed pattern.

Instead, the data line up with a phenomenon called gluon saturation, which quantum chromodynamics — the theory of the strong force — predicts.

"In this regime, gluons become so densely packed that they begin interacting strongly with one another, limiting how many can exist in a given region," Tapia Takaki said.

What are hot spots?

Tapia Takaki has helped pioneer this experimental approach and contributed to theoretical models in which gluons cluster into localized regions of especially high density, known as "hot spots." In the energy-dependent version of the model, these dense regions change as collision energy increases. Their behavior could offer signatures of previously unexplored physics of the strong interaction.

The new findings support that broader picture: at extreme scales, the evidence suggests gluons begin to behave collectively.

How solid is the result?

Some caution is warranted. The suppression carries a statistical significance of about three standard deviations — strong evidence by physics standards, but below the five-standard-deviation threshold researchers traditionally treat as a discovery. The measurement is also the first of its kind in multiple dimensions, which means follow-up measurements will be needed to confirm the pattern.

If confirmed, gluon saturation at these scales would give physicists a sharper handle on how the strong force builds the mass and structure of essentially all visible matter — starting from particles most people have never heard of.