CERN Finds Gluons Behaving Strangely Deep Inside Atomic Nuclei

A CERN experiment has given physicists a much sharper look at how gluons behave inside atomic nuclei, providing new evidence that could help distinguish between two competing explanations of what happens at extremely small scales. The study, conducted as part of the ALICE experiment at CERN's Large Hadron Collider and published in Physical Review Letters, was led in part by University of Kansas physicist Daniel Tapia Takaki, working closely with scientists at the Czech Technical University in Prague.

Gluons are particles that help bind quarks together through the strong force. "Although quarks are often described as the fundamental building blocks of matter, nearly all the mass of the visible universe -- from the atoms in our bodies to the matter inside stars -- actually comes from the energy carried by gluons and the strong force that binds quarks together," said Tapia Takaki, a member of the collaboration.

The researchers report the first multidimensional measurement of a process called incoherent J/psi photonuclear production, using data from Run 2 of the Large Hadron Collider, in which fast-moving lead nuclei pass close to one another without directly colliding. In these encounters, the intense electromagnetic fields around the nuclei act like beams of high-energy photons; when one of these photons strikes another nucleus, it can briefly produce a particle called the J/psi, whose production is a sensitive probe of the underlying gluon structure. Unlike measurements that average gluon distribution across an entire nucleus, this technique can reveal local changes in gluon density at scales even smaller than a proton.

The team measured incoherent J/psi production across photon-nucleus energies ranging from 20 to 633 billion electron volts, at spatial resolutions of 0.6, 0.3 and 0.2 femtometers -- the finest corresponding to structures only about one-quarter the size of a proton. "The results revealed a striking pattern," Tapia Takaki said: at the smallest spatial scales explored, the production rate of J/psi particles was significantly suppressed, with a statistical significance of about three standard deviations.

That suppression challenges a long-standing explanation called "nuclear shadowing," in which gluons inside a nucleus partially overlap and obscure each other, reducing certain particle production processes. Instead, the results are consistent with a different phenomenon called gluon saturation, predicted by quantum chromodynamics, the theory that describes the strong force, in which gluons become so densely packed that they begin interacting strongly with one another, limiting how many can exist in a given region.