CERN Experiment Finds Evidence That Gluons Pack Densely Inside Atomic Nuclei

Deep inside atomic nuclei, particles called gluons bind quarks together and help determine the structure of matter. Nearly all the mass of the visible universe comes not from the quarks themselves but from the energy carried by gluons and the strong force that holds them together, according to Daniel Tapia Takaki, a nuclear physicist at the University of Kansas and member of the ALICE collaboration at CERN. A new study using the ALICE experiment at CERN's Large Hadron Collider, published in Physical Review Letters, has now measured how gluons are distributed inside nuclei with unusual precision.

The researchers used a technique called incoherent J/psi photonuclear production, studying data from Run 2 of the Large Hadron Collider in which fast-moving lead nuclei pass close to one another without colliding. The intense electromagnetic fields around the nuclei act like beams of high-energy photons; when one strikes another nucleus, it can briefly produce a particle called the J/psi, whose production is a sensitive probe of the underlying gluon structure. By varying the momentum transferred in these interactions, the team probed the nucleus at resolutions of 0.6, 0.3 and 0.2 femtometers - the finest corresponding to structures about one-quarter the size of a proton.

"At these extraordinary scales, we observe evidence that the gluons begin to behave collectively, a phenomenon known as gluon saturation," Tapia Takaki said. At the smallest spatial scales in the experiment, the production rate of J/psi particles was significantly suppressed, with a statistical significance of about three standard deviations. That suppression poses a challenge to "nuclear shadowing," a long-standing explanation in which gluons inside a nucleus partially overlap and obscure each other, reducing the chance of certain particle-production processes.

Instead, the researchers say the measurements are consistent with gluon saturation, a phenomenon predicted by quantum chromodynamics, the theory of the strong force, 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 covered photon-nucleus interaction energies ranging from 20 to 633 billion electron volts.