Scientists at CERN’s Large Hadron Collider have conducted a new study examining the behavior of gluons within atomic nuclei, marking a significant advancement in understanding fundamental particle physics. The research, led by Daniel Tapia Takaki of the University of Kansas and published in Physical Review Letters as part of the ALICE experiment, employed a technique called incoherent J/ψ photonuclear production to measure gluon distribution with unprecedented precision.
Gluons are particles responsible for binding quarks together through the strong force, and they account for nearly all the mass of visible matter in the universe. Despite their fundamental importance, physicists have not fully understood how large numbers of gluons behave collectively within nuclei. Traditional measurements typically average gluon distribution across entire nuclei, whereas the incoherent J/ψ production method allows researchers to detect local variations in gluon density at scales smaller than a single proton.
The experiment utilized data from the Large Hadron Collider’s Run 2, where fast-moving lead nuclei passed close to one another without directly colliding. The intense electromagnetic fields surrounding these nuclei acted as beams of high-energy photons, which could briefly produce J/ψ particles. By varying the momentum transfer in these interactions, researchers effectively adjusted the resolution of their observations, probing structures at scales of 0.6, 0.3, and 0.2 femtometers.
The measurements, conducted across photon-nucleus energies ranging from 20 to 633 billion electron volts, revealed unexpected suppression of J/ψ particle production at the smallest spatial scales examined. This finding challenges the long-established “nuclear shadowing” theory, which proposes that overlapping gluons within a nucleus reduce the probability of certain particle production processes. Instead, the results align with predictions of “gluon saturation,” a phenomenon described by quantum chromodynamics in which densely packed gluons interact strongly with one another, limiting their concentration in any given region.
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