Physicists at CERN have uncovered new methods to study the behavior of gluons—particles that bind quarks together within atomic nuclei—using the ALICE experiment at the Large Hadron Collider. This research, conducted as part of the ALICE collaboration, achieved an unprecedented level of spatial detail by measuring particle production at scales as small as one-quarter the size of a proton. The team observed an unexpected decrease in the production of J/ψ particles at the smallest scales, a phenomenon that challenges conventional explanations. Led by physicist Daniel Tapia Takaki from the University of Kansas, the study used a novel technique called incoherent J/ψ photonuclear production. This method allowed researchers to measure both the energy and momentum transfer during interactions, providing a more detailed picture of how gluons are distributed inside atomic nuclei. This approach is a significant advancement in the field, offering insights into the strong force that holds matter together. Gluons are responsible for binding quarks, which are the building blocks of protons and neutrons. While quarks are often thought of as the fundamental components of matter, most of the mass in ordinary matter actually comes from the energy of gluons and the strong force that binds them. Understanding how gluons behave within nuclei is key to unraveling how matter acquires its mass and structure. To probe the distribution of gluons within nuclei, the researchers used data from the Large Hadron Collider's Run 2, where lead nuclei pass close to each other without colliding. The electromagnetic fields around these nuclei act like high-energy photon beams. When a photon from one nucleus interacts with another, it can briefly create a J/ψ particle. This process allows scientists to study the gluon structure in detail. By adjusting the momentum transfer, the researchers effectively zoomed in on different regions of the nucleus, achieving resolutions as fine as 0.2 femtometers—about one-quarter the size of a proton. Tapia Takaki has been instrumental in developing this experimental approach and has contributed to theoretical models suggesting that gluons can gather into dense regions, or "hot spots." These regions are thought to change in response to increasing collision energy, potentially revealing new physics related to the strong force. The study measured J/ψ production across a wide range of photon-nucleus energies and momentum transfers, revealing a significant suppression in J/ψ production at the smallest spatial scales. This finding challenges the conventional "nuclear shadowing" theory, which posits that overlapping gluons reduce particle production. Instead, the results align with the concept of gluon saturation, a prediction of quantum chromodynamics, the theory that describes the strong force.