Physicists at the ALICE experiment, located at CERN’s Large Hadron Collider, have achieved a breakthrough in understanding the fundamental structure of atomic nuclei. Their latest research provides evidence for a state known as gluon saturation, where the density of gluons becomes so high that they begin to interact strongly with one another.
While quarks are often cited as the primary building blocks of matter, the majority of the visible mass in the Universe originates from the energy carried by gluons and the strong force. Understanding how these particles organize themselves within the nucleus is vital for uncovering how matter acquires mass and structure. This new measurement offers a high-resolution view of these internal dynamics.
To capture this data, the team used a method called incoherent J/psi photonuclear production. During LHC Run 2, lead nuclei passed in close proximity without colliding, generating intense electromagnetic fields that acted as high-energy photon beams. When these photons struck a nucleus, they produced the J/psi particle, which serves as a probe for local gluon density.
Unlike traditional measurements that average values across the nucleus, this technique reveals local fluctuations at extremely small scales. The researchers probed regions as small as 0.2 femtometers, or one-quarter the size of a proton. At these dimensions, they observed a suppression in J/psi production that cannot be explained by existing models of nuclear shadowing. Instead, the results align with the theory of quantum chromodynamics, which predicts that gluons reach a saturation point when packed too densely.
This study represents a significant step in mapping the invisible fields that constitute the core of every atom. By varying momentum transfer, the researchers effectively adjusted the focus of their measurements, allowing for a precise observation of how these particles behave under extreme conditions. The team reported their findings in the journal Physical Review Letters.