Physicists working with the ALICE experiment at CERN’s Large Hadron Collider have made the most detailed measurement yet of gluon behavior inside nuclei, finding evidence that the particles may reach a densely packed state known as gluon saturation.
The ALICE detector. Image credit: Mona Schweizer / ALICE / CERN.
“Although quarks are often described as the fundamental building blocks of matter, nearly all the mass of the visible Universe actually comes from the energy carried by gluons and the strong force that binds quarks together,” said University of Kansas Professor Daniel Tapia Takaki, a member of the ALICE Collaboration.
“Understanding how gluons behave inside nuclei is therefore essential to understanding how matter itself acquires its mass and structure.”
To study how gluons fluctuate within nuclei with more spatial resolution than previously possible, the ALICE physicists used an experimental technique called incoherent J/ψ photonuclear production.
“The measurements were performed using data collected during Run 2 of the Large Hadron Collider, where fast-moving lead nuclei pass close to one another without directly colliding,” Professor Tapia Takaki said.
“In these encounters, intense electromagnetic fields surrounding the nuclei behave like beams of high-energy photons.”
“When one of these photons strikes another nucleus, it can briefly produce a particle called the J/ψ, whose production provides a sensitive probe of the underlying gluon structure.”
Unlike other measurements that average over the entire nucleus, incoherent J/ψ production is sensitive to local fluctuations in gluon density, allowing the physicists to probe structures smaller than a proton.
These intense gluon fields, present inside every atomic nucleus, make up nearly all the visible matter in the Universe.
Yet their collective behavior remains one of the greatest challenges in modern physics.
“Our experiments using incoherent production is like switching from a blurry image to a high-resolution microscope,” Professor Tapia Takaki said.
“This process allows us to see how gluons fluctuate and organize themselves inside nuclei.”
“By varying the momentum transfer, our experiment effectively changes the focus of our microscope.”
“At resolutions of 0.6, 0.3 and 0.2 femtometers, ALICE progressively probed smaller regions inside the nucleus.”