LAWRENCE, October 23, 2024 -- Research using the ALICE experiment at CERN’s Large Hadron Collider suggests for the first time the presence of gluonic quantum fluctuations at the subnucleon level in heavy nuclei.

University of Kansas experimental nuclear physicist Daniel Tapia Takaki and his team have published findings detailing the breakthrough discovery in the Editor’s Suggestion of Physical Review Letters.

“Gluons, the elementary particles responsible for ‘gluing’ quarks and anti-quarks to form protons and neutrons, play a crucial role in about 98% of all visible matter in the universe,” Tapia Takaki said. “A new phenomenon called gluon saturation — a dynamic equilibrium between the production and annihilation of gluons — is predicted by quantum chromodynamics, or ‘QCD,’ the prevailing theory of quarks and gluons. Experimental work is still needed to determine the onset of gluon saturation, which is one of the scientific motivations for constructing dedicated QCD facilities such as the future Electron–Ion Collider at Brookhaven National Laboratory.”

Tapia Takai’s team scrutinizes collisions between particles of light (photons) and heavy nuclei to better understand how gluons behave when packed closely together. They’ve focused on creating a specific particle called a ‘J/psi meson,’ made of a charm quark and an antiquark. The more energy in these collisions, the easier it is to detect what's happening inside the nucleus.

One of the key discoveries, based on data from the ALICE project and contributions from Tapia Takaki’s team, suggests these collisions show signs of a process called gluon saturation, where gluons become densely packed. However, other experimental explanations like "shadowing” could also explain the results.

To dig deeper, the KU team explored new ways to study this at CERN. Researchers thought looking at collisions where the J/psi meson is produced in a slightly different way could give more direct evidence of gluon saturation. These different collisions also provided an opportunity to understand the internal structure of protons and nuclei in more detail.

“When the gluon saturation regime is reached, all gluon configurations in the proton or heavy nuclei appear similar, causing the cross section, which is proportional to the variance of the gluon field, to decrease as the energy increases,” Tapia Takaki said.

The newest research in PRL reports for the first time new insights into how incoherent J/psi is produced in collisions between light (photons) and lead nuclei. The KU team at CERN specifically measured how this production changes depending on the amount of momentum transferred during the collision.

In these particle collisions, when Tapia Takaki’s team hit large objects like whole nuclei, the resulting momentum change was small. But when it hit smaller parts, like individual protons or neutrons, the momentum change was bigger. If the team managed to hit even smaller objects inside those particles, the change was larger still.

Spotting these bigger momentum changes could represent strong evidence of gluonic quantum fluctuations at the subnucleon level.

Tapia Takaki's group has made leading contributions in studying this type of reaction known as “ultra-peripheral heavy-ion collisions” (UPCs). He’s supported establishing a research community on UPCs and organized the first international workshop on the physics of UPCs in Mexico last year, partly supported by the Inter-American Network of Networks of QCD Challenges, where he serves as the program lead and principal investigator of this National Science Foundation AccelNet program.

His latest research establishes several measured points of this rare process. The ALICE collaboration has shown that incoherent J/psi meson can only be accurately described when introducing nucleon substructures fluctuations within nuclei in the form of gluonic hotspots into the models. These fluctuations have never been seen before and are likely connected to the presence of gluonic saturated matter.

Moreover, KU students have gained valuable training from participating in these projects, developing skills through international, multi-team collaboration. The research was carried out in collaboration with the Prague ALICE team, led by Guillermo Contreras. KU and Czech Technical University in Prague have an international cooperation agreement, facilitating this partnership. Graduate student David Grund from Prague contributed to this work during a research visit at KU. Currently, Vendulka Filola is visiting KU this fall to continue investigating incoherent J/psi with new data. KU students also plan to visit Prague next year, in addition to their work at CERN.

Future studies, with larger data samples, will provide a deeper understanding of gluon saturation effects.

Tapia Takaki’s research has been funded by the U.S. Department of Energy, Office of Science, Nuclear Physics, Heavy-Ion Physics program.