Physicists Find a Strong Link Between Gluon Saturation and Symmetry Principles

Theorists show gluon fields characterizing gluon saturation can describe a universal pattern of particle emissions—a manifestation of asymptotic symmetry.

As the energy of nuclear collisions increases, more and more gluons (represented as yellow strings) are packed inside a proton. Eventually, they become so crowded that they reach a balanced, dense state called gluon saturation.
Image courtesy of Brookhaven National Laboratory
As the energy of nuclear collisions increases, more and more gluons (represented as yellow strings) are packed inside a proton. Eventually, they become so crowded that they reach a balanced, dense state called gluon saturation.

The Science

Protons are made up of smaller particles called quarks and gluons. These particles follow the rules of Quantum Chromodynamics (QCD), which explains how the strong force causes quarks and gluons to interact to produce larger particles. QCD predicts that as the collision energy in these interactions increases, a state called gluon saturation will form inside a proton. Gluon saturation happens when the processes of gluons splitting into more gluons balance out the process of gluons merging. Discovering and understanding gluon saturation is one of the main goals of the future Electron-Ion Collider, to be built at the Brookhaven National Laboratory. This is because the dense gluon state is a crucial part of the internal structure of the proton. Recently, theorists studied gluon saturation at intermediate collision energy and discovered a surprising link to a special kind of symmetry.

The Impact

Symmetries are the foundation of all the basic laws of nature. These laws are based on a type of symmetry called local gauge symmetry, which means the rules stay the same even if certain changes happen over space and time. Scientists have discovered a link between gluon saturation and a hypothesized special kind of local gauge symmetry known as asymptotic symmetry. This finding opens a new path to study gluon saturation by looking at this type of symmetry, which has also been found in electromagnetic and gravitational systems. One important area of study is how this symmetry links to memory effects. In gravitational systems, these memory effects involve how gravitational waves can change the position of two adjacent masses as the waves pass by. Research on gluon saturation may also provide more experimental proof for the existence of asymptotic symmetry.

Summary

Physicists at the Ohio State University recently found new contributions that can help further describe gluon saturation. Their findings revealed a surprising connection to a universal pattern of particle emissions in nuclear collisions when the emitted particle moves slowly relative to the particle that emitted it. Researchers previously found that asymptotic symmetry is the key principle behind this universal pattern. Combining these two discoveries shows a deep link between gluon saturation and asymptotic symmetry.

This connection could lead to new ways of understanding gluon saturation through the lens of symmetry principles. Asymptotic symmetries are universal and can also be found in other areas, like electromagnetic and gravitational phenomena—especially in the memory effects of gravitational waves. Insights from these fields can enhance our understanding of gluon saturation. Additionally, studies of gluon saturation in future Electron-Ion Collider experiments may offer fresh perspectives on asymptotic symmetry.

Contact

Ming Li
The Ohio State University
[email protected]

Funding

This material is based on work supported by the Department of Energy Office of Science, Office of Nuclear Physics.

Publications

Li, M., Quasiclassical Gluon Fields and Low’s Soft Theorem at Small Momentum-Fraction  x. Physical Review Letters 133, 021902 (2024). [DOI: 10.1103/PhysRevLett.133.021902]

Highlight Categories

Program: NP

Performer: University