Gluons’ Density Isn’t the Same in Bound vs Unbound Protons and Neutrons

Data show the distribution of gluon “glue” in protons and neutrons changes when they are bound together in nuclei.

The Science

Scientists can examine the visible matter that makes up our universe by colliding matter in particle accelerators. Nuclear physicists recently used high-energy particles of light (photons) emitted by an accelerated gold nucleus to probe the inner structure of the protons and neutrons (nucleons) in another gold nucleus. They measured the resulting density of gluons. These are the particles that glue quarks together, and the building blocks of nucleons. The research found that nucleons bound in a nucleus have lower gluon density than free, unbound nucleons.

The Impact

Scientists once viewed the building blocks of matter as static, where all the pieces stay the same no matter how they are arranged. This recent discovery shows that nucleons can change when part of a larger structure. This surprising finding, that gluon density within nucleons changes when they are part of a nucleus, may yield a deeper understanding of the strong force carried by gluons.

Summary

Ultra-peripheral (near-miss) collisions of gold nuclei at the Relativistic Heavy Ion Collider (RHIC), a Department of Energy Office of Science nuclear physics user facility at Brookhaven National Laboratory, offer a powerful method for studying nuclear matter. By tracking the production of J/psi particles in these near-miss collisions, researchers can identify collisions where photons surrounding one accelerated nucleus interacted with gluons in the other nucleus. Researchers then use the energy distribution of the J/psi particles to map out the density of gluons within the nuclear particles.

This study, led by Brookhaven scientists, finds that gluon density within nucleons appears to decrease when the nucleons are part of a heavy nucleus at high energy. This differs from the expectation from theoretical models. The finding, if confirmed, is something of a puzzle because higher energies typically increase gluon density as these particles split and multiply. But at very high densities gluons may also merge, thus reducing the overall gluon density. The observed suppression in gluon density may therefore be a sign that the nuclei have reached this gluon saturation state. Another possible explanation is nuclear shadowing, where tightly packed protons and neutrons block the photons used to make the measurements, causing gluon density to appear to decline. This research was a crucial step toward resolving this debate. More conclusive findings are expected from experiments at the future Electron-Ion Collider.

Contact

Funding

This research was funded by the Department of Energy Office of Science, the National Science Foundation, and a range of international organizations and agencies listed in the scientific paper. The STAR team used computing resources at the Scientific Data and Computing Center at Brookhaven National Laboratory, the National Energy Research Scientific Computing Center at Lawrence Berkeley National Laboratory, and the Open Science Grid consortium.

Publications

Abdulhamid, M.I., et al. (STAR Collaboration), Observation of Strong Nuclear Suppression in Exclusive J/ψ Photoproduction in Au+Au Ultraperipheral Collisions at RHIC. Physical Review Letters 133, 052301 (2024). [10.1103/PhysRevLett.133.052301]

Abdulhamid, M.I., et al. (STAR Collaboration), Exclusive J / ψ , ψ ( 2 s ) , and e + e − pair production in Au + Au ultraperipheral collisions at the BNL Relativistic Heavy Ion Collider, Physical Review C 10, 014911 (2024). [10.1103/PhysRevC.110.014911]

Related Links

Brookhaven National Laboratory news: Physicists 'Shine' Light on Inner Details and Breakup of Simple Nucleus

Highlight Categories

Program: NP

Performer: DOE Laboratory , RHIC