Geneva : Physicists working with the ALICE experiment at CERN’s Large Hadron Collider have obtained new evidence about how gluons behave deep inside atomic nuclei, providing a sharper view of the particles responsible for binding quarks together.
The study, published in Physical Review Letters, uses an advanced measurement of J/ψ production to investigate the distribution of gluons at extremely small spatial scales. Researchers were able to probe structures down to about 0.2 femtometres — roughly one-quarter the size of a proton.
The results revealed an unexpected suppression in J/ψ production at the smallest scales examined. Scientists say the observation is difficult to explain using the conventional theory of nuclear shadowing alone and is instead consistent with a phenomenon known as gluon saturation.
Gluons hold the key to understanding matter
Gluons are elementary particles that carry the strong force, one of the fundamental forces of nature. They bind quarks together to form protons and neutrons.
Although quarks are often described as the building blocks of matter, the energy associated with gluons and the strong interaction accounts for most of the mass of ordinary visible matter. Understanding how gluons behave is therefore central to understanding the structure of protons, neutrons and atomic nuclei.
However, physicists still do not have a complete picture of what happens when huge numbers of gluons occupy extremely small regions of space.
The latest ALICE measurement offers a new way of examining that question.
LHC becomes a microscope for gluons
The researchers used a technique called incoherent J/ψ photonuclear production.
During the experiment, rapidly moving lead nuclei passed close to one another without directly colliding. Their powerful electromagnetic fields could act like beams of high-energy photons. When such a photon interacted with another nucleus, it could briefly produce a J/ψ particle.
The characteristics of the J/ψ particles then provided information about the underlying distribution of gluons inside the nucleus.
Unlike measurements that average gluon behaviour across an entire nucleus, this approach can reveal local variations in gluon density.
Researchers compared measurements at different momentum transfers, effectively changing the spatial resolution of their investigation.
The ALICE experiment examined regions at resolutions of 0.6, 0.3 and 0.2 femtometres. At the highest resolution, scientists were probing structures only about one-quarter the size of a proton.
Scientists observe unexpected suppression
The most striking result appeared at the smallest spatial scales.
Researchers found that the production rate of J/ψ particles was significantly suppressed, with a statistical significance of about three standard deviations.
The observation is important because it provides a potential clue about the collective behaviour of gluons when their density becomes extremely high.
Scientists have traditionally used the concept of nuclear shadowing to explain some changes in particle production inside nuclei. In simplified terms, gluons can overlap in such a way that they reduce the probability of certain interactions.
The latest measurements suggest that nuclear shadowing alone may not be sufficient to explain what the ALICE experiment observed.
Gluon saturation offers another explanation
The alternative explanation is gluon saturation.
According to quantum chromodynamics — the theory describing the strong interaction — gluons can become so densely packed at sufficiently high energies and small distances that they begin interacting strongly with one another.
This collective behaviour can limit the growth of gluon density and produce what physicists describe as a saturated state.
The new results are consistent with this picture, although the findings represent evidence rather than a definitive confirmation that gluon saturation has been observed.
Further measurements and theoretical work will be needed to determine how strongly the data support this interpretation.
Researchers identify possible “hot spots”
The study also relates to theoretical models in which gluons form concentrated regions of particularly high density, sometimes described as gluon hot spots.
These regions may fluctuate inside atomic nuclei and could change as the energy of the interaction increases.
Studying such structures could help researchers understand previously unexplored aspects of the strong interaction and the way gluons organise themselves inside matter.
The ability to investigate these fluctuations at increasingly small scales represents a significant experimental advance.
Why the finding matters
The behaviour of gluons is not merely a question of particle physics. Gluons and the strong force are fundamental to the structure of ordinary matter.
Protons and neutrons make up atomic nuclei, and nuclei make up the atoms found in everything from living organisms to planets and stars.
A better understanding of how gluons behave could therefore improve scientists’ understanding of how nuclear matter is structured and how mass emerges from the strong interaction.
The new measurement also demonstrates the value of examining particle interactions from multiple dimensions rather than relying on a single measurement.
By studying both interaction energy and momentum transfer, the ALICE researchers were able to examine gluon distributions with much greater spatial detail.
A new window into the strong force
University of Kansas physicist Daniel Tapia Takaki played a leading role in the research, working with scientists including collaborators at the Czech Technical University in Prague. The work was carried out within the international ALICE collaboration at CERN.
The measurements used data collected during Run 2 of the Large Hadron Collider. Researchers examined incoherent J/ψ production across photon-nucleus energies ranging from 20 to 633 billion electron volts.
The results do not rewrite the fundamental laws of physics, but they offer a new experimental window into a regime where gluons may behave collectively in ways that are difficult to observe directly.
Further data will be important in determining whether the observed suppression is indeed a clear signature of gluon saturation or whether other effects need to be considered.
For now, the ALICE experiment has provided one of the most detailed looks yet at the microscopic structure of gluons inside atomic nuclei. The finding could help physicists better understand one of nature’s fundamental forces and the complex particles that ultimately shape the matter around us.