Physicists from the ALICE experiment at CERN’s Large Hadron Collider have achieved groundbreaking measurements of gluon behavior within atomic nuclei, uncovering potential signs of gluon saturation—a state where these particles reach maximum density.
ALICE Detector. Image credit: Mona Schweizer / ALICE / CERN.
“While quarks are often seen as the fundamental components of matter, the majority of visible universe mass actually originates from the energy carried by gluons and the powerful forces binding quarks,” explained Professor Daniel Tapia Takaki from the University of Kansas, a member of the ALICE collaboration.
“Understanding gluon behavior within the atomic nucleus is crucial for comprehending how matter acquires mass and structure.”
ALICE physicists utilized an advanced experimental technique known as incoherent J/ψ photonucleation to investigate gluon variations within atomic nuclei at unprecedented spatial resolution.
“These measurements were conducted during Experiment 2 at the Large Hadron Collider, where rapidly moving lead nuclei come close to one another without colliding,” noted Professor Tapia Takaki.
“In such encounters, the intense electromagnetic field surrounding the atomic nucleus acts like a high-energy photon beam.”
“When one of these photons strikes another nucleus, it temporarily generates a J/ψ particle, serving as a sensitive probe of gluon structure.”
Unlike other methods that average over the entire nucleus, incoherent J/ψ production is sensitive to local fluctuations in gluon density, allowing detailed investigations of structures smaller than protons.
These powerful gluon fields exist in every atomic nucleus and constitute nearly all visible matter in the universe; however, their collective behavior poses significant challenges in contemporary physics.
“Our incoherent generation experiments are akin to transitioning from a blurry image to a high-resolution microscope,” said Professor Tapia Takaki.
“This process facilitates the observation of how gluons fluctuate and arrange themselves within the nucleus.”
“By varying the momentum transfer, our experiment effectively adjusts the focus of the microscope.”
ALICE progressively examined smaller regions within the nucleus at resolutions of 0.6, 0.3, and 0.2 femtometers—the highest resolution corresponds to structures roughly a quarter the size of a proton.
At such extraordinary scales, evidence suggests that gluons begin to behave collectively, a phenomenon termed gluon saturation.
Professor Tapia Takaki and colleagues measured incoherent J/ψ production across a wide range of photon nuclear energies, from 20 billion electron volts to 633 billion electron volts, while also assessing interaction changes with different momentum transfers.
“The results revealed a compelling pattern,” remarked Professor Tapia Takaki.
“At the smallest spatial scales examined, the J/ψ particle production rate was significantly suppressed, achieving about three standard deviations in statistical significance.”
This suppression challenges a long-standing theory known as nuclear opacification, previously substantiated by earlier measurements.
In this framework, overlapping gluons in the nucleus obscure each other, akin to a cloud blocking sunlight, thus decreasing certain particle production probabilities.
However, the new measurements indicate that traditional nuclear opacity cannot fully elucidate the observed data.
Rather, this finding aligns with the phenomenon of ‘gluon saturation’ predicted by quantum chromodynamics, which describes the strong force.
In this regime, gluons become densely packed, leading to strong interactions among them and limiting their numbers within a given area.
The team’s research results are published in the journal Physical Review Letters.
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S. Acharya et al. (ALICE Collaboration). 2026. Evidence for 𝐽/𝜓 Suppression in Incoherent Photonucleation. Physical Review Letters 137, 052301; doi: 10.1103/jmwb-75m7
Source: www.sci.news
