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ATLAS researchers at CERN report the first observation that the internal quark and gluon distributions of nucleons differ between peripheral and more inclusive photonuclear collisions in lead nuclei. The measured distributions differed at a significance of 6.0 standard deviations. The result points to a spatial dependence in how a nucleus modifies its nucleons; it does not mean protons or neutrons become different kinds of particles, or that the longstanding EMC effect has been fully explained.
What did CERN researchers observe inside lead nuclei?
The ATLAS Collaboration compared the distributions of quarks and gluons—collectively called partons—inside nucleons in two classes of collisions. The study reports a difference between the classes at 6.0 standard deviations, a statistical measure of how inconsistent the observed difference is with no difference under the analysis assumptions. ATLAS describes this as the first observation that nuclear parton distributions depend on impact parameter, the transverse separation between the colliding nuclei.
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Nucleons are protons and neutrons. The result concerns the partons inside nucleons bound in different nuclear environments, not a change in a proton’s or neutron’s identity. The collaboration’s [1 October 2026 briefing](https://atlas.cern/Updates/Briefing/Edge-Nuclear-Structure) summarizes the finding as nucleons near a nucleus’s edge having a different internal structure from those nearer its centre.
How did ATLAS make the measurement?
Photonuclear collisions and jets
The analysis used ultra-peripheral lead–lead (Pb+Pb) collisions recorded by ATLAS in 2018. In these encounters the ions pass far enough apart to avoid an ordinary collision of their nuclear matter, while a photon from one lead ion can interact with the other ion and produce jets—sprays of particles associated with quarks and gluons. The study used collisions at a nucleon-pair centre-of-mass energy of 5.02 TeV and a dataset with an integrated luminosity of 1.72 nb−1.
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Researchers compared the shape of the measured cross-section as a function of x+, a proxy for the momentum fraction carried by a parton, across different event classes. The full analysis and its stated conclusion appear in the [ATLAS paper on arXiv](https://arxiv.org/abs/2604.20559).
Forward neutrons as an impact-parameter clue
ATLAS used forward-neutron signals in its zero-degree calorimeters to classify events. In the briefing’s notation, 0nXn events have forward neutrons, while 0n0n events lack them on the relevant side. The 0n0n class served as a proxy for a more peripheral interaction in which the struck nucleus remained intact; the comparison included a more inclusive event sample.
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This is an inference from event signatures, not a direct image or position measurement of an individual nucleon. Forward-neutron activity helps distinguish collision conditions, and the resulting classes are used to probe how the measured distributions vary with impact parameter.
What does the result say about the EMC effect?
The EMC effect is the broader observation that quark distributions in nucleons bound inside nuclei differ from those in free nucleons. First observed by the European Muon Collaboration in the 1980s, it remains an open nuclear-physics question. ATLAS adds evidence about one aspect of that problem: the modifications are not necessarily uniform across a nucleus, but can depend on impact parameter.
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ATLAS reports that, at large x+, its pattern is consistent with large-impact-parameter collisions showing no modifications of the kind seen in hard scattering involving nuclei at smaller impact parameters. That interpretation supports spatial variation; it does not identify every mechanism responsible for the EMC effect or settle the broader debate.
A separate line of work offers context but measures a different question. The U.S. Department of Energy’s [2022 account of MARATHON data and a JAM global analysis](https://www.energy.gov/science/np/articles/protons-and-neutrons-things-arent-same-inside-nuclei) says the EMC effect may influence down-quark distributions more than up-quark distributions. That analysis concerned helium-3 and tritium and possible flavour dependence. ATLAS instead used photonuclear jets in lead–lead collisions to examine spatial dependence; the findings are complementary, not competing measurements of the same quantity.
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What remains uncertain, and what comes next?
The event classes provide proxies for impact parameter rather than a map of where individual nucleons sit. The result establishes a significant difference between the studied classes and supports impact-parameter-dependent nuclear parton distributions, but it does not by itself provide a complete microscopic explanation for that dependence.
ATLAS says larger lead–lead datasets from LHC Run 3 and the future High-Luminosity LHC programme may enable more precise follow-up measurements. Such data could sharpen how the dependence varies across the nucleus and help distinguish possible explanations.
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