ATLAS researchers at CERN report the first observation that the internal distributions of quarks and gluons in nucleons vary with their position-related environment inside a lead nucleus. Comparing two classes of photonuclear lead-ion collisions, the collaboration found a difference at a reported significance of 6.0 standard deviations. The result points to spatial variation in nuclear effects; it does not mean protons or neutrons change identity, or that researchers directly saw individual nucleons.
What did CERN researchers observe inside lead nuclei?
In its 1 October 2026 briefing, the ATLAS Collaboration described the result as the first observation that nucleons near a nucleus’s edge have a different internal structure from those nearer its centre. More precisely, ATLAS found that the measured distributions of quarks and gluons—collectively called partons—differ between event classes associated with different impact parameters. The paper reports the difference at 6.0 standard deviations. ATLAS briefing · ATLAS paper on arXiv
A nucleon is a proton or neutron. The finding concerns the distribution of its constituent partons in the nuclear environment, not a basic distinction between protons and neutrons. Nor did ATLAS photograph or pinpoint a nucleon: its conclusion comes from comparing collision classes and the jets produced in them.
How did ATLAS compare different nuclear environments?
The analysis used ultra-peripheral collisions of lead ions: the ions passed close to one another without a central collision, and a photon emitted by one interacted with the other nucleus, producing jets. ATLAS analysed data recorded in 2018, corresponding to an integrated luminosity of 1.72 nb⁻¹, at a nucleon-pair centre-of-mass energy of 5.02 TeV.
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Researchers used forward-neutron signals in ATLAS’s zero-degree calorimeters to sort events into classes. The 0n0n class, with no relevant forward-neutron signal, served as a proxy for a peripheral interaction in which the struck nucleus remained intact. The 0nXn class included events with forward neutrons and provided a more inclusive comparison. These signals allow an inference about impact parameter—the transverse separation of the passing nuclei—not a direct map of where a particular nucleon sat.
ATLAS compared the shape of the cross-section as a function of x₊, a proxy for the parton momentum fraction, between the peripheral and inclusive samples. Their distributions differed. The collaboration says the pattern at large x₊ is consistent with large-impact-parameter collisions showing no modifications of the kind found in hard scattering involving nuclei at smaller impact parameters. That interpretation supports spatial dependence in nuclear parton distributions; it does not establish every mechanism responsible for the effect.
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What is the connection to 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 a question in nuclear physics. ATLAS addresses one dimension of it: whether nuclear modifications depend on impact parameter. The result is a new piece of evidence, not a complete explanation of the EMC effect.
A separate line of work should not be confused with this measurement. A 2022 U.S. Department of Energy account described MARATHON data and a JAM global analysis of helium-3 and tritium, suggesting the EMC effect may influence down-quark distributions more than up-quark distributions. That is a question about possible flavour dependence, studied with different targets and data; ATLAS used photonuclear jets in lead-ion collisions to investigate spatial dependence. U.S. Department of Energy background on the EMC effect and MARATHON/JAM
What does the result establish—and what comes next?
The observation strengthens the case that nuclear parton-distribution modifications are not uniform across a nucleus. It does not, by itself, settle why those modifications occur or resolve the entire EMC-effect puzzle. ATLAS says larger Run 3 lead-lead datasets and the future High-Luminosity LHC programme may enable more precise follow-up measurements.
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