Protons and neutrons inside a nucleus do not contribute to scattering exactly as if they were isolated particles. In deep-inelastic scattering, the measured quark distributions in nuclei differ from the simple sum expected from free protons and neutrons. This observed nuclear modification is called the EMC effect. It shows that a nucleon’s internal structure, as inferred from scattering, depends on its nuclear surroundings—but it does not yet reveal one settled explanation for why.
What is the EMC effect?
The EMC effect is a difference between the structure measured in a nucleus and the structure expected by adding up the contributions of its constituent protons and neutrons. It was first identified by comparing scattering per nucleon from iron with scattering from deuterium, a nucleus often used as an approximate proton-plus-neutron reference. The historical comparison found suppression in the region 0.3 < x < 0.8.
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Here, “suppression” means that the measured nuclear structure-function values in that range were lower than the reference expectation. The effect is not a photograph of quarks or a direct measurement of one proton changing shape. It is an inference from how high-energy particles scatter from a target.
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How scattering reveals nuclear structure
In deep-inelastic scattering, a high-energy lepton—such as an electron—strikes a target. The scattering data are used to determine structure functions, which encode information about the target’s quarks and other partons. Comparing those functions for different targets reveals whether their internal distributions match a simple combination of free-nucleon contributions.
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Bjorken x is the momentum-fraction variable used to describe the part of the target’s momentum carried by the struck parton in the scattering analysis. The result is a distribution inferred from many scattering events, not a view of individual quarks moving inside one proton.
So, when people say that a proton “changes” inside a nucleus, they mean that the proton’s inferred internal parton distribution differs in the nuclear setting. The proton remains a proton; the shorthand describes a change in measured structure, not a conversion into another particle.
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Why the size of the effect varies among nuclei
If nuclear modification depended only on the total number of nucleons or on the nucleus’s average density, measurements of different nuclei should follow relatively simple patterns. Comparisons among light nuclei challenged those expectations.
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Beryllium-9 helps illustrate why an average can conceal important local structure. Jefferson Lab describes it as two alpha-like clusters with an additional neutron. The clusters hold nucleons in locally denser regions, even though the nucleus’s relatively large radius gives it a modest overall average density. This makes local nuclear environments a plausible part of the explanation, but the beryllium result does not prove a single mechanism.
What competing explanations propose
Binding and nucleon motion, including Fermi motion, contribute to nuclear effects. Researchers also disagree about which nucleons are modified and which features of the nuclear environment matter most. These explanations are not all necessarily mutually exclusive; the measured effect is firmer than any one account of its cause.
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| Explanatory approach | What it proposes | What evidence bears on it | Status |
|---|---|---|---|
| Modification of bound nucleons | Bound nucleons are modified in broadly similar ways by being inside a nucleus. | Inclusive structure-function ratios across nuclei and light-nucleus comparisons test how the effect scales with nuclear properties. | A class of models; no unique explanation is established. |
| Short-range-correlated pairs | Many nucleons behave nearly as if free, while a smaller fraction—especially protons and neutrons in short-range-correlated pairs—are modified more strongly. | A Jefferson Lab 2019 report described a reanalysis of 2004 CEBAF data for carbon, aluminum, iron and lead relative to deuterium. The researchers derived a common pair-modification pattern and applied it to EMC measurements. | An influential proposed account, not a consensus or definitive proof. |
| Average-density scaling | The size of the effect tracks the nucleus’s average density. | Light-nucleus measurements, including beryllium-9, test this against nuclei with different structures. | Simple average-density scaling does not describe the cited beryllium-9 result. |
| Local structure and position | Modification may depend on local clustering or on where a nucleon lies within a nucleus. | Clustered light nuclei probe local environments; ATLAS’s lead analysis compared more inclusive and peripheral event classes. | Supported as important questions by measurements, but not a complete account of the EMC mechanism. |
Lawrence Weinstein, lead coauthor of the short-range-correlation report and an Old Dominion University professor and eminent scholar, characterized that proposal this way: “This one points strongly to an answer, but it’s not definitive.” Jefferson Lab likewise states: “Despite much theoretical work, no unique and universally accepted explanation of this difference, known as the ‘EMC effect’, has emerged.”
What the newest position-dependent result adds
On 2026-10-01, the ATLAS Collaboration reported a first observation that nucleons near the edge of a lead nucleus have different parton distributions from nucleons near its centre. The analysis used 2018 lead-lead ultra-peripheral collision data with an integrated luminosity of 1.72 nb⁻¹. In these collisions, photons emitted by one ion probe the other. ATLAS compared event classes with and without forward neutrons to distinguish more inclusive interactions from peripheral ones; the measured cross-section ratio showed a difference with a reported statistical significance of 6.0 standard deviations.
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- The shapes of electron clouds on the S layer in different electron layers are all spherical, and the only difference is that the area with the largest electron cloud density is different from the nucleus.
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This adds spatial position within the nucleus to the ways researchers can investigate nuclear parton distributions. It does not settle the EMC mechanism: ATLAS described the origin of the difference as an open question and presented the measurement as new information for understanding how nuclear modifications occur.
What flavor-sensitive analyses can—and cannot—say
Not all protons and neutrons contribute identical quark flavors, so global analyses of nuclear parton distributions can examine possible flavor or isospin dependence. A Jefferson Lab JAM overview reported a first indication of an isovector EMC effect in light nuclei from a global analysis that included MARATHON helium-3 and tritium structure-function ratios.
That result is an indication from a particular analysis, not an established general conclusion. The same overview says that constraints on neutron-to-proton structure-function ratios and on the d/u quark-distribution ratio remain relatively weak. Flavor-sensitive fits therefore add a useful line of inquiry without removing the uncertainty about the mechanism.
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What the effect establishes—and what it does not
- Established: nuclear structure functions, and the parton distributions inferred from them, do not reduce to a simple sum of free proton and neutron contributions.
- Not established: one universally accepted microscopic reason for that difference, or a single account that explains every nucleus and measurement.
- Useful shorthand: saying a proton or neutron “changes” in a nucleus refers to its inferred internal distribution, not to the particle turning into something else.
- Active questions: how binding, motion, short-range correlations, local clustering, flavor and position contribute—and how those contributions relate to one another.
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