Scientists study protons and neutrons inside nuclei by firing particles at nuclear targets, measuring how they scatter and what new particles emerge, then comparing those results with calculations. They do not photograph a nucleus’s interior or pull out and inspect a quark. Different experiments reveal different layers: how nucleons interact, how quarks are distributed inside them, and how gluons and nuclear shape behave at finer scales.
What scientists mean by “inside” a nucleus
At one level, a nucleus is a collection of protons and neutrons, called nucleons. At a finer level, each proton and neutron is a composite particle made of quarks and gluons. Quantum chromodynamics (QCD) describes how quarks and gluons interact through the strong force. Because that force confines quarks, they cannot be extracted as isolated particles for direct inspection. Scientists instead infer internal structure from measurable scattering and collision products, interpreted with theory and computation. The U.S. Department of Energy explains the quark-and-gluon picture in its quarks and gluons explainer and its QCD explainer.
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The scale of the question matters. A study of proton-neutron pairs at close range tests the force between nucleons; it does not directly map the quarks inside each one. Deep-inelastic scattering, by contrast, can probe quark distributions. Researchers choose a probe, target and collision energy suited to the property they want to learn about.
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Electron scattering and deep-inelastic scattering
In electron scattering, an energetic electron interacts electromagnetically with the target through a virtual photon. “Virtual” describes the interaction in the calculation; it is not a tiny camera flash. Detectors record the outgoing electron and other reaction products, including their directions and energies. The pattern of results constrains the target’s internal structure.
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At sufficiently high momentum transfer, deep-inelastic scattering is sensitive to quark-level structure. To investigate nucleons bound in a nucleus, researchers compare results from nuclear targets with those from free protons and neutrons and use global QCD analyses to disentangle the nuclear environment from the nucleons’ own structure. The resulting distributions are analysis products, not images. The DOE describes this approach and the Jefferson Lab MARATHON measurements in its account of protons and neutrons inside nuclei.
Mirror nuclei and the EMC effect
The EMC effect is the observed difference between quark distributions in nucleons inside nuclei and those in free nucleons. It was first observed by the European Muon Collaboration at CERN in the 1980s, according to the DOE. One way to constrain the neutron’s structure is to compare deep-inelastic scattering from mirror nuclei whose proton and neutron counts are exchanged: helium-3 has two protons and one neutron, while tritium has one proton and two neutrons.
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In the MARATHON program at Jefferson Lab, those measurements were analyzed alongside other data. A subsequent Jefferson Lab Angular Momentum (JAM) global QCD analysis reported that down-quark distributions may be more modified by the nuclear environment than up-quark distributions. That is a result of the cited analysis, not a settled, universal explanation of the EMC effect; the DOE account says further investigation is needed to characterize it. It lists the JAM analysis by Cocuzza et al., Isovector EMC Effect from Global QCD Analysis with MARATHON Data, Physical Review Letters 127, 242001 (2022), and the MARATHON structure-function-ratio measurement by Abrams et al., Physical Review Letters 128, 132003 (2022).
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Different experiments answer different questions
| Approach | What is measured or calculated | What it can reveal | Important distinction |
|---|---|---|---|
| Electron and deep-inelastic scattering | Scattered electrons and reaction products from proton, neutron or nuclear targets | Quark distributions and how they differ when nucleons are bound in nuclei | Distributions are inferred through analysis rather than photographed directly. |
| Mirror-nucleus comparison, including MARATHON | Deep-inelastic scattering from helium-3 and tritium | Constraints on neutron structure and tests of the EMC effect | Interpretation depends on global QCD analysis; the DOE says further investigation is needed to characterize the effect. |
| Short-range nucleon scattering | Data on close proton-neutron or proton-proton configurations, compared with strong-force models | How the nuclear force behaves at very short distances | This tests interactions between nucleons, not a direct quark map. |
| Heavy-ion collisions and particle tracking | Particles emerging from collisions, including momentum, angle and interference information | In specialized analyses, constraints on gluon distributions and studies of hot, dense matter | Interpretation depends on collision kinematics and theory. |
| Exclusive meson production at a future Electron-Ion Collider | Electron-ion events producing a single meson and their measured cross sections | Potential sensitivity to nuclear shape and gluon distributions | This is a proposed future capability, not a completed EIC measurement. |
| QCD computation and simulation | Numerical calculations of quark and gluon interactions, compared with experimental results | Tests of whether theory reproduces nucleon properties and collision data | These are calculations, not direct observations, and require substantial computing and approximations. |
Evidence beyond quark distributions
Close encounters between nucleons
When researchers examine proton-proton or proton-neutron pairs at very short distances, they are asking how the nuclear force behaves, rather than directly measuring the quark distributions in each nucleon. A DOE account describes a study using Jefferson Lab data across nuclei from carbon to lead, compared with strong-force models. The study reported that the strongest model was developed at Argonne National Laboratory and described a repulsive core at the shortest distances. This is a model comparison about nucleon-nucleon interactions, not a direct picture of a nucleon’s interior: DOE’s account of the study.
Gluons in nuclear collisions
Some studies use heavy-ion collisions and track many particles emerging from them. In a specialized method described by the DOE, polarized photons interact with gluons, while the STAR detector tracks outgoing particles. Their velocities and angles help constrain photon polarization and, through the analysis, gluon distributions in nuclei. The signal is reconstructed from particle patterns; it is not a matter of isolating a gluon. The DOE describes this approach in its account of the entanglement method.
Why computation is part of the experiment
Scattering data do not interpret themselves. Researchers use QCD calculations and simulations to connect the observed products to quark and gluon behavior, then compare theoretical predictions with measurements. QCD is difficult to solve, so simulations rely on computational methods and approximations. The DOE has described a method that enabled calculations with lighter quarks than earlier approaches and explains how simulated particle properties can be checked against experiment in its article on computers and particle simulations. As Argonne’s Kawtar Hafidi puts it, “You can’t isolate quarks to study them,” in a DOE interview about proton structure.
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What the Electron-Ion Collider may add
The DOE describes the Electron-Ion Collider at Brookhaven as a future facility. One proposed approach is to collide electrons with nuclei and select events that produce a single meson. The measured cross section could provide sensitivity to nuclear shape and gluon distributions. The meson’s momentum affects the length scale probed: higher momentum corresponds to shorter length scales, where quark-and-gluon structure becomes more relevant. This is a theoretical framework for future EIC research, not a report of measurements already made there. See the DOE’s account of how researchers may see the shape of atomic nuclei.
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How to interpret a claim about the inside of a nucleus
- Identify the target: Is the experiment using a free proton, a nuclear target or colliding nuclei?
- Ask what property is measured: A charge distribution, quark distribution, gluon distribution and nucleon-nucleon force are different quantities.
- Check the scale and collision conditions: The probe and energy determine which structures the experiment can resolve.
- Separate data from interpretation: Detectors record particles and their properties; theory and simulations connect those records to internal structure.
- Check whether the result is measured or projected: Proposed EIC capabilities should not be confused with existing measurements.
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