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Scientists study protons and neutrons inside nuclei by firing particles at nuclear targets, measuring what scatters or emerges, and using quantum chromodynamics (QCD) calculations to interpret the results. They do not photograph a nucleus’s interior or pull out and inspect an isolated quark. Instead, different experiments reveal different features: how nucleons interact, how quarks are distributed, or how gluons and nuclear shape behave.
What does “inside a nucleus” mean?
There are two related but distinct levels of structure. At the nuclear scale, a nucleus is made of protons and neutrons, called nucleons. At finer resolution, each proton and neutron is itself a composite particle containing quarks and gluons. QCD describes how quarks and gluons interact through the strong force.
Because the strong force confines quarks, scientists cannot isolate one and examine it on its own. They infer internal structure from how particles scatter, what products appear, and how those observations compare with theory and simulation. As Argonne National Laboratory physicist Kawtar Hafidi puts it, “You can’t isolate quarks to study them.” (U.S. Department of Energy interview; DOE explanation of quarks and gluons)
So a result about the force between a proton and a neutron is not automatically a map of the quarks inside either one. The probe, collision energy, and analysis determine which question an experiment can answer.
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How scattering turns measurements into evidence
In electron scattering, an energetic electron interacts electromagnetically with a target through a virtual photon. The photon is an exchange in the interaction, not a tiny camera flash. Detectors measure the deflected electron and, in many experiments, other reaction products. From their energies, angles, and rates, researchers infer properties of the target.
Deep-inelastic scattering uses energetic collisions that can probe quark-level structure. Experiments can use free-proton targets or nuclei; comparing the results helps researchers determine how quark distributions differ when nucleons are bound in a nucleus. Those distributions are extracted through analysis, often using global QCD fits that combine data and theory, rather than directly photographed.
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Resolution depends on the interaction’s energy and kinematics: different conditions are sensitive to different length scales and internal features. A useful way to interpret any result is to ask what target was used, what property was measured, what scale was probed, and how much theoretical analysis was needed.
Which methods answer which questions?
| Method | What researchers measure or calculate | What it can reveal | Important distinction |
|---|---|---|---|
| Electron scattering and deep-inelastic scattering | Scattered electrons and reaction products from proton, neutron, or nuclear targets | Quark distributions and changes associated with nucleons bound in nuclei | Distributions are inferred from measurements and analysis, not photographed directly. (DOE account of MARATHON and the EMC effect) |
| Mirror-nucleus comparison, including MARATHON | Deep-inelastic scattering from helium-3 and tritium | Helps constrain neutron structure and investigate the EMC effect | The conclusions depend on global QCD analysis; the DOE account says more investigation is needed to characterize the effect. (DOE account of MARATHON and the EMC effect) |
| Short-range nucleon scattering | Evidence about close proton-neutron or proton-proton configurations, compared with strong-force models | How the nuclear force behaves at very short distances | This concerns interactions between nucleons, not a direct map of their quark distributions. (DOE account of short-range nucleon studies) |
| Heavy-ion collisions and particle tracking | Particles emerging from collisions, including momentum, angle, and interference information | Can constrain gluon distributions and probe hot, dense nuclear matter | Interpretation depends on collision conditions and theory; the cited entanglement technique is a specialized method. (DOE account of the RHIC entanglement method) |
| Exclusive meson production in electron-ion collisions | Events producing a single meson and the measured cross section | A proposed way to study nuclear shape and gluon distributions | The DOE describes a future Electron-Ion Collider capability, not a completed EIC measurement. (DOE account of nuclear shape and the future EIC) |
| QCD computation and simulation | Numerical calculations of quark and gluon interactions | Tests whether theory can reproduce nucleon properties and experimental results | Simulations are calculations, not observations; QCD is difficult to solve and computing methods use approximations. (DOE account of QCD computing; DOE explanation of QCD) |
What the EMC effect reveals about nucleons in nuclei
The EMC effect is the observed difference between the quark distributions of nucleons inside nuclei and those of free nucleons. The European Muon Collaboration first observed it at CERN in the 1980s, raising the question of how the nuclear environment changes a nucleon’s internal structure.
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In the MARATHON program at Jefferson Lab, researchers used deep-inelastic scattering data from two mirror nuclei: helium-3, with two protons and one neutron, and tritium, with one proton and two neutrons. Reversing the proton-neutron counts helps constrain neutron structure, which is harder to measure directly than proton structure.
A later 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 finding of the cited analysis, not a settled, universal explanation of the EMC effect: the DOE’s 2022 account says further investigation was needed to characterize it. The publications listed there include Cocuzza et al., “Isovector EMC Effect from Global QCD Analysis with MARATHON Data,” Physical Review Letters 127, 242001 (2022), and Abrams et al., “Measurement of the Nucleon F2n/F2p Structure Function Ratio by the Jefferson Lab MARATHON Tritium/Helium-3 Deep Inelastic Scattering Experiment,” Physical Review Letters 128, 132003 (2022). (DOE account and publication list)
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How force studies differ from quark-distribution studies
Scientists also study what happens when two nucleons come very close together. A DOE-described analysis compared Jefferson Lab observations of short-range proton-neutron and proton-proton configurations across nuclei from carbon to lead with models of the strong force. The study reported that the strongest model in its comparison, developed at Argonne National Laboratory, included a repulsive core at the shortest distances.
This line of work tests the nucleon-nucleon force: how protons and neutrons affect one another inside a nucleus. It is related to nuclear structure, but it does not directly map the quark distribution within each nucleon. (DOE account of short-range nucleon studies)
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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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How collisions and computation extend the picture
Inferring gluon distributions from collision products
At the Relativistic Heavy Ion Collider (RHIC), researchers have used particle tracking and quantum interference to constrain gluon distributions in nuclei. In the specialized method described by DOE, polarized photons interact with gluons and the STAR detector tracks emerging particles. Their measured velocities and angles help constrain the photon polarization and, in turn, the gluon distributions. The detector records particles; the internal distribution is reconstructed from the pattern and interpreted with theory. (DOE account of the RHIC method)
Testing QCD with computation
Numerical simulation complements accelerator experiments by calculating strong-force dynamics and comparing predicted nucleon properties with measurements. A DOE-described computational method enabled simulations with lighter quarks than earlier approaches. These calculations help test whether QCD can reproduce observed properties; they do not provide a direct view of an isolated quark. QCD remains notoriously difficult to solve, so such work requires substantial computing and carefully chosen approximations. (DOE account of QCD computing; DOE explanation of QCD)
What the future Electron-Ion Collider may add
The U.S. Department of Energy describes Brookhaven’s Electron-Ion Collider (EIC) as a future facility. One proposed approach is to collide electrons with nuclei and select exclusive 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 being 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 at the collider. (DOE account of nuclear shape and the future EIC)
Quick Recap
How to read claims about nuclear interiors
- Identify the level of structure. A claim about proton-neutron forces is not necessarily a claim about quarks inside a proton or neutron.
- Check the probe and target. Electron scattering from a free proton, scattering from a nucleus, and collisions between heavy nuclei test different properties.
- Separate measurements from interpretation. Detectors record outgoing particles and their kinematics; distributions and internal structure are inferred with analysis and theory.
- Distinguish completed results from proposed capabilities. In particular, exclusive meson production at the EIC is described as future research.
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