For Protons and Neutrons, Things Aren’t the Same Inside Nuclei

Nuclear physicists find that the internal structures of protons and neutrons may be altered in different ways inside nuclei.

Image courtesy of Christopher Cocuzza, Temple University
Electrons probe the structure of a proton or neutron by way of a virtual photon. This image shows a virtual photon (γ*) interacting with a proton or neutron inside a tritium nucleus, which contains one proton and two neutrons.

The Science

The building blocks of protons and neutrons—quarks—are distributed differently in free protons and neutrons versus inside nuclei. Nuclear physicists call this difference “the EMC effect.” Each proton is made of three quarks, with two called up quarks and one called a down quark. Neutrons have two down quarks and one up quark. Scientists previously thought that the EMC effect treated the up and down quarks equally. New high-precision data from the MARATHON experiment made possible a new global analysis of experimental data on this phenomenon. The complex analysis indicates that the EMC effect may exert more influence on the distribution of down quarks compared to up quarks inside nuclei.

The Impact

Prior to this result, nuclear physicists thought they could treat protons and neutrons, and their quarks, similarly in certain cases. This allowed a simpler understanding of how up and down quarks arrange themselves inside protons and neutrons, without the need to account for confounding effects of the environment inside nuclei. The new results from MARATHON appear to contradict this simple picture. Nuclear physicists need to conduct further investigations of this phenomenon to better characterize this effect. If confirmed, the result could affect experiments in neutrino physics, heavy-ion physics, astrophysics, and other fields.


When protons and neutrons live inside an atom’s nucleus, their internal quarks are distributed differently versus those inside protons or neutrons that roam free. This effect was first observed by the European Muon Collaboration at CERN in the 1980s, and it has remained a mystery for decades. The MARATHON collaboration has now collected new data on this phenomenon in an experiment carried out at Thomas Jefferson National Accelerator Facility’s Continuous Electron Beam Accelerator Facility particle accelerator, a Department of Energy (DOE) user facility. The data came from helium-3 and tritium nuclei. Helium-3 has two protons (each with two up quarks and one down quark) and one neutron (with two down quarks and one up quark). Tritium has one proton and two neutrons. Helium-3 and tritium have the same number of up quarks compared to the other nucleus’ down quarks. This new data enabled a sophisticated global analysis by the Jefferson Lab Angular Momentum (JAM) collaboration. The JAM analysis revealed that the distributions of down quarks may be more modified by the environment inside nuclei compared to the up quark distributions. This means that experiments seeking to reveal new information about the quark structure of nucleons will need to account for the nuclear environment.


Cynthia Keppel
Thomas Jefferson National Accelerator Facility (Jefferson Lab)  


This research was supported by the Department of Energy Office of Science, Office of Nuclear Physics, the National Science Foundation, the University of Adelaide, and the Australian Research Council.


Cocuzza C., et al., (Jefferson Lab Angular Momentum [JAM] Collaboration), Isovector EMC Effect from Global QCD Analysis with MARATHON Data. Physical Review Letters 127, 242001 (2022). [DOI: 10.1103/PhysRevLett.127.242001]

Abrams, D., 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). [DOI: 10.1103/PhysRevLett.128.132003]

Related Links

Marathon Measures Mirror Nuclei, Jefferson Lab news

Different Particles Get Different Treatment Inside Nuclei, Jefferson Lab news

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