Fast-Moving Pairs May Solve 35-Year-Old Mystery

Physicists develop a universal mathematical description that suggests that proton-neutron pairs in a nucleus may explain why their associated quarks have lower average momenta than predicted.

The team’s data came from Jefferson Lab's Experimental Hall B, featuring the Continuous Electron Beam Accelerator Facility Large Acceptance Spectrometer.

The Science

Fleeting partnerships form between protons and neutrons inside an atom’s nucleus, according to a careful re-analysis of data from an unrelated experiment conducted in 2004. Researchers extracted a mathematical equation that describes these short-range partnerships for the nuclei studied. The re-analysis also suggests that the connections may explain a 35-year-old mystery in nuclear physics known as the EMC Effect. The mystery involves these particle’s quarks, the building blocks of protons and neutrons. This new work shows why quarks in an atom’s core don’t have the same momenta (mass x velocity) as those in free protons and neutrons.

The Impact

First discovered over 35 years ago by the European Muon Collaboration, the EMC Effect was an unexpected discovery that quarks inside nuclei have lower average momenta than predicted. The new mathematical equation (termed a universal function) describes the EMC Effect. The universal function suggests that the effect can be explained by proton-neutron pairs in the nucleus.


The European Muon Collaboration discovered the EMC Effect in data taken at CERN. The collaboration found that quarks inside a nucleus appeared different from those found in free protons and neutrons. Physicists developed two main models to explain this phenomenon. One model is that all protons and neutrons in a nucleus, and thus their quarks, are modified in the same way by being bound inside a nucleus. The other model is that many protons and neutrons behave as if they are free, while about one-fifth are involved in short-range correlations and are highly modified. The latter model is now supported by a new result from a careful re-analysis of data from an experiment conducted in 2004 using Jefferson Lab’s Continuous Electron Beam Accelerator Facility (CEBAF). Researchers used CEBAF to probe the nuclei of carbon, aluminum, iron, and lead as compared to deuterium (an isotope of hydrogen containing a proton and neutron in its nucleus). Comparing the data, a pattern emerged, and the researchers derived from this information a universal modification function for short-range correlations, the brief pairing of protons and neutrons in nuclei. They then applied the function to the nuclei used in measurements of the EMC Effect, and they found that it was the same across all measured nuclei. The physical picture of short-range correlations as the cause of the EMC Effect also accomplishes another step toward a long-time goal of nuclear and particle physicists to connect two different views of the atom’s nucleus. That is, the nucleus is made up of protons and neutrons versus their constituent quarks. The universal function will be tested directly in the future using lattice quantum chromodynamics calculations and by a future experiment in Hall C at Jefferson Lab that will study directly the relationship of the EMC Effect and short-range correlations in the deuteron.


Or Hen
Massachusetts Institute of Technology

Larry Weinstein
Old Dominion University


The research was supported by the Department of Energy’s Office of Science, Office of Nuclear Physics. It was also supported by the National Science Foundation, the Israel Science Foundation, the Chilean Comisión Nacional de Investigación Científica y Tecnológica, the French Centre National de la Recherche Scientifique and Commissariat a l’Energie Atomique, the French-American Cultural Exchange, the Italian Istituto Nazionale di Fisica Nucleare, the National Research Foundation of Korea, and the United Kingdom’s Science and Technology Facilities Council.


The CLAS Collaboration, “Modified structure of protons and neutrons in correlated pairs.” Nature 566, 354 (2019). [DOI: 10.1038/s41586-019-0925-9]

Related Links

Jefferson Lab press release: Correlated Nucleons May Solve 35-Year-Old Mystery

Jefferson Lab press release: Protons Get Zippier in Neutron-Rich Nuclei

Jefferson Lab press release: Protons Hog the Momentum in Neutron-Rich Nuclei

Jefferson Lab press release: Protons Pair Up with Neutrons

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