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Modified structure of protons and neutrons in correlated pairs


The atomic nucleus is made of protons and neutrons (nucleons), which are themselves composed of quarks and gluons. Understanding how the quark–gluon structure of a nucleon bound in an atomic nucleus is modified by the surrounding nucleons is an outstanding challenge. Although evidence for such modification—known as the EMC effect—was first observed over 35 years ago, there is still no generally accepted explanation for its cause1,2,3. Recent observations suggest that the EMC effect is related to close-proximity short-range correlated (SRC) nucleon pairs in nuclei4,5. Here we report simultaneous, high-precision measurements of the EMC effect and SRC abundances. We show that EMC data can be explained by a universal modification of the structure of nucleons in neutron–proton SRC pairs and present a data-driven extraction of the corresponding universal modification function. This implies that in heavier nuclei with many more neutrons than protons, each proton is more likely than each neutron to belong to an SRC pair and hence to have distorted quark structure. This universal modification function will be useful for determining the structure of the free neutron and thereby testing quantum chromodynamics symmetry-breaking mechanisms and may help to discriminate between nuclear physics effects and beyond-the-standard-model effects in neutrino experiments.

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Fig. 1: DIS and quasi-elastic (e,e′) cross-section ratios.
Fig. 2: Universality of SRC-pair quark distributions.
Fig. 3: EMC and universal modification function slopes.
Fig. 4: Growth and saturation of the EMC effect for protons and neutrons.

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The raw data from this experiment are archived in Jefferson Laboratory’s mass storage silo.


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We acknowledge the efforts of the staff of the Accelerator and Physics divisions at Jefferson Laboratory that made this experiment possible. The analysis presented here was carried out as part of the Jefferson Laboratory Hall B Data-Mining project, supported by the US Department of Energy (DOE). The research 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 UK Science and Technology Facilities Council. The research of M.S. was supported by the US DOE, Office of Science, Office of Nuclear Physics, under award number DE-FG02- 93ER40771. Jefferson Science Associates operates the Thomas Jefferson National Accelerator Facility for the DOE, Office of Science, Office of Nuclear Physics under contract DE-AC05-06OR23177.

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Authors and Affiliations



CLAS was designed and constructed by the CLAS Collaboration and Jefferson Laboratory. Data acquisition, processing and calibration, Monte Carlo simulations of the detector and data analyses were performed by a large number of CLAS Collaboration members, who also discussed and approved the scientific results. The analysis presented here was performed by B.S. and A.S. with input from S.G., O.H., E. Piasetzky and L.B.W. and reviewed by the CLAS collaboration.

Corresponding author

Correspondence to O. Hen.

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Extended data figures and tables

Extended Data Fig. 1 models.

\({{\boldsymbol{F}}}_{2}^{{\boldsymbol{n}}}\,{\rm{/}}\,{{\boldsymbol{F}}}_{2}^{{\boldsymbol{p}}}\) Ratio of neutron to proton structure functions, \({F}_{2}^{n}\,{\rm{/}}\,{F}_{2}^{p}\), derived from the SRC-driven EMC model (blue band) used in the isoscalar corrections of refs 9 (‘SLAC’, red line) and 10 (‘JLab Hall C’, green line) and derived in the CTEQ-14 global fit, shown here for Q2 = 10 GeV2 (grey band). The large spread among the various models shows the uncertainty in \({F}_{2}^{n}\), a key ingredient in the isoscalar corrections previously applied to the EMC effect data.

Extended Data Table 1 SRC scaling coefficients
Extended Data Table 2 EMC slopes
Extended Data Table 3 Per nucleon, per-proton and per-neutron EMC slopes

Supplementary information

Supplementary Information

This file contains Supplementary Text and Data, which includes Supplementary Figures 1-4, Supplementary Tables 1-7 and additional references

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The CLAS Collaboration. Modified structure of protons and neutrons in correlated pairs. Nature 566, 354–358 (2019).

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