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# Probing high-momentum protons and neutrons in neutron-rich nuclei

## Abstract

The atomic nucleus is one of the densest and most complex quantum-mechanical systems in nature. Nuclei account for nearly all the mass of the visible Universe. The properties of individual nucleons (protons and neutrons) in nuclei can be probed by scattering a high-energy particle from the nucleus and detecting this particle after it scatters, often also detecting an additional knocked-out proton. Analysis of electron- and proton-scattering experiments suggests that some nucleons in nuclei form close-proximity neutron–proton pairs1,2,3,4,5,6,7,8,9,10,11,12 with high nucleon momentum, greater than the nuclear Fermi momentum. However, how excess neutrons in neutron-rich nuclei form such close-proximity pairs remains unclear. Here we measure protons and, for the first time, neutrons knocked out of medium-to-heavy nuclei by high-energy electrons and show that the fraction of high-momentum protons increases markedly with the neutron excess in the nucleus, whereas the fraction of high-momentum neutrons decreases slightly. This effect is surprising because in the classical nuclear shell model, protons and neutrons obey Fermi statistics, have little correlation and mostly fill independent energy shells. These high-momentum nucleons in neutron-rich nuclei are important for understanding nuclear parton distribution functions (the partial momentum distribution of the constituents of the nucleon) and changes in the quark distributions of nucleons bound in nuclei (the EMC effect)1,13,14. They are also relevant for the interpretation of neutrino-oscillation measurements15 and understanding of neutron-rich systems such as neutron stars3,16.

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## Acknowledgements

This work was supported by the US Department of Energy (DOE), contract number DEAC05-06OR23177, under which Jefferson Science Associates, LLC, operates the Thomas Jefferson National Accelerator Facility; 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.

### Reviewer information

Nature thanks T. Aumann, D. Phillips and the other anonymous reviewer(s) for their contribution to the peer review of this work.

## Author information

### Contributions

The CEBAF large acceptance spectrometer was designed and constructed by the CLAS Collaboration and Jefferson Laboratory. Data 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 M. Duer with input from O. Hen, E. Piasetzky and L. B. Weinstein and reviewed by the CLAS Collaboration.

### Corresponding author

Correspondence to O. Hen.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

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## Supplementary information

### Supplementary Information

This file contains Supplementary Text and Data, Supplementary Figures 1–31, Supplementary Tables 1–11 and Supplementary References.

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The CLAS Collaboration., Duer, M., Hen, O. et al. Probing high-momentum protons and neutrons in neutron-rich nuclei. Nature 560, 617–621 (2018). https://doi.org/10.1038/s41586-018-0400-z

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