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# Revealing the short-range structure of the mirror nuclei 3H and 3He

## Abstract

When protons and neutrons (nucleons) are bound into atomic nuclei, they are close enough to feel significant attraction, or repulsion, from the strong, short-distance part of the nucleon–nucleon interaction. These strong interactions lead to hard collisions between nucleons, generating pairs of highly energetic nucleons referred to as short-range correlations (SRCs). SRCs are an important but relatively poorly understood part of nuclear structure1,2,3, and mapping out the strength and the isospin structure (neutron–proton (np) versus proton–proton (pp) pairs) of these virtual excitations is thus critical input for modelling a range of nuclear, particle and astrophysics measurements3,4,5. Two-nucleon knockout or ‘triple coincidence’ reactions have been used to measure the relative contribution of np-SRCs and pp-SRCs by knocking out a proton from the SRC and detecting its partner nucleon (proton or neutron). These measurements6,7,8 have shown that SRCs are almost exclusively np pairs, but they had limited statistics and required large model-dependent final-state interaction corrections. Here we report on measurements using inclusive scattering from the mirror nuclei hydrogen-3 and helium-3 to extract the np/pp ratio of SRCs in systems with a mass number of three. We obtain a measure of the np/pp SRC ratio that is an order of magnitude more precise than previous experiments, and find a marked deviation from the near-total np dominance observed in heavy nuclei. This result implies an unexpected structure in the high-momentum wavefunction for hydrogen-3 and helium-3. Understanding these results will improve our understanding of the short-range part of the nucleon–nucleon interaction.

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## Data availability

The raw data from this experiment were generated at the Thomas Jefferson National Accelerator Facility and are archived in the Jefferson Lab mass storage silo. Access to these data and relevant analysis codes can be facilitated by contacting the corresponding author.

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## Acknowledgements

We acknowledge discussions with O. Benhar, C. C. degli Atti, W. Cosyn, A. Lovato, N. Rocco, M. Sargsian, M. Strikman and R. Wiringa, and the contribution of the Jefferson Lab target group and technical staff for design and construction of the tritium target and their support running this experiment. This work was supported in part by the Department of Energy’s Office of Science, Office of Nuclear Physics, under contracts DE-AC02-05CH11231, DE-FG02-88ER40410, DE-SC0014168 and DE-FG02-96ER40950, the National Science Foundation, including grant NSF PHY-1714809, and DOE contract DE-AC05-06OR23177 under which JSA, LLC operates JLab.

## Author information

Authors

### Contributions

J.A., D.Day, D.W.H., P.S. and Z.H.Y. were the experiment co-spokespersons. S.L., N.S., R.C.-T., Z.H.Y., R.E.M. and F.H. made significant contributions the set-up of the experiment and/or data analysis. R.J.H., D.M. and P.S. contributed to the design and operation of the tritium target. The full collaboration participated in the data collection and/or detector calibration and data analysis.

### Corresponding author

Correspondence to J. Arrington.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

## Peer review

### Peer review information

Nature thanks Patrick Achenbach, Ivica Friščić and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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## Extended data figures and tables

### Extended Data Fig. 1 Target window contamination.

Number of events versus position in the target along the beamline for the 3H cell (blue) and for the empty target (black) after scaling to the same luminosity as the target windows. The shaded region indicates the region used in the analysis.

### Extended Data Fig. 2 3He/2H per-nucleon cross-section ratios.

3He/2H ratio for this work and ref. 11 are shown. Error bars show the combined statistical and uncorrelated systematic uncertainty (1σ range); the normalization uncertainties are 1.18% for this work, 1.8% for E02-019.

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Reprints and Permissions

Li, S., Cruz-Torres, R., Santiesteban, N. et al. Revealing the short-range structure of the mirror nuclei 3H and 3He. Nature 609, 41–45 (2022). https://doi.org/10.1038/s41586-022-05007-2

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• DOI: https://doi.org/10.1038/s41586-022-05007-2