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# New directions in hypernuclear physics

## Abstract

A hypernucleus, a subatomic bound system with at least one hyperon, is a great test ground to investigate nuclear forces and general baryonic interactions with up, down and strange quarks. Hypernuclei have been extensively studied for almost seven decades in reactions involving cosmic rays and with accelerator beams. In recent years, experimental studies of hypernuclei have entered a new stage using energetic collisions of heavy-ion beams. However, these investigations have revealed two puzzling results related to the lightest three-body hypernuclear system, the so-called hypertriton, and the unexpected existence of a bound state of two neutrons with a Λ hyperon. Solving these puzzles will not only impact our understanding of the fundamental baryonic interactions with strange quarks but also of the nature of the deep interior of neutron stars. In this Perspective, we discuss approaches to solving these puzzles, including experiments with heavy-ion beams and the analysis of nuclear emulsions using state-of-the-art technologies. We summarize ongoing projects and experiments at various facilities worldwide and outline future perspectives.

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## Acknowledgements

Discussions presented for the WASA-FRS experiment here are based on the experiment S447, which is currently scheduled in 2022 at the FRS at the GSI Helmholtzzentrum fuer Schwerionenforschung, Darmstadt (Germany) in the context of FAIR Phase 0. The authors thank the accelerator departments at GSI and IMP, the FRS department at GSI and the Experiment Electronics department at GSI for the technical support. The authors thank the J-PARC E07 collaboration for providing us with the nuclear emulsion data. The authors thank Luise Doersching-Steitz of GSI, Rita Krause of GSI, Yukiko Kurakata of RIKEN, Daniela Press of GSI, Miao Yang of IMP and Xiaohua Yuan of IMP for supporting the projects, including the administrative works. The authors also thank Risa Kobayashi of RIKEN and Yoko Tsuchii of Gifu University for their technical support in mining hypertriton events in the E07 nuclear emulsions. K.N., J.Y. and M.Y. acknowledge support by JSPS KAKENHI grant numbers JP23224006, JP16H02180, JP20H00155 and JP20J00682, and MEXT KAKENHI grant numbers JP24105002 (Grant-in-Aid for Scientific Research on Innovative Areas 2404), JP18H05403 and JP19H05147 (Grant-in-Aid for Scientific Research on Innovative Areas 6005). S.E. and C.R. are supported by the grant 2019-T1/TIC-13194 of the program ‘Atracción de Talento Investigador’ of the Community of Madrid.

## Author information

Authors

### Contributions

All authors contributed to the manuscript. T.R.S., V.D., H.E., S.E., N.K.-N., A.K., M.K., E.L., S.M., A.M., M.N., C.R., N.S., C.S., M.T., Y.K.T., J.Y. and H.W. contributed to the WASA-FRS experiment at GSI and will contribute to future hypernuclear projects at FAIR. T.R.S., W.D., H.E., A.K., E.L., A.M., M.N., K.N., C.R., N.S., M.T., Y.K.T., J.Y., M.Y. and H.W. contributed to the development of the method to analyse the nuclear emulsion with machine learning. T.R.S., H.E., Y.H., E.L., Y.M., M.N., C.R., H.W. and X.Z. are working on the hypernuclear project at HIAF.

### Corresponding author

Correspondence to Takehiko R. Saito.

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### Competing interests

The authors declare no competing interests.

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Nature Reviews Physics thanks Yu-Gang Ma, Stefano Trogolo, Ed Hungerford and the other, anonymous, reviewer for their contribution to the peer review of this work.

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Saito, T.R., Dou, W., Drozd, V. et al. New directions in hypernuclear physics. Nat Rev Phys (2021). https://doi.org/10.1038/s42254-021-00371-w