Abstract
According to quantum chromodynamics, vacuum is not an empty space, because it is filled with quark–antiquark pairs. The pair has the same quantum numbers as the vacuum and forms a condensate because the strong interaction of the quantum chromodynamics is too strong to leave the vacuum empty. This quark–antiquark condensation, the chiral condensate, breaks the chiral symmetry of the vacuum. The expectation value of the chiral condensate is an order parameter of the chiral symmetry, which is expected to decrease at high temperatures or high matter densities where the chiral symmetry is partially restored. Head-on collisions of nuclei at ultra-relativistic energies have explored the high-temperature regime, but experiments at high densities are rare. Here we measure the spectrum of pionic 121Sn atoms and study the interaction between the pion and the nucleus. We find that the expectation value of the chiral condensate is reduced at finite density compared to the value in vacuum. The reduction is linearly extrapolated to the nuclear saturation density and indicates that the chiral symmetry is partially restored due to the extremely high density of the nucleus.
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Data availability
Raw data were generated at the RI Beam Factory. Source data are provided with this paper.
Code availability
The computer codes used to generate results are available from the corresponding author upon reasonable request.
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Acknowledgements
We thank the staff of the RI Beam Factory for stable operation of the facility. This experiment was performed at the RI Beam Factory, operated by RIKEN Nishina Center and CNS, University of Tokyo. This work is partly supported by the MEXT Grant-in-Aid for Scientific Research on Innovative Areas (nos. JP22105517, JP24105712 and JP15H00844 to K.I.), JSPS Grant-in-Aid for Scientific Research (B) (nos. JP16340083 and JP18H01242 to K.I.), (A) (no. JP16H02197 to K.I. and T.U.) and (C) (nos. JP16K05355 and JP24540274 to S.H.), Grant-in-Aid for Early-Career Scientists (no. JP19K14709 to N. Ikeno), Grant-in-Aid for JSPS Research Fellow (no. JP12J08538 to T.N.), JSPS Fund for the Promotion of Joint International Research (Fostering Joint International Research (B); no. JP20KK0070 to K.I.), Institute for Basic Science (IBS-R031-D1 to D.A.), the Bundesministerium für Bildung und Forschung (H.G., E.H. and H.W.), the National Science Foundation through grant no. Phys-0758100, and the Joint Institute for Nuclear Astrophysics (grants nos. Phys-0822648 and PHY-1430152; JINA Center for the Evolution of the Elements to G.P.A.B.).
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T.N. and K.I. designed the experimental concepts and performed experiments, developed detectors and ion optics, analysed data, performed theoretical calculations, and wrote the paper. G.P.A.B., M.D., H.F., N. Fukuda, N. Fukunishi, H.G., E.H., K. Kusaka, N.S., H.S., K.S., H.T., Y.K.T., T.U., Y.W. and H.W. developed the ion optics and performed experiments. S.H. and N. Ikeno designed experimental concepts, performed theoretical calculations and wrote the paper. N.N.-T. performed theoretical calculations and wrote the paper. H.M. developed the detectors. H.N. and M.I. designed the experimental concepts. All other authors performed experiments.
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Nishi, T., Itahashi, K., Ahn, D. et al. Chiral symmetry restoration at high matter density observed in pionic atoms. Nat. Phys. 19, 788–793 (2023). https://doi.org/10.1038/s41567-023-02001-x
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DOI: https://doi.org/10.1038/s41567-023-02001-x
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