Probing the Symmetry Energy with the Spectral Pion Ratio

(SπRIT Collaboration)

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78 Citations (Scopus)


Many neutron star properties, such as the proton fraction, reflect the symmetry energy contributions to the equation of state that dominate when neutron and proton densities differ strongly. To constrain these contributions at suprasaturation densities, we measure the spectra of charged pions produced by colliding rare isotope tin (Sn) beams with isotopically enriched Sn targets. Using ratios of the charged pion spectra measured at high transverse momenta, we deduce the slope of the symmetry energy to be 42<L<117 MeV. This value is slightly lower but consistent with the L values deduced from a recent measurement of the neutron skin thickness of Pb208.

Original languageEnglish
Article number162701
JournalPhysical review letters
Issue number16
Publication statusPublished - 2021 Apr 19

Bibliographical note

Funding Information:
The authors would like to thank Professor Hermann Wolter, Professor Che-Ming Ko, and Professor Pawel Danielewicz and the TMEP Collaboration for many fruitful discussions. This work was supported by the U.S. Department of Energy under Awards No. DE-SC0021235, No. DE-NA0003908, No. DE-FG02-93ER40773, No. DE-FG02-93ER40773, No. DE-SC0019209, No. DE-SC0015266, No. DE-AC02-05CH11231, U.S. National Science Foundation Grant No. PHY-1565546, the Robert A. Welch Foundation (A-1266 and A-1358), the Japanese MEXT, Japan KAKENHI (Grant-in-Aid for Scientific Research on Innovative Areas) Grant No. 24105004, JSPS KAKENHI Grants No. JP16H02179 and No. JP18H05404, the National Research Foundation of Korea under Grants No. 2016K1A3A7A09005578, No. 2018R1A5A1025563, No. 2013M7A1A1075764, the Polish National Science Center (NCN) under Contracts No. UMO-2013/09/B/ST2/04064, No. UMO-2013/-10/M/ST2/00624, the National Natural Science Foundation of China under Grant No. 11375094 and the Tsinghua University Initiative Scientific Research Program, the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany’s Excellence Strategy – EXC-2094–390783311 (ORIGINS), Project-ID 279384907 - SFB1245, the BMBF via Projects No. 05P15RDFN1, No. EXC-2094-390783311 and the Romanian Ministry of Education and Research through Contract No. PN 19 06 01 01/2019-2022. Computing resources were provided by the NSCL, the HOKUSAI-Great Wave system at RIKEN, and the Institute for Cyber-Enabled Research at Michigan State University.

Publisher Copyright:
© 2021 American Physical Society.

ASJC Scopus subject areas

  • Physics and Astronomy(all)


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