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  • Letter
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Measurement of the mass difference and the binding energy of the hypertriton and antihypertriton


According to the CPT theorem, which states that the combined operation of charge conjugation, parity transformation and time reversal must be conserved, particles and their antiparticles should have the same mass and lifetime but opposite charge and magnetic moment. Here, we test CPT symmetry in a nucleus containing a strange quark, more specifically in the hypertriton. This hypernucleus is the lightest one yet discovered and consists of a proton, a neutron and a Λ hyperon. With data recorded by the STAR detector1,2,3 at the Relativistic Heavy Ion Collider, we measure the Λ hyperon binding energy BΛ for the hypertriton, and find that it differs from the widely used value4 and from predictions5,6,7,8, where the hypertriton is treated as a weakly bound system. Our results place stringent constraints on the hyperon–nucleon interaction9,10 and have implications for understanding neutron star interiors, where strange matter may be present11. A precise comparison of the masses of the hypertriton and the antihypertriton allows us to test CPT symmetry in a nucleus with strangeness, and we observe no deviation from the expected exact symmetry.

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Fig. 1: A typical \(_{\bar{\Lambda }}^{3}\overline{{\rm{H}}}\) 3-body decay in the detectors.
Fig. 2: Particle identification and the invariant mass distributions for \(_{\Lambda }^{3}{\rm{H}}\) and \(_{\bar{\Lambda }}^{3}\overline{{\rm{H}}}\) reconstruction.
Fig. 3: Measurements of the relative mass-to-charge ratio differences between nuclei and antinuclei.
Fig. 4: Measured Λ binding energy in the hypertriton compared to earlier results and theoretical calculations.

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Data availability

All raw data for this study were collected by the STAR detector at Brookhaven National Laboratory. Data tables for the results reported in this Letter are publicly available on the STARwebsite ( or from the corresponding authors upon reasonable request.


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The STAR Collaboration acknowledges contributions from V. Dexheimer, F. Hildenbrand and H.-W. Hammer. We thank the Relativistic Heavy Ion Collider (RHIC) Operations Group and the RHIC Computing Facility (RCF) at Brookhaven National Laboratory (BNL), the National Energy Research Scientific Computing (NERSC) Center at Lawrence Berkeley National Laboratory and the Open Science Grid Consortium for providing resources and support. This work was supported in part by the Office of Nuclear Physics within the US Department of Energy Office of Science, the US National Science Foundation, the Ministry of Education and Science of the Russian Federation, the National Natural Science Foundation of China, Chinese Academy of Science, the Ministry of Science and Technology of China and the Chinese Ministry of Education, the National Research Foundation of Korea, the Czech Science Foundation and Ministry of Education, Youth and Sports of the Czech Republic, the Hungarian National Research, Development and Innovation Office, the New National Excellency Programme of the Hungarian Ministry of Human Capacities, the Department of Atomic Energy and Department of Science and Technology of the Government of India, the National Science Centre of Poland, the Ministry of Science, Education and Sports of the Republic of Croatia, RosAtom of Russia and the German Bundesministerium für Bildung, Wissenschaft, Forschung und Technologie (BMBF) and the Helmholtz Association.

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All authors made important contributions to this publication, in one or more of the areas of detector hardware and software, operation of the experiment, acquisition of data and data analysis. All STAR collaborations who are authors reviewed and approved the submitted manuscript.

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Correspondence to J. H. Chen or P. Liu.

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Peer review information Nature Physics thanks Stefan Ulmer and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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The STAR Collaboration. Measurement of the mass difference and the binding energy of the hypertriton and antihypertriton. Nat. Phys. 16, 409–412 (2020).

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