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# Observation of the antimatter helium-4 nucleus

## Abstract

High-energy nuclear collisions create an energy density similar to that of the Universe microseconds after the Big Bang1; in both cases, matter and antimatter are formed with comparable abundance. However, the relatively short-lived expansion in nuclear collisions allows antimatter to decouple quickly from matter, and avoid annihilation. Thus, a high-energy accelerator of heavy nuclei provides an efficient means of producing and studying antimatter. The antimatter helium-4 nucleus (), also known as the anti-α (), consists of two antiprotons and two antineutrons (baryon number B = −4). It has not been observed previously, although the α-particle was identified a century ago by Rutherford and is present in cosmic radiation at the ten per cent level2. Antimatter nuclei with B < −1 have been observed only as rare products of interactions at particle accelerators, where the rate of antinucleus production in high-energy collisions decreases by a factor of about 1,000 with each additional antinucleon3,4,5. Here we report the observation of , the heaviest observed antinucleus to date. In total, 18 counts were detected at the STAR experiment at the Relativistic Heavy Ion Collider (RHIC; ref. 6) in 109 recorded gold-on-gold (Au+Au) collisions at centre-of-mass energies of 200 GeV and 62 GeV per nucleon–nucleon pair. The yield is consistent with expectations from thermodynamic7 and coalescent nucleosynthesis8 models, providing an indication of the production rate of even heavier antimatter nuclei and a benchmark for possible future observations of in cosmic radiation.

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## Acknowledgements

We thank the RHIC Operations Group and RACF at BNL, the NERSC Center at LBNL and the Open Science Grid consortium for providing resources and support. This work was supported in part by the Offices of NP and HEP within the US DOE Office of Science, the US NSF, the Sloan Foundation, the DFG cluster of excellence ‘Origin and Structure of the Universe’ of Germany, CNRS/IN2P3, FAPESP CNPq of Brazil, the Ministry of Education and Science of the Russian Federation, NNSFC, CAS, MoST and MoE of China, GA and MSMT of the Czech Republic, FOM and NWO of the Netherlands, DAE, DST and CSIR of India, the Polish Ministry of Science and Higher Education, the Korea Research Foundation, the Ministry of Science, Education and Sports of Croatia, and RosAtom of Russia.

Correspondence and requests for materials should be addressed to The Star Collaboration (star-antihe4-l@lists.bnl.gov).

## Author information

### Contributions

All authors contributed equally.

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### Competing interests

The author declare no competing financial interests.

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The STAR Collaboration. Observation of the antimatter helium-4 nucleus. Nature 473, 353–356 (2011). https://doi.org/10.1038/nature10079

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• DOI: https://doi.org/10.1038/nature10079

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