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 Bang; 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 level. 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 antinucleon. 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 200GeV and 62GeV per nucleon-nucleon pair. The yield is consistent with expectations from thermodynamic and coalescent nucleosynthesis models, providing an indication of the production rate of even heavier antimatter nuclei and a benchmark for possible future observations of in cosmic radiation.
- Publication:
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Nature
- Pub Date:
- May 2011
- DOI:
- 10.1038/nature10079
- arXiv:
- arXiv:1103.3312
- Bibcode:
- 2011Natur.473..353S
- Keywords:
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- Nuclear Experiment
- E-Print:
- 19 pages, 4 figures. Submitted to Nature. Under media embargo