Observation of the antimatter helium-4 nucleus

摘要

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 (~4He), also known as the anti-a (a), consists of two antiprotons and two antineutrons (baryon number B = - 4). It has not been observed previously, although the a-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 antinucleon. Here we report the observation of ~4He, the heaviest observed antinucleus to date. In total, 18 ~4He counts were detected at the STAR experiment at the Relativistic Heavy Ion Collider (RHIC; ref. 6) in 10~9 recorded gold-on-gold (Au+Au) collisions at centre-of-mass energies of 200 GeV and 62 GeV per nuc-leon-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 ~4He in cosmic radiation.