Observation of the antimatter helium-4 nucleus

Antimatter worth the weight The α-particle — the helium nucleus consisting of two protons and two neutrons — was identified a century ago by Ernest Rutherford. Its antimatter counterpart of two antiprotons and two antineutrons has now been detected by the Relativistic Heavy Ion Collider at Brookhaven National Laboratory in Upton, New York. The STAR Collaboration has detected anti-α-particles — the heaviest antinuclei observed to date — at a yield that is consistent with expectations from thermodynamic and coalescent nucleosynthesis models. This discovery provides an indication of the likely production rates of even heavier antimatter nuclei, and serves as a benchmark for possible future observations of anti-α-particles in the cosmos. High-energy nuclear collisions create an energy density similar to that of the Universe microseconds after the Big Bang 1 ; 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 2 . Antimatter nuclei with B