Physics Colloquium

High-energy nuclear collisions create an energy density similar to that of the universe microseconds after the Big Bang, and 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 is an efficient means of producing and studying antimatter. The antimatter helium-4 nucleus, also known as the anti-$\alpha$, consists of two antiprotons and two antineutrons (baryon number $B = - 4$). It has not been observed previously, although the $\alpha$ particle was identified a century ago by Rutherford and is present in cosmic radiation at the 10\% 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 about $1000$ with each additional antinucleon. We present the observation of the antimatter helium-4 nucleus, the heaviest observed antinucleus. In total 18 anti helium-4 counts were detected at the STAR experiment at RHIC in $10^{9}$ recorded gold on gold (Au+Au) collisions at center-of-mass energies of 200 GeV and 62 GeV per nucleon-nucleon pair. The yield is consistent with expectations from thermodynamic and coalescent nucleosynthesis models,which has implications for future production of even heavier antimatter nuclei, as well as for experimental searches for new phenomena in the cosmos.