Figure. Coalescence parameters B2 and B3 excitation functions for semicentral Au+Au or Pb+Pb collisions. The nuclei are plotted using hollow markers, and the antinuclei are plotted using solid markers.

The production of antinuclei at RHIC energies is possible via two mechanisms. The first mechanism is direct production of nucleus-antinucleus pairs in elementary nucleon-nucleon or parton-parton interactions. Because of their small binding energies, nuclei or antinuclei produced via early direct production are likely to be dissociated in the medium before escaping. The second, and presumably dominant, mechanism for antinucleus production is via final-state coalescence. In this picture, produced antinucleons merge to form light antinuclear clusters during the final stages of kinetic freeze-out. The measured yield of nuclei or antinuclei with nucleon number A and momentum P is related to the primordial nucleon invariant yield at momentum p = P/A through a coalescence parameter B_A.

The first measurements of the production of light antinuclei (dbar and anti-³He)in Au+Au collisions at sqrt(s_nn) = 130 GeV were done by STAR. A large enhancement in production rate is observed compared to lower energies. The coalescence parameters B₂ and B₃ were extracted by combining the measured light antinuclei yields with the measurements of pbar production. The qualitative trend for B₂ and B₃ is very similar (see Figure). The coalescence factor is independent of energy for pA collisions, and it decreases with increasing collision energy for AA collisions. This decrease can be understood by noting that once the collision region is larger than the intrinsic size of the produced (anti)nucleus, (anti)nucleons of equal velocity are not always in close proximity and hence do not always form a bound state. Quantitative comparisons to SPS results indicate little or no increase of the antinucleon freeze-out volume. The anti-³He were also found to be produced from a smaller volume than dbar.