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 (4He), also known as the anti-α (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 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 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 4He in cosmic radiation.

Antimatter production in proton-proton and heavy-ion collisions at ultrarelativistic energies

“…From Fig. 3 it clear that, using the thermal parameters at chemical freeze-out obtained from the analysis of particle yields at RHIC, there is an excellent description of measured ratios of 3 He. In elementary collisions, nuclei and antinuclei as well as hypernuclei and antihypernuclei can be produced by direct pair production.…”

“…which then is ∼ (p/p) 3 . If the nuclei carry strangeness, this leads to an extra term μ S and the antihypertriton/hypertriton ratio reads…”

“…While several hypernuclei have been found since the first observation, no antihypernucleus has ever been observed until the recent discovery of the antihypertriton in Au + Au collisions at √ s NN = 200 GeV by the STAR Collaboration at the Relativistic Heavy Ion Collider (RHIC) [9]. The yield of (anti)hypernuclei measured by STAR is very large; in particular they seem to be produced with a yield similar to that of other (anti)nuclei, such as (anti-) 3 He. This abundance is much higher than that measured for hypernuclei and nuclei at lower energies [10].…”

“…The data on the 3 He and 3 He yields have been corrected for contamination from hypertriton and antihypertriton decays, assuming a decay branching ratio of 25%. Consequently, in the model calculations, such decays have not been included.…”

“…It remains a mystery how this symmetry got lost in the evolution of the universe, reaching a stage with no visible amounts of antimatter being present. Closely related to the matter-antimatter symmetry is the production of light antinuclei, hypernuclei, and antihypernuclei at high energies, especially in view of the recent observation of the anti- 4 He nucleus by the STAR Collaboration [3].…”