Members of the international STAR collaboration at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory have observed antihelium-4. This is the heaviest antinucleus detected so far, following the discovery of the first antihypernucleus (an antiproton, an antineutron and a Λ) by the same collaboration just a year ago. After sifting through 0.5 × 1012 tracks in data for 109 gold–gold collisions at centre-of-mass energies of 200 GeV and 62 GeV per nucleon–nucleon pair, the STAR collaboration found 18 events with the signature of the antihelium-4 nucleus, which is distinguished by its mass together with its charge of -2.
While the curvature of the tracks in the magnetic field of the STAR detector provide a momentum measurement, key information also comes from the mean energy-loss per unit track length, 〈dE/dx〉, in the gas of the TPC and from the time of flight of particles arriving at the time-of-flight barrel that surrounds the TPC. The 〈dE/dx〉 information helps in identification by distinguishing particles with different masses or charges, the time of flight being needed for identification at higher momenta, above 1.75 GeV/c. The figure shows the identification of isotopes based on energy loss and mass calculated from momentum in the region of helium-3 and helium-4 for both positive and negative particles, with 18 counts for antihelium-4.
The team used this observation to calculate the antimatter yield at RHIC and found that the production rate falls by a factor of 1.6 +1.0/–0.6 × 103 (1.1 +0.3/–0.2 × 103) for each additional antinucleon (nucleon). This is in line with the expectations from coalescent nucleosynthesis models, as well as from thermodynamic models.
The finding ties in with the scientific goals of the Alpha Magnetic Spectrometer launched on 16 May (AMS takes off), which will search for antimatter in space. It also nicely marks the centenary of the paper by Ernest Rutherford in which he analysed the scattering of helium nuclei (alpha particles) on gold and first established the existence of the atomic nucleus.
Further reading
STAR collaboration 2011 Nature online doi:10.1038/nature10079.
