Anti-d and Anti-He Production in Au+Au Collisions at RHIC

The STAR experiment at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory, New York, USA, investigates novel QCD phenomena at high density and high temperature in ultra-relativistic Au+Au collisions. RHIC has a diameter of 3.8 km, and in total 1740 superconducting magnets [ 1]. The highest center-of-mass energy ( √ s=200 GeV) is about a factor 10 higher than past fixed target experiments (e.g. CERN SPS Ebeam=160 GeV, equivalent to √ s≃18 GeV). The main STAR subdetector is a midrapidity (|η|≤1.6) Time Projection Chamber (TPC, R=2 m, L=4 m) [ 2] with ≃48,000,000 pixels. In 3 years of data taking, high statistics (106≤N≤107 events on tape) for Au+Au, p+p and d+Au at √ s=200 GeV and Au+Au at √ s=130 GeV were recorded, which enabled analyses of rare events. The particle identification of anti-particles is performed by energy loss dE/dx, corresponding to charged particle ionization in the TPC gas (10% methane, 90% argon). As production of Anti-He is a rare process (dN/dy∼10−6), a level-3 trigger system [ 3] was used for realtime anti-nuclei identification. One central Au+Au collision contains about ≃130,000 TPC hits and ≃6,500 TPC tracks, which were fully reconstructed in realtime (t≤100 ms) by 432 Intel i960 CPUs and 48 ALPHA 21264 CPUs, respectively. The trigger signature was a tag onto a negative charge Q=-2, by usage of the TPC dE/dx in realtime. As no other particles in nature are expected to have the same signature, the signature is clean. For a single track, ≤45 dE/dx samples (corresponding to 45 TPC hits) were used for the calculation of a 70% truncated mean (i.e. excluding samples in the Landau tail of the dE/dx distribution). A track quality cut of NHit≥23 hits per track on the level-3 trigger and NHit≥30 in the offline analysis was applied. Anti-He candidate tracks were also required to have a distance-of-closest-approach to the event collision