Measuring the heaviest anti-nucleus observed so far

The event display of the ALICE experiment shows an anti-alpha candidate track among the many other particles produced in a Pb-Pb collision (Image: ALICE collaboration)

In a recent paper*, the ALICE collaboration has released the measurement of the integrated production yield of the alpha particle as well as of its anti-matter partner, the anti-alpha nucleus, in heavy-ion collisions. This new analysis of the anti-alpha, the heaviest anti-nucleus observed so far, is based on data which was obtained during the run 1 of LHC. The first anti-alpha candidates were seen by the STAR collaboration at the Relativistic Heavy-Ion Collider at the Brookhaven National Laboratory in 2011.

The powerful particle identification capabilities of ALICE allow for a direct observation of rarely produced anti-nuclei by distinguishing them from the large background of the bulk of particles produced in Pb-Pb collisions, posing one of the main challenges in the data analysis. The present analysis is based on the larger data set from the 2011 data taking campaign and exploits the combination of two detectors, the Time Projection Chamber (TPC) and the Time-of-Flight detector (TOF), for a nearly background-free identification. In addition, the unknown hadronic interaction cross-section of the anti-alpha nucleus with the detector material needed to be carefully addressed in simulation studies by a comparison of two alternative codes, namely GEANT3 and GEANT4, used to transport the particles through the detector.

The new results allow for the first time a direct comparison of the anti-alpha production yields with statistical-thermal models. The measured yield agrees with the previously found trend that the production of (anti-)nuclei is suppressed by a factor of about 330 for each additional nucleon in Pb-Pb collisions. While about 30 protons are produced per unit rapidity in a central Pb-Pb collision, anti-alphas occur only once every millionth event. At LHC energies, the statistical-thermal model is known to successfully describe the production of light flavour hadrons with only one parameter, the temperature of about 156 MeV corresponding to about 1.8 trillion Kelvin. Surprisingly, the new results show that this description remains valid also for fragile objects such as the anti-alpha nucleus with a binding energy as low as 7.1 MeV per nucleon.

The ALICE collaboration is now looking forward to the analysis based on the entire run 2 Pb-Pb data sample as well as to the measurement of the transverse momentum distribution and the elliptic flow of anti-alphas with the largely increased statistics which is expected in the LHC run 3 and 4.

*Acharya, S. et al. (ALICE Collaboration), Production of 4He and anti-4He in Pb–Pb collisions at sqrt(sNN) = 2.76 TeV, arXiv:1710.07531, CERN-EP-2017-266, submitted to NPA