ALICE’s wonderland reveals the heaviest antimatter ever observed

Producing and observing antiparticles is part of everyday life for many physics laboratories around the world, including CERN. However, recreating and observing the anti-nuclei of complex atoms is a much more difficult task. Analysing data collected in a run of just one month, ALICE has recently found evidence of the formation of four anti-nuclei of Helium 4, the heaviest antimatter ever created in a laboratory.


The STAR experiment at RHIC came first and published the result in March: they presented evidence of 18 anti-nuclei of Helium 4 collected over several years of data taking. “ALICE came second but it's amazing to see how fast the results came,” exclaims Paolo Giubellino, the experiment’s spokesperson. “We were able to confirm the observation of 4He anti-nuclei with data collected in November 2010.”

Scientists agree on the fact that antimatter was created in the Big Bang together with matter. However, today we do not observe antimatter outside the Earth’s atmosphere, where antimatter particles are created by the collisions of cosmic rays with the gas molecules. The disappearance of antimatter from the Universe is one of the big mysteries researchers are also trying to solve at the LHC. “In particle collisions, it is not rare to observe the production of antimatter particles,” says Giubellino. “However, things change a lot when we try to form anti-nuclei. The RHIC experiments and ALICE had to analyse large amounts of data to be able to spot the rare anti-helium nuclei, which contain two antiprotons and two antineutrons.”

“The STAR and ALICE results clearly demonstrate that, in colliders, matter and antimatter are produced symmetrically,” says Peter Braun-Munzinger, from the ExtreMe Matter Institute, GSI, Darmstadt, Chair of the ALICE Collaboration Board. “We will have to collect more data and study the fine properties of the various physics processes involved in the production of anti-nuclei. This is in our plans for 2011.”

For ALICE, the search for anti-nuclei produced in lead-lead collisions at the LHC is part of a larger campaign to look for all sorts of matter and antimatter ‘conglomerates’, which might come out of the hot and dense quark-gluon plasma. “The whole collaboration is now concentrating on completing the analysis of the 2010 data set. The rich harvest of results will be presented in May at the Quark Matter Conference, which will be held in Annecy (France),” concludes Giubellino.

by CERN Bulletin