Anti-Alpha Particles Observed for the First Time

Observation of these particles in cosmic rays with space based detectors would imply large amounts of anti-matter somewhere in the universe.

Image courtesy of Brookhaven National Laboratory
Anti-nuetrons and anti-protons formed in very high energy collisions at RHIC are observed to combine to form anti-alpha particles.

The Science

Anti-Alpha particles, the nucleus of anti-helium made of anti-neutrons and anti-protons, have been observed for the first time in very high energy collisions of gold nuclei with the STAR detector at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory. This is the heaviest anti-nucleus ever produced.

The Impact

The number of anti-alpha particles produced is consistent with a model of chance formation from the large number of anti-protons and anti-neutrons produced in the very high energy gold collisions. The relatively small number of anti-alpha particles produced, 18 observed in 1 billion collisions, implies that the observation of such particles in space based cosmic ray detectors, such as the AMS attached to the International Space Station, would imply large amounts of anti-matter, possibly an anti-star, somewhere in the universe.


The world we live in is made of what we call “matter,” and our current understanding of physics tells us that when matter is produced an equal amount of “antimatter” should be produced. Observations of gold ions colliding at very high energies at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory in fact show that particles are produced as matter-antimatter pairs. The STAR detector at this facility has observed the heaviest anti-nucleus ever found: anti-helium, also known as the anti-alpha particle. Finding these anti-alphas among many hundreds of particles produced in each collision was exceptionally difficult, as they are produced very rarely at the rate of only 18 being found in over 1 billion collisions of gold nuclei. This low yield of anti-alpha particles is explained well by a hypothesis where anti-neutrons and anti-protons produced in the collision of two gold nuclei are by chance close enough to combine to form the anti-alpha particle. The relatively small number of anti-alphas produced in such collisions implies that if they are observed in cosmic rays with detectors like the Alpha Magnetic Spectrometer attached to the International Space Station, the result would imply large quantities of anti-matter, such as an anti-star, somewhere in the universe.


Jamie Dunlop
Brookhaven National Laboratory


This work was supported in part by the Offices of NP and HEP within the U.S. DOE Office of Science, the U.S. NSF, the Sloan Foundation, CNRS/IN2P3, FAPESP CNPq of Brazil, Ministry of Ed. and Sci. of the Russian Federation, NNSFC, CAS, MoST, and MoE of China, GA and MSMT of the Czech Republic, FOM and NWO of the Netherlands, DAE, DST, and CSIR of India, Polish Ministry of Sci. and Higher Ed., National Research Foundation (NRF-2012004024), Ministry of Sci., Ed. and Sports of the Rep. of Croatia, and RosAtom of Russia.


H. Agakishiev et al., “Observation of the Antimatter Helium-4 Nucleus.” Nature 473 353 (2011).

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