Smashing time (Image: STAR/RHIC)

A newly created form of antimatter is the heaviest and most complex anti-thing ever seen. Anti-helium nuclei, each containing two anti-protons and two anti-neutrons, have been created and detected at the Relativistic Heavy Ion Collider (RHIC) in Upton, New York.

Anti-particles have the opposite electrical charge to ordinary matter particles (anti-neutrons, which are electrically neutral, are made up of antiquarks that have the opposite charge to their normal quark counterparts). They annihilate on contact with matter, making them notoriously tricky to find and work with. Until recently, the most complex unit of antimatter ever seen was the counterpart of the helium-3 nucleus, which contains two protons and one neutron.

But experiments at RHIC are changing that. RHIC collides heavy atomic nuclei such as lead and gold to form microscopic fireballs, where energy is so densely packed that many new particles can be created.


Last year RHIC announced the creation of a new variety of antimatter. Called the anti-hypertriton, it is made of one anti-proton, one anti-neutron and one unstable particle called an anti-lambda. The anti-hypertriton was then the heaviest antiparticle known, but the 18 nuclei of anti-helium-4 seen at RHIC now takes the record.

Anti-periodic table

“They have moved us up to the next element in the anti-periodic table,” says Frank Close of the University of Oxford in the UK.

But he adds, “It doesn’t take us nearer to the big question of why is the universe at large not full of antimatter?” Indeed, standard theories say that matter and antimatter were created in equal amounts in the universe’s first instants, but for unknown reasons, matter prevailed.

An experiment called the Alpha Magnetic Spectrometer, due to launch to the International Space Station in April, will try to chip away at the problem. Anti-protons are known to occur naturally in small quantities among the high-energy particles called cosmic rays that hit Earth.

The AMS will search for heavier anti-particles. But if anti-helium is produced only rarely in collisions, as shown by RHIC, then the AMS should see no anti-helium. If it finds higher levels of anti-helium, that could bolster a theory that antimatter was not all destroyed in the early universe but merely separated in a different part of space, where it would not come into contact with matter.

The next heaviest anti-element, anti-lithium, could in theory form solid antimatter at room temperature – but it will be much harder to make. The RHIC team calculates that it will occur in their collisions less than one-millionth as often as anti-helium, putting it beyond the reach of today’s colliders.

Journal reference: arxiv.org/abs/1103.3312