A NEWLY created form of antimatter is the heaviest and most complex anti-thing ever seen. Anti-helium nuclei, each containing two antiprotons and two antineutrons, have been created and detected at the Relativistic Heavy Ion Collider (RHIC) in Upton, New York.
Antiparticles have the opposite electrical charge to ordinary matter particles. So antiprotons have negative charge, while antineutrons, which are electrically neutral, are made up of antiquarks that have the opposite charge to their normal quark counterparts.
Antimatter annihilates on contact with matter, making it notoriously tricky to find and work with. Until recently, the most complex unit ever seen was the counterpart of the helium-3 nucleus, which contains two protons and one neutron.
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Experiments in New York are changing that. RHIC collides heavy atomic nuclei such as lead and gold to form microscopic fireballs, in which 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 antiproton, one antineutron 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 take the record ().
鈥淭hey have moved us up to the next element in the anti-periodic table,鈥� says Frank Close of the University of Oxford. But he adds: 鈥淚t doesn鈥檛 take us nearer to the big question of why the universe at large is not full of antimatter.鈥�
Standard theories say that matter and antimatter were created in equal amounts in the universe鈥檚 first instants, but for unknown reasons, matter prevailed. An experiment called the Alpha Magnetic Spectrometer, due to launch to the International Space Station on 19 April, will try to chip away at the problem (see next week鈥檚 New Scientist).
Antiprotons 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 antiparticles. If anti-helium is produced only rarely in collisions, as shown by the RHIC, then the AMS should see none. If it does find some 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.
鈥淭he nuclei of anti-helium take the record for the heaviest antiparticles known so far鈥�
The next heaviest anti-element, anti-lithium, could in theory form solid antimatter at room temperature 鈥� but it will be much harder to make. The RHIC team calculates that it will occur in its collisions less than one-millionth as often as anti-helium, putting it beyond the reach of today鈥檚 colliders.