Stephen Battersby writes on New Scientist:
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.