Brookhaven National Laboratory's Relativistic Heavy Ion Collider in Upton, N.Y., has found the heaviest atom of antimatter ever, a bit of anti-helium.
Until now the heaviest antimatter atoms created were helium-3, which has two protons and one neutron. Creating helium-4, which has two protons and two neutrons, requires a lot more energy.
The antimatter was made by firing gold nuclei into each other at nearly the speed of light. The collisions recreate the kind of conditions found near the beginning of the universe, and the resulting explosions yield a zoo of subatomic particles. Some are more common than others, so scientists need to create a billion collisions to see the rarer kinds of particles. Anti-matter helium 4 is so rare that out of a billion collisions, only 18 atoms were seen.
Aihong Tang, a physicist at Brookhaven and one of the leaders of the research team, noted that while creating antimatter is interesting in itself, there are some fundamental questions that it answers.
One is how often one might expect to find antihelium - or any other kind of antimatter - in the universe. Many natural processes produce subatomic particles of antimatter. Anti-electrons, or positrons, are used in positron emission tomography (PET) scans in hospitals.
But creating antihelium takes a lot of energy. The production rate of the anti-helium gives an idea of how much one might expect to find in nature if there were large amounts of antimatter in the universe. If later observations pick up anti-helium, it means there is either something very energetic going on or there is a large block of antimatter somewhere very far away.
The lack of antimatter in the universe that we see is one of the big questions scientists have tried to answer for decades. Antimatter and matter are exactly the same, except the electrical charges in the atoms are reversed. An antimatter proton has a negative charge, and an anti-electron has a positive charge. A block of antimatter would (scientists think) look just like normal matter. But the two annihilate when they touch, turning into gamma-ray photons. That means that if there were roughly equal amounts of matter and anti-matter in the universe when it started, there should be nothing left but photons today.
Since there is matter to begin with, that means there was some imbalance of matter and antimatter in the early universe, leaving enough matter behind to form stars, planets, and us. Why that imbalance occurred is still a mystery.
One thing that won't happen soon is the creation of the next heaviest element of antimatter, antilithium. The amount of energy needed for that is even bigger than that which the Large Hadron Collider in Switzerland could produce.