By Phillip F. Schewe, Inside Science News Service
(ISNS)—Scientists at the Relativistic Heavy Ion Collider, an accelerator complex at the Brookhaven National Laboratory on Long Island, N.Y., have on multiple occasions produced the heaviest antimatter nucleus ever observed. Each of the particles, the antimatter equivalent of a helium-4 nucleus, is really a four-particle parcel consisting of two antiprotons and two antineutrons.
Physicists use powerful beams of particles to explore matter at the finest possible level. They often do this by smashing together beams of particles accelerated to very high speeds, approaching the speed of light. Then they collide these particles to see what comes out—as if they smashed together two Swiss watches to learn how they work.
In this case each "watch" was a gold nucleus, an atom of gold with its entire complement of electrons stripped off. The moving parts inside a nucleus are not springs and wheels, but rather protons and neutrons. Each gold nucleus contains 197 protons and neutrons. And when two such gold nuclei, moving within two beams zipping around the RHIC machine at very high speed, finally collide, they create a fireball involving nearly 400 protons and neutrons.
The surplus energy of this fireball is so high that some of it can be converted into new particles on the spot. In past collision studies at RHIC, antimatter particles have been seen to spring out of the fireball. These included anti-protons, anti-deuterons (each consisting of an anti-proton and an anti-neutron), anti-tritons (consisting of two anti-neutrons and one anti-proton), and even an antimatter counterpart of the helium-3 nucleus (consisting of two anti-protons and one anti-neutron).
In a paper posted online on April 24 by Nature magazine, the RHIC collaboration of scientists announced the first sighting of anti-helium-4 nuclei, the heaviest antimatter particle yet observed. In the experiment 18 traces of the heavy antiparticles were seen. Each time a particle with composite mass of the anti-helium was recorded in surrounding detectors. Even as its presence was being sensed, the anti-helium annihilated with ordinary matter in the detectors, releasing a characteristic short burst of energy to signify the destruction of the new antimatter.
One of the lead authors on the paper, Aihong Tang of Brookhaven, said that if the anti-helium didn't exist then only one or two events with that particular energy would have been observed. The fact that 18 events were actually seen with that amount of energy gives the scientists confidence that they had indeed discovered anti-helium.
Antimatter is made in various violent events in distant celestial objects. These are expected to send some antimatter particles in the direction of Earth, where, with orbiting telescopes stationed above the Earth’s atmosphere, they have been observed.
"The fact that so few antimatter helium nuclei have been seen in our detectors on Earth suggests that if any antimatter helium is seen on those orbiting detectors it will mean that it will not be coming from collisions among ordinary matter particles, but from distant bulk antimatter sources in the sky," said Tang.
Tang points out a nice symmetry involved in the new discovery. It was exactly 100 years ago that the nucleus itself was discovered in experiments conducted in Britain by Ernest Rutherford. Before that time, the composition of atoms was unknown. Long before there were accelerators available, Rutherford contrived to collect a faint beam of helium-4 nuclei, which are also called alpha particles, from a naturally radioactive material. Then he directed these alphas into a thin gold foil. Instead of all passing through the foil, some of the alpha particles rebounded backwards.
Scientists quickly deduced that atoms consisted of a heavy inner part, now called the nucleus, and a lighter outer part consisting of one or more electrons. In 1911, Rutherford had discovered the nature of gold nuclei using an alpha beam. Now, 100 years later, in a nice feat of turnabout, RHIC has discovered anti-alphas by colliding gold nuclei at high energies.
Jeffrey Hangst is a spokesperson for the ALPHA experiment at the Large Hadron Collider, an even more powerful instrument at the CERN lab in Geneva, Switzerland. He says that even though the discovery of anti-helium at RHIC was not unexpected, everything in science needs to be confirmed. "What if it hadn’t been there?" Hangst asked, adding that the new finding provides an experimental benchmark for the rarity of anti-helium nucleus production in the universe.
High energy collisions have also been carried out at by another group, the ALICE detector group at the LHC, where the beams consisted of lead nuclei rather than gold nuclei. Has any anti-helium been seen at LHC? "There is a recent unpublished report of some events at ALICE," Hangst said.
Hangst's own work resulted last year in the first ever making and trapping of anti-hydrogen atoms.
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