19 November 2008

Laser creates billions of antimatter particles

By shooting a laser through a gold disc no bigger than the head of a drawing pin, physicists have created more than 100 billion particles of antimatter.
LLNL laser facility

Positron factory: Physicist Hui Chen sets up targets for the anti-matter experiment at the LLNL laser facility. Credit: LLNL

SYDNEY: By shooting a laser through a gold disc no bigger than the head of a drawing pin, physicists have created more than 100 billion particles of antimatter.

The ability to create vast numbers of positrons in the laboratory opens the door to new avenues of research, they say. These include an understanding of the physics behind black holes, gamma ray bursts and why more matter than antimatter survived the Big Bang.

Super-sized portion of positrons

“We’ve detected far more antimatter than anyone else has ever measured in a laser experiment,” said Hui Chen, a physicist at the Lawrence Livermore National Laboratory (LLNL) in California, U.S., who led the experiment. “We’ve demonstrated the creation of a significant number of positrons using a short-pulse laser.”

Previous experiments made smaller quantities of positrons using lasers and paper-thin targets – but new simulations showed that millimetre-thick gold could be a far more effective source, said the researchers, who report their finding this week at the American Physical Society’s Division of Plasma Physics Meeting in Dallas, South Carolina.

Chen and her team used a short, ultra-intense laser to irradiate a millimetre-thick gold target.

In the experimental set-up, the laser ionises and accelerates electrons, which are driven right through the gold target. On their way, the electrons interact with the gold nuclei, which serve as a catalyst to create positrons.

Electron’s opposite number

The electrons give off packets of pure energy, which decay into matter and antimatter, following the predictions of Einstein’s famous equation that relates matter and energy. By concentrating the energy in space and time, the laser produces positrons more rapidly and in greater density than ever before in the laboratory.

Positrons are the antimatter equivalent to the electron, and behave in a similar way, though they have the opposite charge (see, New twist to matter-antimatter mystery, Cosmos Online).

The researchers took advantage of this property to detect them, by using a typical device to detect electrons (a spectrometer) and equipping it to detect particles with opposite polarity as well.

“By creating this much antimatter, we can study in more detail whether antimatter really is just like matter, and perhaps gain more clues as to why the universe we see has more matter than antimatter,” said LLNL team member Peter Beiersdorfer.

“We’ve entered a new era,” Beiersdorfer added. “Now, that we’ve looked for it, it’s almost like it hit us right on the head. We envision a centre for antimatter research, using lasers as cheaper antimatter factories.”

Particles of antimatter are almost immediately annihilated by contact with normal matter, and converted to pure energy in the form of gamma rays.

There is considerable speculation as to why the observable universe appears to be almost entirely matter, whether other universes could be almost entirely antimatter, and what might be possible if antimatter could be harnessed.

Product of energetic celestial events

Normal matter and antimatter are thought to have been in balance in the very early universe, but, due to a mysterious ‘asymmetry’, the antimatter decayed or was annihilated, and today very little remains.

Over the years, physicists had theorised about antimatter, but it wasn’t confirmed to exist experimentally until 1932.

High-energy cosmic rays impacting Earth’s atmosphere produce minute quantities of antimatter in the resulting jets, and physicists have learned to produce modest amounts of anti-matter using traditional particle accelerators and smaller laser set-ups in the lab.

Antimatter may also be churned our in regions like the centre of the Milky Way and other galaxies, where very energetic celestial events occur. The presence of the resulting antimatter is detectable by the gamma rays produced when positrons are destroyed when they come into contact with nearby matter.

With the Lawrence Livermore National Laboratory.

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