Photon Collider Theoretically Produces Matter

Producing matter from light via two-photon collision was originally proposed in 1934, but there has never been a viable experimental method to prove this fundamental prediction of quantum electrodynamics – until now. The 80-year-old theory, proposed by John Wheeler and Gregory Breit, says that because annihilating electron-positron pairs produces two or more photons, colliding photons should produce electron-positron pairs.

Experimental verification would not only help validate QED, but also could be applied to the study of elastic photon-photon scattering and quark resonances, according to researchers at Imperial College London and the Max Planck Institute for Nuclear Physics.

Model of the proposed photon collider. Courtesy of Nature Photonics.

They have proposed a method to prove the theory that involves passing gamma rays through a laser-heated vacuum cavity to trigger photon collisions within. A single shot would produce hundreds of thousands of electron-positron pairs, which could be captured by magnets and detected using Cerenkov radiation or other methods.

In the experiment, a high-intensity laser would be used to generate an ultra-relativistic electron beam. The electrons would then strike a roughly 5-mm-thick gold target and generate gamma rays via bremsstrahlung. The gamma rays would enter the vacuum cavity, or hohlraum, where they would interact with a high-temperature thermal radiation field.

“Within a few hours of looking for applications of hohlraums outside their traditional role in fusion energy research, we were astonished to find they provided the perfect conditions for creating a photon collider,” said Oliver Pike, a doctoral candidate in plasma physics and the lead researcher.

Magnets placed near the gold target would keep any electrons and positrons created there from interfering with the gamma ray beam; another set of magnets would intercept electrons and positrons produced by photon collisions within the hohlraum.

A 2-GeV electron beam and a 400-eV hohlraum would yield more than 100,000 positrons, the theorists predicted, adding that an experiment using higher electron energies could produce hadrons.

“Despite all physicists accepting the theory to be true, when Breit and Wheeler first proposed the theory, they said that they never expected it be shown in the laboratory,” said ICL physics professor Dr. Steve Rose. “What was so surprising to us was the discovery of how we can create matter directly from light using the technology that we have today in the U.K.”

Pike, Rose and his colleagues are now seeking others “who can use our ideas to undertake this landmark experiment.”

The research was funded by the Engineering and Physical Sciences Research Council, the John Adams Institute for Accelerator Science and the Atomic Weapons Establishment. It is published in Nature Photonics (doi: 10.1038/nphoton.2014.95).

For more information, visit www3.imperial.ac.uk