The title begs the question – did older ways to turn light into matter exist? Well, that is exactly what photo-electric effect does! So what’s new here is that we are talking about using direct, pairwise, photon-photon interactions to produce electron-positron pairs, which has not been possible to achieve, experimentally. Writing in Nature Photonics, O.J. Pike et al of Imperial College, London and Max-Planck Institut, Heidelberg, have come up with a suggestion for an experiment in which this can be achieved. They further reiterate their idea by running a Monte Carlo simulation which demonstrates that the dominant channel, with proper design of the experimental apparatus, is the two-photon (Breit-Wheeler) process of pair production.

Quantum electrodynamics does allow for the interconversion of photons and matter, namely, electron-positron pairs can annihilate to give photons; pairs of photons can interact to give electron-positron pairs and even vacuum, or empty space, can undergo polarisation to give out electron-positron pairs.

There are various energy constraints that make some of these processes difficult to achieve experimentally, especially that of two photons colliding to give out a pair.

Pike et al have proposed an experimental scheme to build a table-top photon-photon collider which will produce electron-positron pairs. They suggest shooting ultra-relativistic electrons into a gold target where, due to retardation, the electrons emit high-energy photons. This beam is fired into a vacuum holhlraum (a hohlraum is a cavity whose walls are in equilibrium with the radiation inside. So that the radiant energy that comes out of a small hole in it is like a blackbody radiation.) Inside the vacuum holhlraum, the light beam interacts and produces the electron-positron pairs, which, in turn, can be picked up using a magnetic field outside the cavity.

The authors suggest that the dominant mechanism in the pair production will be the two-photon collision process, and this is an experimental feat that has not been achieved so far. Further they perform Monte Carlo simulations and solve the cross-section equations to corroborate this.

Though this simulation does not bring in anything new theoretically, it will be quite a feat to have a table-top photon-photon collider as they appear to suggest.

However, since they are suggesting that the energies of the pairs produced are in the range of 100 MeV to a GeV, it is debatable how useful this would be in high-energy physics experiments. It may be useful to probe quark resonances, but then, these have already been probed in other particle accelerators.


The science of science writingMay 26, 2014

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