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Almost every particle-physics experiment ever performed can be explained by a single theory called the standard model of elementary particles. But while that’s a great triumph for particle physicists, it also makes life rather boring. So the search is on for experiments that will reveal physics beyond the standard model, and one of the most exciting has just been switched on at the DESY particle accelerator in Germany.

The new experiment is searching for an entirely new form of tunneling, the weird quantum process by which a quantum particle passes through a potential barrier that a classical particle could not traverse. This behavior is the result of the Heisenberg uncertainty principle, which gives a finite probability for a quantum particle to cross any barrier of specific height and thickness.

But there are other kinds of tunneling too, as we recently saw. One of the most intriguing is that quantum mechanics allows photons to change into particles and then change back again. If those particles can pass through a barrier, then turn back into photons, it would look as if the photons were tunneling.

Experiments to measure this shining-a-light-through-a-wall effect are exciting because they could reveal particles that are not predicted by the standard model. For example, the particle that the folks at DESY are looking for is called the WISP (weakly interacting sub-eV particle), which could be a major component of dark matter. The standard model does not account for dark matter, which is why any discovery in this area would be a jaw-dropper for particle physicists.

The DESY experiment announced on the arXiv is essentially a high-tech wall. On one side of the wall is a laser, and on the other a detector. The trouble is that photons transform into WISPs only very rarely. So you need a huge number of photons to see the effect. The DESY team has a powerful laser producing some 10^19 photons per second, but even that isn’t enough. So the researchers have built themselves a couple of mirrors to reflect the photons back and forth, so that each photon approaches the wall around 200 times.

The DESY team calls itself the Axion-Like Particle Search Collaboration, or ALPS Collaboration. Having built their experiment, the researchers’ task now is to sit back and watch a blank wall.

And if they spot one or two photons popping through, you’ll be hearing a whole lot more about ALPS.

Ref: arxiv.org/abs/0905.4159: Resonant laser power build-up in ALPS: a light-shining-through-walls experiment

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