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Light through the wall – take 2

Preparations for ALPS II are in full swing

| 15 September 2011

ALPS: a still-small version. The huge one is yet to come. Image: DESY

Bending a HERA magnet straight may sound bizarre, but it is an important prerequisite for ALPS II. Last year, the “light shining through a wall” experiment ALPS at DESY in Germany did not find the lightweight particles it was looking for, but it did obtain the best ­exclusion limits worldwide for hidden light particles. Thus, for the ALPS collaboration, there is no doubt that “ALPS must become larger and more precise.”

ALPS stands for “any light particle search”. Since 2007, it has searched for very light particles such as ­axions. To find them, laser light is sent through a HERA dipole with a wall in its centre. The general assumption is obvious: light shining through a wall – no way! However, where axions do exist, the light’s behaviour would be completely different. The laser light could transform into an axion in the magnetic field, traverse the wall and transform back into a photon again behind the wall. The result is light ­shining through the wall.

“A whole series of axion-like particles just tumble out of string theory,” ­explains ALPS spokesperson Axel Lindner. These extremely light particles – should they exist – could give answers to some open questions of particle physics. ­Extremely light particles could make up dark matter or explain dark energy – 95 percent of our ­universe that is so far unknown to us. More­over, the longer that high-energy ­experiments as the LHC run without detecting particles that might explain such pheno­mena, the ever more fascinating the low-energy range ­becomes.

The final location for the installation of ALPS II, planned for the period after the completion of the European XFEL, will be in the HERA tunnel. A giant version of ALPS is to increase the current experiment’s measuring precision: twelve superconducting magnets in front and twelve behind the wall – instead of one magnet in total – form the basic structure. Yet this presents the ALPS collaboration – and the participating ­scientists from the ­Albert Einstein Institute Hannover, the Sternwarte Bergedorf Observatory and the University of Hamburg – with challenges, because the beam pipe of the HERA dipoles is bent. In order to send light through all magnets, some of them will have to be straightened! In addition to the magnets, ALPS II includes a new superconducting detector, a stronger ­laser and a much more complex optical structure. In this final stage, ALPS would be able to search for axions and other super-light particles with more precision than the current astrophysical ­experiments in this field.

Until then, many further steps are required. First, the scientists have to find out whether a straightened magnet works as well as a bent one. Parallel to this, the laser technology will be tested – initially on a smaller scale, at 20 ­instead of 200 metres, in a new laser laboratory in the HERA hall west, and ­without magnets. This, however, is not only an ordinary test run; at this stage ALPS will also search for so-called ­hidden photons. These are other ­particles that might exist in the low-­energy range and that, in contrast to axion-like ­particles, might also appear without a magnetic field. “As soon as it becomes clear that the optics work on this small scale, we will expand the whole structure to 200 metres in the HERA tunnel – again without magnets at first,” Lindner says about the later-stage ­procedure. Only when everything is ­running well there will the DESY ­directorate be asked to decide on the ­con­struction of ALPS II with a total of­ 24 magnets.

ALPS is not the only experiment searching for light particles in Hamburg. DESY is also collaborating in the Solar Hidden Photon Search experiment – SHIPS for short – at the Sternwarte Bergedorf Observatory. Possible cooperation with CERN scientists is also in the air. “The axion research field becomes more and more exciting,” says Lindner. “DESY’s outstanding infrastructure and know-how provide the chance to continue to be in the front line of this new particle physics research field.”

This article originally appeared in the 1 September issue of DESY InForm.

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