Researchers at DESY are interested not only in extremely heavy particles, which have to be generated by means of large high-energy accelerators; very light particles at the lower end of the energy scale can also point the way toward unknown physics phenomena. The ALPS experiment, which is tiny compared to the vast LHC and ILC facilities, is helping DESY researchers to track down such lightweights of the subatomic world.

With large high-energy accelerators such as the LHC in Geneva or the planned ILC linear collider, physicists are searching for the heavyweights among subatomic particles – particles that had previously escaped them because the energies reached by former accelerators were never high enough to generate particles of such a mass. Most of the theories that take us beyond the Standard Model of particle physics predict the existence of new particles that are around 1000 times heavier than protons. However, recent theoretical works, along with some still poorly understood experimental observations, suggest that the “new physics” could also include a host of extremely light particles.

These so-called WISPs (weakly interacting sub-eV particles) rarely react with matter and are therefore seldom generated, which means that their tracks would simply be drowned out by the flood of standard reactions in large high-energy accelerators. To detect these hypothetical WISPs at the lower end of the energy scale, it is therefore necessary to use other means. These include the experiment ALPS (Any Light Particle Search) at DESY.

Light through a wall

The ALPS experiment involves physicists from DESY, the Albert Einstein Institute Hannover and the Universities of Hamburg and Mainz. Their seemingly absurd proposal is to make “light shine through a wall”. To achieve this, they first direct a laser beam through the powerful magnetic field generated by decommissioned dipole magnets from the HERA ring accelerator. In the event that WISPs do actually exist, some of the photons (particles of light) in the laser beam should disappear and change into these mysterious lightweight particles. Installed in the middle between the magnets is a wall that stops the laser beam. The theory is that any WISPs produced would be able to pass through the wall – since they react so rarely with other particles, solid matter is no obstacle for them. Once they re-enter the magnetic field beyond the wall, the WISPs could then change back into particles of light, which would then show up in a photon detector. The light from the laser beam thus would have effectively passed through the wall.

Yet the expected yields from such photon regeneration experiments are extremely low. At best, scientists predict that one in a billion photons will change into a WISP; and of those, only one in a billion will ever change back into a photon. The experimental setup thus has to be extremely sensitive for researchers to even have a chance of tracking down the quick-change artists.

Small but sharp

In the first phase of the ALPS experiment from 2007 to 2010, the physicists used a single HERA magnet. As the world's most sensitive experiment in this field, ALPS provided the most precise limits on the existence of WISPs, improving previous results by a factor of ten. To further increase the sensitivity of the measurements by several orders of magnitude, the ALPS scientists plan to repeat the experiment with 20 HERA dipoles, in the middle of which the wall is to be located.

It may well turn out that the ALPS physicists with their HERA magnets will be quicker to deliver some long hoped-for signs of new physics than the scientists working on hugely expensive accelerator facilities. In any case, the hunt for extremely lightweight particles in the low-energy region ideally complements the large-scale experiments conducted at the highest energies. In combination, the results of each approach will make a vital contribution toward increasing our knowledge of the elementary building blocks of the universe and the way in which they interact with one another.