Magnet row of the ALPS experiment in the HERA tunnel: In this part of the magnets, intense laser light is reflected back and forth, from which axions are supposed to form. Credit: DESY, Marta Maye
Topics: Dark Matter, Materials Science, Particle Physics, Quantum Mechanics
The ALPS (Any Light Particle Search) experiment, which stretches a total length of 250 meters, is looking for a particularly light type of new elementary particle. The international research team wants to search for these so-called axions or axion-like particles using twenty-four recycled superconducting magnets from the HERA accelerator, an intense laser beam, precision interferometry, and highly sensitive detectors.
Such particles are believed to react only extremely weakly with known kinds of matter, which means they cannot be detected in experiments using accelerators. ALPS is therefore resorting to an entirely different principle to detect them: in a strong magnetic field, photons—i.e., particles of light—could be transformed into these mysterious elementary particles and back into [light] again.
"The idea for an experiment like ALPS has been around for over 30 years. By using components and the infrastructure of the former HERA accelerator, together with state-of-the-art technologies, we are now able to realize ALPS II in an international collaboration for the first time," says Beate Heinemann, Director of Particle Physics at DESY.
Helmut Dosch, Chairman of DESY's Board of Directors, adds, "DESY has set itself the task of decoding matter in all its different forms. So ALPS II fits our research strategy perfectly, and perhaps it will push open the door to dark matter."
The ALPS team sends a high-intensity laser beam along a device called an optical resonator in a vacuum tube, approximately 120 meters in length, in which the beam is reflected backward and forwards and is enclosed by twelve HERA magnets arranged in a straight line. If a photon were to turn into an axion in the strong magnetic field, that axion could pass through the opaque wall at the end of the line of magnets.
Once through the wall, it would enter another magnetic track almost identical to the first. Here, the [axion] could then change back into a photon, which would be captured by the detector at the end. A second optical resonator is set up here to increase the probability of an [axion[ turning back into a photon by a factor of 10,000.
This means if [light] does arrive behind the wall, it must have been an axion in between. "However, despite all our technical tricks, the probability of a photon turning into an axion and back again is very small," says DESY's Axel Lindner, project leader and spokesperson of the ALPS collaboration, "like throwing 33 dice and them all coming up the same."
In order for the experiment to actually work, the researchers had to tweak all the different components of the apparatus to maximum performance. The light detector is so sensitive that it can detect a single photon per day. The precision of the system of mirrors for the light is also record-breaking: the distance between the mirrors must remain constant to within a fraction of an atomic diameter relative to the wavelength of the laser.
World's most sensitive model-independent experiment starts searching for dark matter, Deutsches Elektronen-Synchrotron, Phys.org.