Any Light Particle Search (ALPS) II
ALPS II (Any Light Particle Search II) is looking for a new, light-weight, set of particles called axion-like-particles. These could explain dark matter and other phenomena that the standard model of particle physics cannot describe. The experiment relies on a high power laser, high finesse cavities and superconducting dipole magnets. It is located underground at DESY in Hamburg and AEI Hannover is providing major parts of the experimental setup while conducting pathfinder laboratory experiments.
What are axion-like-particles?
Axions are hypothetical particles, which were first postulated to explain the non-observation of the violation of time reversal invariance in strong interactions. They later also became a candidate for a constituent of cold dark matter. Besides the axion, there are several other candidates for similar “Weakly Interacting Sub-eV Particles” (WISPs): axion-like particles, “hidden sector photons” or light minicharged particles.
How to search for axion-like particles
Axions couple weakly to photons in the presence of magnetic fields, and many experiments use this interaction to search for these hypothetical particles. ALPS I has and ALPS II will use the “light shining through a wall” approach. Light (here high power laser light) is sent through a strong magnetic field in order to convert some of the photons into axions. Because the hypothetical axions are weakly interacting they would then pass through a wall. On the other side of this barrier, the axions are re-converted into photons in another strong magnetic field.
The ALPS II experiment
ALPS II is a light-shining through a wall experiment to search for WISPs. It will consist of two 122 meter long optical cavities, with a light tight wall in between them preventing any light in the first cavity from getting into the second. The light circulating in both cavities will travel through 100 meters of strong magnetic fields (5.3 Tesla), which are generated by superconducting dipole magnets from the former HERA particle accelerator. The high power pre-stabilized laser (70 W at 1064 nm) injected into the cavity will meet the required input power to reach our initial goal of 150 kW of laser power circulating in the cavity in front of the wall. Behind the wall, ALPS Il will use two different detection systems, to pick up any axions re-converted into photons in a second cavity. One is a heterodyne detector, the other a transition-edge sensor.
Overall, the ALPS II experimental goal is to achieve a higher sensitivity to increase the probability of detecting axions or axion-like particles.
ALPS II pathfinder experiments at AEI
Contributions to ALPS II from the AEI
The AEI contribution to the ALPS II experiment includes the generation system consisting of the high power laser system, the input optics system to guide the laser beam to the 122 meter optical cavity in front of the light tight wall. Furthermore, control electronics were developed and tested that stabilize the high power laser to match and track the resonance frequency of the cavity. The AEI ALPS II group leads the design efforts for the generation system. Laboratory experiments were conducted to project the behavior of the control loops in the ALPS II environment. In addition AEI personell will contribute to the commissioning activities of the ALPS II experiment at DESY.