It is our exciting quest to look for new particles beyond the Standard Model. In addition to the known elementary particles, there may exist also further ones that have a tiny mass and very feeble interactions with the remaining matter particles. So-called WISPs (very Weakly Interacting Slim Particles) arise naturally in many extensions of the Standard Model and might also explain observations that are not accounted for within the particle physics known today.

How do we intend to observe axions and axion-like particles ?

A photon can be converted into an axion or ALP and vice versa inside a magnetic field. This key characteristic is used in all our experiments to observe and eventually prove the existence of axions and ALPs.

Haloscopes such as  MADMAX look for axions that would be the dominant constituents of the dark matter halo of our galaxy by using microwave resonators immersed in a magnetic field. These experiments benefit from a high postulated density of axion-like particles and therefore achieve better sensitivities than the other types experiments we will discuss. However, they can only search for a relatively narrow band of masses at a time and their expected detection rate as well as the  interpretation of their results are highly dependent on cosmological assumptions.

Helioscopes such as IAXO use the sun as a source of axions that are converted to x-rays by magnetic fields within terrestrial x-ray telescopes. While the helioscopes have the advantage that the sun is a less model dependent source than the dark matter halo, it is also a weaker source of axions. Helioscopes are, however, sensitive to a much broader range of masses as they search for ultra-relativistic particles.

Light-shining through wall (LSW) experiments take place entirely in the laboratory using a high power laser propagating through a magnetic field. This will generate a beam of axions-like particles that travel through  a wall which blocks the laser light. A second magnetic field behind the wall will cause some of these axion-like particles to reconvert back to photons which can then be measured. Of the three types of axion searches mentioned so far, the LSW experiments are the least model dependent, but also engage the weakest source of axions. Examples for LSW experiments are ALPS I and ALPS II.