Axion search: crucial progress in the MADMAX experiment

So far, the axion has only existed in theoretical models. What makes the extremely light particle so interesting is that it could significantly advance two research topics in particle physics. One of these is the composition of dark matter. The second is a specific, but not yet understood, feature of the strong interaction which makes quarks stick together in protons and neutrons, thus ensuring stable atomic nuclei.

Particle physics unambiguously predicts that inside a magnetic field cosmic axions trigger an oscillation of the electric field. This prediction is the basis for the MADMAX experiment, initiated by MPP: Using a very strong magnet, scientists try to make the oscillation detectable as microwave radiation. However, the theory does not make precise predictions about the frequency of the expected extremely faint microwave signal. “You can think of axion experiments like a radio receiver,” says Béla Majorovits, scientist at the Max Planck Institute for Physics and spokesperson for the MADMAX Collaboration. “The axion transmits its signal at an unknown frequency and we have to tune our radio precisely to this frequency to detect it.”

Current experiments are looking for axions in the range of several hundred megahertz, i.e. in the radio wave spectrum. However, plausible theoretical models predict that the oscillation caused by axions is at a significantly higher frequency. “With our experiment, we will search the bandwidth from 10 to 100 gigahertz,” says Béla Majorovits. “To do this, we are using a so-called booster that amplifies the conversion of vacuum oscillations into microwaves, increasing the signal strength by many orders of magnitude.”

This innovative booster consists of several disks that are positioned in front of a mirror and are permeable to microwaves. The vacuum vibrations are converted into microwaves at the surfaces of the mirror and the disks. The multiple reflections of the waves between the mirror and the disks generate resonances and thus amplify the signal. In order to obtain reliable and reproducible results with MADMAX, it is important to determine the exact ‘boost factor’.

The research team has now succeeded in determining the amplification effect of the booster for the first time. To do this, the scientists used two complementary methods. "When we irradiate our booster with microwaves, similar resonances arise as if they were excited by axions. If we measure the strength of these resonances, we can directly determine the amplification factor we are looking for," explains Béla Majorovits [1]. “The second method is based on the reflection behavior of the booster. This can be used to determine the key parameters required to calculate the amplification effect.

Thanks to this preliminary work, it has now been possible to search for dark matter axions with a prototype booster [2]. For this purpose, the booster was brought to CERN and the measurements were carried out in the 1.6 Tesla magnetic field of the MORPURGO magnet. Although the research team did not find the axion they were able to top the most sensitive measurements to date in two frequency bands.

Having reached this important milestone, the international research group is confident that it will be able to further optimize the booster and the detection methods in the coming years. As a next step, further measurements are planned for 2027 to 2029 in the MORPURGO magnet at CERN, which will use a further developed prototype booster. The final experiment will then be set up at the DESY laboratory in Hamburg.