PARTICLE PHYSICS

A mysterious particle called axion has the potential to solve two of the most pressing open questions in particle physics: dark matter could be composed of axions, and axions could resolve a mystery in our understanding of the strong nuclear force. The international research collaboration MADMAX aims to find the axion. In the current project phase, the research team is testing and evaluating various approaches for detecting the axion. The new developments and findings have now been published in two articles in Physical Review Letters.

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 – one of the four forces in nature – 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 the Max Planck Institute for Physics (MPP) and with participation of DESY and University of Hamburg: 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.”

Focus on a previously unmeasured frequency range
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 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’.

First measurements with a booster prototype
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 Majorovits. “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.

The first method was developed by DESY researcher Jacob Egge, now a fellow in the ALPS group, during his PhD at the University of Hamburg. Together with the resonance test, it enabled a first search for so-called “dark photons”, another candidate for dark matter. “Unfortunately, we were not yet able to detect any dark photons in this first test run,” says Jacob Egge, first author of the first of the two publications in the open access journal Physical Review Letters. “However, we were able to improve the sensitivity by almost a factor of 1000 compared to all previous experiments and are therefore well on the way to either detecting axions or dark photons in the next measurements or excluding them in our measured bandwidth.”

Thanks to the preliminary work, it has now also been possible to search for axions with a prototype booster. For this purpose, the booster was brought from Hamburg 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 also 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 with the MORPURGO magnet at CERN, which will use a further developed prototype booster. The final experiment will then be set up at DESY in Hamburg.

Background information:

The weakness of the strong force
The axion could not only explain dark matter, but also solve a central problem in particle physics: the so-called strong CP problem. This concerns the question of why the strong force that holds the quarks and gluons together in the atomic nucleus is symmetrical when the direction of time is reversed. The Standard Model of Particle Physics actually predicts the opposite.

The problem can be illustrated using the example of a glass of water. A glass of water remains a glass of water, regardless of when you look at it. Even if you rewind time a few minutes like in a video, it remains a glass of water. It is different with a glass containing an ice cube: at room temperature, it would melt over time. If you now reverse the direction of time, the melted ice cube will freeze again - at room temperature that does not actually allow this. In this case, we would speak of an asymmetry with reversed time direction.

According to the Standard Model, the strong force is not symmetrical when the direction of time is reversed. However, there is no evidence for this. The axion could resolve this contradiction between theory and practice: The particle could combine with the quarks and gluons and thus prevent asymmetry from appearing.

References
First Search for Dark Photon Dark Matter with a MADMAX Prototype; J. Egge, D. Leppla-Weber, S. Knirck, B. Ary dos Santos Garcia, D. Bergermann, A. Caldwell, V. Dabhi, C. Diaconu, J. Diehl et al. (MADMAX Collaboration); Phys. Rev. Lett. 134, 151004 - DOI: 10.1103/PhysRevLett.134.151004

First Search for Axion Dark Matter with a MADMAX Prototype; B. Ary dos Santos Garcia, D. Bergermann, A. Caldwell, V. Dabhi, C. Diaconu, J. Diehl, G. Dvali, J. Egge, E. Garutti et al. (MADMAX Collaboration); Phys. Rev. Lett. 135, 041001 - DOI: 10.1103/PhysRevLett.135.041001


Website of the MADMAX Collaboration