First Limits on Axion Dark Matter from MADMAX

20 September 2024

Photo: Ary dos Santos Garcia, B., et al. (MADMAX Collaboration (2024)

The axion dark matter search result (orange filled region) from the MADMAX prototype. The axion-photon coupling (in inverse giga-electron-Volts ) is on the vertical axis and axion dark matter mass (in micro-electron-volts) is on the horizontal axis. Right inset: one of the two experimental setups placed inside the magnetic field - booster of sapphire disks & mirror with focusing mirror to horn antenna.    

For its first time, the MADMAX (Magnetized Disc and Mirror Axion eXperiment) collaboration has set limits on axion dark matter with its prototype configuration. The University of Hamburg & DESY are playing a major role in developing MADMAX to search for the sought after QCD axion particle - at once both a dark matter candidate and solving the strong CP problem in physics. This is the first axion search by any dielectric-boosted haloscope and has covered higher axion masses than other haloscopes.

To search for axion dark matter, the MADMAX prototype experiment required a strong external static magnetic field, for which the collaboration  utilized the 1.4 Tesla Morpurgo dipole magnet at CERN. Previously, just last month, MADMAX had announced dark photon dark matter constraints derived from running their dielectric-boosted haloscope without any applied magnetic field.

The MADMAX team have submitted a paper reporting axion limits where a key role in the University of Hamburg's contribution was played by Excellence Cluster Quantum Universe's Master's student David Leppla-Weber. The authors have been able to place limits on the axion-photon coupling over two narrow ranges at relatively high axion masses. The MADMAX prototype search for axions used two different booster configurations resulting in axion dark matter constraints over two separate narrow axion mass ranges: 76.56 to 76.82 micro-electron-Volts (μeV) and 79.31 to 79.53 μeV. It is their novel boosted dielectric design which enabled limits in the new territory of higher axion masses within 75-80 micro-electron-Volts.

MADMAX’s dielectric haloscope concept has the distinct advantage of decoupling the volume where axions convert to microwave radio photons from the frequency or axion mass at which the search is performed. This is achieved by placing resonant dielectric disks to amplify the axion-to-microwave photon converted signal. By controlling the spacing between the disks, the boost factor can be tuned allowing both broadband and resonant searches for dark matter particles. This opens up a previously inaccessible range of masses for axion dark matter searches.

By collecting experimental data adding up to around two weeks of running their prototype they have already been able to improve our constraints on the axion-photon coupling by up to a factor of a few better than the benchmark CAST helioscope experiment. It is also an improvement upon astrophysical constraints derived from globular clusters. MADMAX has calculated their coupling limits assuming the standard local axion dark matter density of 0.3 Giga-electron-Volt per cubic cm (GeV cm-3) and velocity dispersion of 218 km/s.

This relatively ‘heavy’ axion mass region, above 40 micro-electron-Volts or so, is especially important in the hunt for axions because that is the mass rage where we predict axions if they were formed after cosmic inflation. This is described in the literature as the post-inflationary Peccei-Quinn symmetry breaking scenario.

Many important advancements have been achieved with this science run. Detailed understanding of the radio frequency response of a booster system with closed boundary conditions was demonstrated. The booster could be quickly tuned and re-calibrated to different frequencies needed for future larger-range frequency scans. The data from the prototype test campaign using a small booster system inside a modest strength magnetic field allow future probes for axion dark matter in previously uncharted territory.

The new results yield a firm basis for the future research and development program towards a competitive dielectric haloscope, including more and larger dielectric disks, calibration and operation of boosters at cryogenic temperatures and the implementation of tuning mechanisms via motorized disk position controls. With ongoing development of a future, unique, large-bore 9 Tesla dipole magnet, the MADMAX collaboration is on track towards probing dark matter QCD axions in the 100 micro-electron-Volt mass region.