⚛️ high-energy experiments

A new Limit for Axion Dark Matter with SPACE

The SPACE experiment, Germany's first axion dark matter search, utilized a 14 T resonant cavity to probe the mass range of 16.626–16.653 μeV and established a new, record-breaking limit on the axion-photon coupling that surpasses previous constraints by over two orders of magnitude.

Original authors: M. A. Akgümüs, N. Salama, J. Egge, E. Garutti, M. Maroudas, L. H. Nguyen, D. Leppla-Weber

Published 2026-10-20

📖 4 min read🧠 Deep dive

CC BY 4.0

Original authors: M. A. Akgümüs, N. Salama, J. Egge, E. Garutti, M. Maroudas, L. H. Nguyen, D. Leppla-Weber

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The universe is filled with matter that we cannot see. Astronomers know this invisible substance, called dark matter, exists because its gravity holds galaxies together and bends the light of distant stars, yet it does not emit or reflect light in any way we can detect. For decades, scientists have searched for the particle that makes up this cosmic shadow, testing many different theories. One leading idea suggests that dark matter consists of axions, tiny, ghostly particles that were proposed to solve a deep puzzle about why the strong nuclear force behaves differently than expected. If these axions exist, they should be everywhere, passing through us and the Earth constantly, but they interact so weakly with normal matter that they have remained invisible. The challenge for physicists is to find a way to catch one of these elusive particles, turning the invisible into something we can measure.

A team of researchers in Germany has taken a significant step toward this goal with a new experiment called SPACE, the Student Project for an Axion Cavity Experiment. This is the first experiment of its kind in Germany, and it represents a major leap forward in the sensitivity of our search. The researchers built a specialized device designed to listen for the faint whisper of an axion converting into a photon, a particle of light. They focused their search on a very specific range of masses, looking for axions that would correspond to a frequency between 4.020 gigahertz and 4.027 gigahertz. To make this conversion possible, they placed a hollow metal cylinder inside a powerful superconducting magnet that generates a magnetic field of 14 tesla, roughly 280,000 times stronger than the Earth's magnetic field. In this intense magnetic environment, if an axion were to pass through the cylinder, the theory predicts it would transform into a tiny burst of radio waves that the cylinder could resonate with, much like a bell ringing at a specific pitch.

The experiment ran for 71 hours in April 2024, during which the team collected billions of measurements of the radio signals coming from the cavity. They used extremely sensitive amplifiers and filters to isolate the specific frequency range they were interested in, carefully tracking how the temperature and physical properties of the equipment changed over time to ensure their data remained accurate. Despite the high sensitivity of their equipment and the strength of the magnetic field, the researchers did not find any signal that stood out from the background noise. They did not hear the bell ring. However, this lack of a signal is a powerful result in itself. By not finding the axion, the team was able to rule out the existence of axions with certain properties in that specific mass range. They determined that if axions exist with a mass between 16.626 microelectronvolts and 16.653 microelectronvolts, their interaction with light must be weaker than a specific limit.

The team calculated that any axion in this mass range must have an interaction strength with photons smaller than 14.6 times 10 to the negative 13 inverse gigaelectronvolts, and at their most sensitive point, they could exclude values larger than 2.8 times 10 to the negative 13 inverse gigaelectronvolts. This new limit is more than one hundred times stricter than previous constraints in this mass range, effectively closing the door on a wide swath of possibilities that other experiments had left open. The researchers are confident in this result, having accounted for various sources of error and uncertainty in their equipment and analysis. While they did not discover the axion, they have successfully narrowed the search space, telling the scientific community that if the axion exists in this mass range, it is even more elusive than previously thought. This work, conducted by students and researchers at the University of Hamburg and DESY, pushes the boundaries of what is possible in the hunt for dark matter, proving that even a null result can provide profound clarity about the nature of the universe.

Technical Summary: A New Limit for Axion Dark Matter with SPACE

Problem Statement
Dark matter constitutes the majority of matter in the universe yet remains undetected due to its weak interaction with ordinary matter. The axion, a hypothetical particle arising from the Peccei-Quinn theory to resolve the strong CP-problem, is a leading candidate for dark matter. While its mass ( $m_a$ ) is theoretically unknown, astrophysical constraints suggest an upper bound of order O(eV). In the presence of a static external magnetic field ( $B_e$ ), axions are predicted to convert into detectable photons. This conversion is governed by modified Maxwell's equations, where the axion-photon coupling constant ( $g_{a\gamma\gamma}$ ) determines the strength of the interaction. The challenge lies in detecting the extremely weak signal power expected from this conversion within a specific mass range, requiring high sensitivity and precise control of experimental parameters.

Methodology and Experimental Setup
This work presents the "Student Project for an Axion Cavity Experiment" (SPACE), the first axion search of its kind in Germany. The experiment utilizes a haloscope approach, searching for axion dark matter in the mass range of 16.626 $\mu$ eV to 16.653 $\mu$ eV (corresponding to a frequency range of 4.020 GHz to 4.027 GHz).

Resonant Cavity: The core detector is a cylindrical resonant cavity made of oxygen-free high-conductivity copper (inner radius $r = 28.483$ mm, length $l = 280$ mm). It is designed to support the TM $_{010}$ mode, which is predominantly excited by the axion-induced current density aligned with the cavity axis.
Magnetic Field: The cavity is situated within a superconducting solenoid electromagnet providing a peak magnetic field of 14 T, with a root-mean-square value ( $B_{rms}$ ) of 12.66 T over the cavity volume.
Receiver Chain: Signals are coupled via a critically dipole antenna ( $\beta = 1.07$ ) and amplified by a three-stage low-noise amplifier (LNA) chain. The system includes bandpass filters to isolate the frequency of interest and a mixer to down-convert the signal to an intermediate frequency (115–122 MHz) for digitization by an ADC.
Calibration: The system gain (113.83 dB) and noise temperature were calibrated using the Y-Factor method with a calibrated noise diode. The system noise temperature ( $T_{sys}$ ) ranged from 36 K to 367 K during the run.
Data Acquisition: The experiment ran for 71 hours (April 19–22, 2024), generating 1.82 billion power spectra. To account for environmental drifts affecting the cavity's resonance frequency and quality factor ( $Q_L$ ), a Savitzky-Golay filter was applied to each raw spectrum to normalize the data. The processed spectra were combined using a weighted mean, accounting for variations in signal-to-noise ratio.
Analysis: The combined spectrum was cross-correlated with a theoretical signal shape derived from a boosted Maxwell-Boltzmann distribution, assuming a local dark matter velocity dispersion of 154 km/s and a laboratory velocity of 242.1 km/s. The analysis accounted for the resolution bandwidth being larger than the axion signal bandwidth.

Key Results
No significant signal was detected in the searched mass range. The largest observed excess was 3.24 $\sigma$ (global significance of 0.84 $\sigma$ ), consistent with random fluctuations. Consequently, the authors established exclusion limits on the axion-photon coupling constant ( $g_{a\gamma\gamma}$ ) at a 95% confidence level (CL):

Full Mass Range: $g_{a\gamma\gamma} = 14.6 \times 10^{-13}$ GeV $^{-1}$ is excluded for the entire range of 16.626 $\mu$ eV to 16.653 $\mu$ eV.
Peak Sensitivity: At the most sensitive point within the range, couplings greater than $g_{a\gamma\gamma} = 2.8 \times 10^{-13}$ GeV $^{-1}$ are excluded.

Systematic uncertainties, including those from the quality factor, antenna coupling, and system temperature, were propagated to the final limit, resulting in a total relative uncertainty of approximately 5.4% on the coupling constant.

Significance and Claims
The authors claim that this result represents a significant advancement in the search for axion dark matter within the specific mass range investigated. The exclusion limit surpasses previous constraints in this region, including those from the CAST experiment, by more than two orders of magnitude. While the achieved sensitivity remains a factor of 44 away from the KSVZ benchmark sensitivity, the work demonstrates the successful operation of a student-led axion search in Germany and establishes a new, tighter constraint on axion-photon coupling for masses around 16.64 $\mu$ eV. The paper emphasizes that these limits are derived assuming a local dark matter density of $\rho_a = 0.45$ GeV/cm $^3$ .

Technical Summary: A New Limit for Axion Dark Matter with SPACE

More like this