First Axion Search Results of the SUPAX Prototype Experiment

The SUPAX prototype experiment successfully operated a 2 K copper cavity in a 12 T magnetic field to exclude axion-photon couplings down to $1.6 \times 10^{-13}$GeV$^{-1}$and dark photon kinetic mixing parameters above$1.4 \times 10^{-12}$in the mass range around$34\,\mu$eV, demonstrating the viability of the full-scale haloscope design.

The Gist

Imagine the universe is filled with a mysterious, invisible "fog" called Dark Matter. Scientists have long suspected that this fog isn't made of ordinary stuff like dust or gas, but of tiny, ghostly particles called Axions. These particles are so light and elusive that they have never been directly seen. However, there's a clever theory: if you shine a very strong magnetic "flashlight" on them, they might turn into tiny flashes of light (photons) that we can catch.

This paper is the report card for a prototype experiment called SUPAX (SUPerconduction AXion search). Think of it as the "beta test" or the "pilot episode" for a much bigger, future experiment designed to hunt these ghost particles.

Here is the story of what they did, explained simply:

1. The Hunting Ground: A Giant Microwave Oven

To catch an axion, you need a special trap. The scientists built a copper box (a resonant cavity) that acts like a high-tech musical instrument.

• The Analogy: Imagine a guitar string. If you pluck it at the exact right speed, it sings loudly. The copper box is tuned to "sing" at a very specific frequency (8.3 GHz).
• The Magic: They put this box inside a super-strong magnet (12 Tesla, which is about 200,000 times stronger than a fridge magnet).
• The Cold: To stop the box from making its own "static noise" (like a radio tuned between stations), they froze it down to 2 Kelvin (colder than outer space, just above absolute zero).

2. The Strategy: Tuning the Radio

The problem is, we don't know the exact "note" (mass) the axions are singing. They could be anywhere in a specific range.

• The Challenge: Usually, to change the note a box sings, you have to physically move parts of it (like sliding a tuning rod). But at near-freezing temperatures, metal shrinks and gets brittle, making this hard.
• The Clever Trick: The SUPAX team used Helium gas pressure as a tuning knob. By slightly changing the pressure of the helium gas inside the box, they could shift the frequency of the "song" without touching the metal. It's like changing the pitch of a flute just by blowing harder or softer, rather than moving your fingers.

3. The Hunt: Listening for a Whisper

They ran the experiment for about 3 hours, scanning a tiny slice of the frequency spectrum (around 34 micro-electron-volts, or a mass of about 34 µeV).

• The Process: They listened to the box. If an axion turned into a photon inside the magnet, it would add a tiny bit of extra power to the signal, creating a "blip" on their graph.
• The Result: They listened very carefully, but they didn't hear a blip. The box remained silent.

4. The Victory: "We Didn't Find It, But We Know Where It Isn't"

In science, finding nothing is still a huge win. It's like searching for a lost key in a room. If you look in the corner and don't find it, you know the key isn't in that corner.

• The Exclusion: Because they didn't find a signal, they can say with 95% confidence: "Axions with this specific mass and this specific strength of interaction do not exist (or are much weaker than we thought)."
• The New Limits: They set a new "fence" on the map of the universe. They proved that if axions exist in this mass range, they are even more "ghostly" (harder to detect) than their previous best guess. They also used the same data to rule out another ghostly particle called a Dark Photon.

5. Why This Matters (The "So What?")

This prototype was a success story for three reasons:

1. It Worked: The copper box, the super-cold fridge, and the helium-pressure tuning trick all worked perfectly together.
2. It Was Fast: They covered a useful range of frequencies in just a few hours, proving their "tuning knob" idea is efficient.
3. It Paves the Way: This was just the dress rehearsal. The team is now building the real, full-scale experiment (two of them, actually) in Mainz and Bonn. These new versions will be even colder (10 millikelvin!) and use quantum amplifiers to listen even more quietly.

In a nutshell: The SUPAX team built a super-sensitive, super-cold radio receiver to listen for the universe's most elusive particles. They didn't catch the fish this time, but they proved their fishing rod works, showed exactly where the fish aren't, and are now building a bigger boat to go fishing again.

Technical Summary

Here is a detailed technical summary of the paper "First Axion Search Results of the SUPAX Prototype Experiment."

1. Problem Statement

The paper addresses the search for Axions and Axion-Like Particles (ALPs), which are hypothetical pseudoscalar particles proposed to solve the Strong-CP problem in Quantum Chromodynamics (QCD) and serve as candidates for Cold Dark Matter (CDM).

• Target Mass Range: The full-scale SUPerconduction AXion search (Supax) experiment aims to probe the mass range of 8 µeV to 30 µeV.
• The Challenge: Detecting axions requires converting them into microwave photons via the inverse Primakoff effect in a strong magnetic field. This signal is extremely weak, requiring:
  • Ultra-low noise temperatures.
  • High-quality factor ($Q$) resonant cavities.
  • Strong magnetic fields.
  • Precise frequency tuning to scan the narrow axion mass window.

2. Methodology and Experimental Setup

The authors constructed and operated a prototype haloscope to validate the design and performance of the future full-scale experiment.

Experimental Configuration

• Location: Helmholtz-Institut Mainz (Mainz, Germany).
• Magnet: A 14.1 T superconducting solenoid (operated at 12.1 T for the prototype).
• Cavity: A precision-machined copper cavity optimized for the $TM_{010}$ mode.
  • Dimensions: $\approx 160 \times 40 \times 26.8$ mm$^3$ (usable volume $\approx 102.6$ cm$^3$).
  • Quality Factor ($Q_0$): Measured at $5.2 \times 10^4$ (unloaded) at cryogenic temperatures.
  • Resonance Frequency: Tuned to 8.27 GHz (corresponding to an axion mass of $\approx 34$ µeV).
• Cryogenics: The system was cooled to 2 K using liquid helium (LHe) to minimize thermal noise.
• Readout System:
  • Amplification: A cryogenic Low-Noise Amplifier (LNA) with 36 dB gain, followed by a room-temperature LNA (38 dB).
  • Detection: A Real-Time Spectrum Analyzer (RSA) sampling at 28 MS/s with a 1 kHz readout bandwidth.
  • Tuning Mechanism: A novel method was employed where helium gas pressure inside the cavity was varied (30–90 mbar). The dielectric properties of the helium vapour compensated for the thermal contraction of the copper, allowing the resonance frequency to be tuned across a 1.4 MHz range without mechanical movement.

Data Analysis Strategy

1. Grand Unified Spectrum (GUS): Raw spectra were integrated, filtered using a Savitzky–Golay (SG) filter to remove cavity resonance and electronic artifacts, and rescaled by the cavity's Lorentzian response.
2. Weighting: Spectra were combined using inverse variance weighting to create a single, high-sensitivity spectrum.
3. Signal Search: The team searched for a narrow-band excess power peak (axion signal) against a white noise background. The signal shape was modeled based on the Maxwell–Boltzmann velocity distribution of galactic dark matter.
4. Statistical Limits: Upper limits were set at a 95% confidence level ($\alpha = 0.05$) using a $\chi^2$ likelihood function.

3. Key Contributions

• Prototype Validation: Successfully demonstrated the feasibility of the Supax design, including the integration of a high-field magnet, a high-$Q$ copper cavity, and a low-noise readout chain.
• Novel Tuning Mechanism: Validated the use of helium gas pressure to tune the cavity resonance frequency. This non-mechanical tuning method allows for rapid and precise scanning of the frequency band, a critical requirement for future axion searches.
• Dual Search Capability: The analysis framework was applied to exclude not only axions but also Dark Photons (a different dark matter candidate) in the same mass range.

4. Results

The prototype operated for approximately 3 hours of effective data taking, scanning a frequency range centered at 8.2662 GHz (corresponding to $m_a \approx 34$ µeV).

• Axion-Photon Coupling Limits:

  • No excess signal was detected.
  • The experiment excluded axion-photon couplings down to $|g_{a\gamma\gamma}| > 1.6 \times 10^{-13} \text{ GeV}^{-1}$ at 95% CL.
  • This limit is competitive with existing experiments in this specific mass range despite the short integration time.

• Dark Photon Limits:

  • The data was re-analyzed to constrain the kinetic mixing parameter ($\chi$) of dark photons.
  • Random Polarization Scenario: Excluded $\chi < 2 \times 10^{-14}$ to $6.3 \times 10^{-14}$.
  • Fixed Polarization Scenario: Excluded $\chi < 5.2 \times 10^{-14}$ to $1.6 \times 10^{-13}$.

• System Performance:

  • System noise temperature ($T_{sys}$): $\approx 10$ K.
  • Loaded quality factor ($Q_L$): $\approx 27,600$.
  • The measured noise fluctuations agreed with theoretical expectations, confirming the stability of the setup.

5. Significance and Outlook

• Proof of Concept: The successful operation of the Supax prototype confirms the viability of the experimental concept, particularly the helium-pressure tuning mechanism and the data acquisition pipeline.
• Future Roadmap: Based on these results, the collaboration is developing two next-generation Sikivie-type haloscopes:
  • Locations: One at the University of Mainz and one at the University of Bonn.
  • Upgrades: These will operate at 10 mK (significantly lower than the prototype's 2 K) and utilize quantum-limited readout to drastically improve sensitivity.
  • Timeline: Commissioning is targeted for the end of 2026.
• Broader Impact: Beyond axion searches, the future sensor network (GravNet) will be utilized to search for high-frequency gravitational waves, expanding the scientific reach of the facility.

In summary, the paper presents a successful first step in the Supax program, establishing a robust experimental foundation and demonstrating competitive exclusion limits for axions and dark photons in the 34 µeV mass range.