A SQUID-based microwave cavity search for dark-matter axions (0910.5914v1)

Abstract: Axions in the micro eV mass range are a plausible cold dark matter candidate and may be detected by their conversion into microwave photons in a resonant cavity immersed in a static magnetic field. The first result from such an axion search using a superconducting first-stage amplifier (SQUID) is reported. The SQUID amplifier, replacing a conventional GaAs field-effect transistor amplifier, successfully reached axion-photon coupling sensitivity in the band set by present axion models and sets the stage for a definitive axion search utilizing near quantum-limited SQUID amplifiers.

• The paper demonstrates a two-order-of-magnitude increase in scan rate using a SQUID amplifier to enhance axion detection sensitivity.
• The experiment employs a 7.6-tesla superconducting solenoid and a copper-plated microwave cavity to target axion masses around 3.3 μeV.
• The study highlights how further noise reduction and advanced cryogenics could extend detection limits and refine axion dark matter models.

Overview of SQUID-Based Microwave Cavity Search for Dark-Matter Axions

The paper discusses a novel approach for detecting axions, hypothetical particles proposed as candidates for dark matter, through their conversion to microwave photons in a resonant cavity. The methodology involves using a Superconducting QUantum Interference Device (SQUID) as a first-stage amplifier in the Axion Dark Matter eXperiment (ADMX) setup, significantly enhancing the sensitivity and scanning speed over previous methods. The axion's low mass, theoretically in the $\mu$eV range, makes it a plausible form of cold dark matter, with implications in particle physics, astrophysics, and cosmology when detected.

Experimental Setup and Methodology

The experiment, conducted at Lawrence Livermore National Laboratory, centers on detecting axion-photon coupling in a high magnetic field environment. The ADMX configuration integrates a 7.6-tesla superconducting solenoid and a cylindrical, copper-plated microwave cavity. Axions passing through this field could resonate, converting into microwave photons. The power generated from axion-photon conversions depends on several parameters, including the axion-photon coupling constant $g_{a\gamma\gamma}$, cavity volume, magnetic field strength, and the local dark matter density $\rho_a$.

The paper details the use of a SQUID-based amplifier, which offers improved performance over previous gallium arsenide (GaAs) field-effect transistor amplifiers by approaching quantum-limited noise temperatures. The noise temperature reduction enhances the signal-to-noise ratio (SNR), which is critical for detecting faint axion signals amidst background noise. The system noise temperature $T_S$, combining contributions from physical cavity and amplifier noise, directly influences the scanning speed and sensitivity of the detector.

Results and Implications

Employing the dc SQUID as a first-stage amplifier allows for a two-order-of-magnitude improvement in scan rate compared to prior setups, significantly enhancing the experiment's ability to probe the vast parameter space of axion models. The specific axion mass range targeted was from 3.3 $\mu$eV to values exceeding previous experiments, further narrowing the parameter space where dark matter axions could exist. No persistent axion signals were detected within this exploration, providing a 90% confidence exclusion for certain axion models.

The results confirm the potential for SQUID-based systems to operate effectively in high-field environments, paving the way for future experiments with even greater sensitivity. By cooling the detector further using a dilution refrigerator, subsequent phases of ADMX may reduce detection limits to encompass weaker axion-photon couplings and further expand the search range.

Future Prospects

The demonstrated efficacy of SQUID amplifiers in axion detection suggests several avenues for advancement. Continued reduction of system noise temperatures and increased scanning speeds may allow researchers to cover more expansive mass ranges, refining the exclusion regions for axion models. Future experiments might integrate improved amplification technologies and precision cryogenic systems, facilitating unprecedented exploration of the axion parameter space. The ongoing developments in this experimental approach hold promise for contributing valuable insights into the nature of dark matter and the role of axions within the broader framework of the cosmos.