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Register a new userA SQUID-based microwave cavity search for dark-matter axions
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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.
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The axion is a light pseudoscalar particle which suppresses CP-violating effects in strong interactions and also happens to be an excellent dark matter candidate. Axions constituting the dark matter halo of our galaxy may be detected by their resonant conversion to photons in a microwave cavity permeated by a magnetic field. The current generation of the microwave cavity experiment has demonstrated sensitivity to plausible axion models, and upgrades in progress should achieve the sensitivity required for a definitive search, at least for low mass axions. However, a comprehensive strategy for scanning the entire mass range, from 1-1000 $mu$eV, will require significant technological advances to maintain the needed sensitivity at higher frequencies. Such advances could include sub-quantum-limited amplifiers based on squeezed vacuum states, bolometers, and/or superconducting microwave cavities. The Axion Dark Matter eXperiment at High Frequencies (ADMX-HF) represents both a pathfinder for first data in the 20-100 $mu$eV range ($sim$5-25 GHz), and an innovation test-bed for these concepts.
Non-virialized dark-matter axions may be present in the Milky Way halo in the form of low-velocity-dispersion flows. The Axion Dark Matter eXperiment performed a search for the conversion of these axions into microwave photons using a resonant cavity immersed in a strong, static magnetic field. The spread of photon energy in these measurements was measured at spectral resolutions of the order of 1 Hz and below. If the energy variation were this small, the frequency modulation of any real axion signal due to the orbital and rotational motion of the Earth would become non-negligible. Conservative estimates of the expected signal modulation were made and used as a guide for the search procedure. The photon frequencies covered by this search are 812$-$852 and 858$-$892 MHz, which correspond to an axion mass of 3.36$-$3.52 and 3.55$-$3.69 {mu}eV. No axion signal was found, and limits were placed on the maximum local density of non-virialized axions of these masses.
In dark matter axion searches, quantum uncertainty manifests as a fundamental noise source, limiting the measurement of the quadrature observables used for detection. We use vacuum squeezing to circumvent the quantum limit in a search for a new particle. By preparing a microwave-frequency electromagnetic field in a squeezed state and near-noiselessly reading out only the squeezed quadrature, we double the search rate for axions over a mass range favored by recent theoretical projections. We observe no signature of dark matter axions in the combined $16.96-17.12$ and $17.14-17.28spacemutext{eV}/c^2$ mass window for axion-photon couplings above $g_{gamma} = 1.38times g_{gamma}^text{KSVZ}$, reporting exclusion at the 90% level.
The microwave cavity experiment is the most sensitive way of looking for axions in the 0.1-10 GHz range, corresponding to masses of 0.5 - 40 $mu$eV. The particular challenge for frequencies greater than 5 GHz is designing a cavity with a large volume that contains a resonant mode that has a high form factor, a high quality factor, a wide dynamic range, and is free from intruder modes. For HAYSTAC, we have designed and constructed an optimized high frequency cavity with a tuning mechanism that preserves a high degree of rotational symmetry, critical to maximizing its figure of merit. This cavity covers an important frequency range according to recent theoretical estimates for the axion mass, 5.5 - 7.4 GHz, and the design appears extendable to higher frequencies as well. This paper will discuss key design and construction details of the cavity, present a summary of the design evolution, and alert practitioners of potentially unfruitful avenues for future work.
We suggest a new approach to search for galactic axions or other similar exotica. Streaming dark matter (DM) could have a better discovery potential because of flux enhancement, due to gravitational lensing when the Sun and/or a planet are aligned with a DM stream. Of interest are also axion miniclusters, in particular, if the solar system has trapped one during its formation. Wide-band axion antennae fit this concept, but also the proposed fast narrow band scanning. A network of detectors can provide full time coverage and a large axion mass acceptance. Other DM searches may profit from this proposal.
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