Session C12: Axion and Dark Matter I

Status and Overview of the Axion Dark Matter Experiment (ADMX)

The axion is a hypothetical particle that both explains why the strong force is CP invariant and answers the question of what constitutes the dark matter in the universe. ADMX is a direct search for dark-matter axions. The search technique is based on the resonant conversion of axions into photons with an apparatus consisting of a microwave cavity threaded by a magnetic field, and quantum-limited microwave amplification. This experiment, in several different configurations, is now beginning operations with enough sensitivity to either detect the ``QCD axion'' or reject that hypothesis at high confidence. This talk is an overview of the ADMX project and its search capability.   

Design of Axion Dark Matter Experiment (ADMX) Upgrade

Axions are hypothetical elementary particles that may provide the answer as to why QCD preserves the discrete symmetries P and CP and may also be the dark matter of the universe. The ADMX experiment has been at the forefront of the search for dark-matter axions for over a decade, and has recently undergone upgrades to dramatically improve its sensitivity. Construction of the first phase of the upgrade (dubbed Phase IIa) is now complete and we will be data taking for much of the upcoming year. I will give a very brief motivation for axions, then go into the mechanical details of ADMX, and finally look at the upgrades we plan to make in the near future.   

First results from ADMX Experiment: Phase IIa

The Axion,a light pseudo-scalar particle predicted as a consequence of the Peccei-Quinn solution to the Strong CP problem, is a compelling dark matter candidate with a well predicted coupling to the photon. The Axion Dark Matter Experiment (ADMX) is a microwave cavity experiment that has recently completed a full system upgrade and begun a comprehensive search for dark matter axions. The sensitivity of the newly upgraded experiment will be discussed and early results will be shown.   

The ADMX ultra-low noise receiver

The Axion Dark Matter eXperiment (ADMX) searches for dark-matter axions by looking for their resonant conversion to microwave photons in a strong magnetic field. Given the current experimental setup the axion-photon conversion power is expected to be below a yoctowatt ($< 10^{-24}$ W). Detecting such feeble signals above the thermal and electronic noise background requires a very sensitive microwave receiver. To ensure a fully characterized data pipeline, synthetic axion waveforms are simulated and periodically injected through the cavity and receiver chain. Here I discuss the calibration of the ADMX receiver and real-time analysis performed by the DAQ.   

A tunable microstrip SQUID amplifier for the Axion Dark Matter eXperiment (ADMX)

We describe a microstrip SQUID (Superconducting QUantum Interference Device) amplifier (MSA) used as the photon detector in the Axion Dark Matter eXperiment (ADMX). Cooled to 100 mK or lower, an optimized MSA approaches the quantum limit of detection. The axion dark matter is detected via Primakoff conversion to a microwave photon in a high-Q ($\approx $ 10$^{5})$ tunable microwave cavity, currently cooled to about 1.6 K, in the presence of a 7-tesla magnetic field. The MSA consists of a square loop of thin Nb film, incorporating two Josephson tunnel junctions with resistive shunts to prevent hysteresis in the current-voltage characteristic. The microstrip is a square Nb coil deposited over an intervening insulating layer. Since the photon frequency is determined by the unknown axion mass, the cavity and amplifier must be tunable over a broad frequency range. Tunability is achieved by terminating the microstrip with a GaAs varactor diode with a voltage-controlled capacitance that enables us to vary the resonance from nearly 1/2 to 1/4 of a wavelength. With the SQUID current-biased in the voltage state, we demonstrate a gain of typically 20 dB over nearly one octave, 415 MHz to 800 MHz.   

Design considerations for extending ADMX to temperatures below 1K

Phase II of the ADMX experiment is a large-scale upgrade with the objective of integrating the state-of-the-art in microwave detection and in cryogenic technologies. From its initial operations with pumped liquid $^{4}$He at $\sim$1.5K, a further reduction in physical temperature to the targeted 100~mK would improve its sensitivity more than twentyfold, extending the search below the DFSZ limit. But the cooling of a large microwave cavity to millikelvin temperatures in a high magnetic field poses some new challenges with no turnkey solutions from commercial cryogenic technologies. In this talk, we address the issue of incorporating current commercial technologies within our custom made insert to construct a dilution refrigeration system with a cooling power of 800~$\mu$W at 100~mK. Additionally, we describe a separate homemade pumped liquid $^{3}$He system with a 2-3~mW cooling power at 0.5K, which will be used as a bridge between the current $^{4}$He system at 1.5K and the planned 100~mK dilution system.   

Seaching for axions with ADMX: Higher Order Microwave Cavity Modes

The ADMX experiment searches for axions by looking for their resonant conversion to detectable photons with a frequency that directly corresponds to the axion mass (a currently unknown value). Though initial phases of the experiment only collected data at the fundamental frequency of the tunable cavity ADMX now includes all the necessary hardware and electronics to conduct simultaneous axion searches at two frequency regimes. ADMX researchers are investigating the mode structure of the cavity in operation to identify optimal modes and frequency regions for simultaneous data collection at the fundamental frequency mode and at a higher frequency mode. As these structures are understood, strategies of operation will be developed. In addition, in the summer of 2013 smaller high frequency cylindrical cavities were designed, constructed, and tested to allow ADMX to perform searches at higher frequencies than the large volume cavity that is currently installed. The cavities are essentially the same geometry as the current ADMX cavity scaled down, and an adapter plate to attach the cavities to the current hardware was also built to simplify integration in the current system and allow a quick move to a higher frequency search.   

Results from a modulation sensitive search for non-virialized halo axions

Flows of non-virialized axions may exist within the Milky Way halo. These flows are expected to have very low velocity dispersions, leading to correspondingly narrow peaks in the measured power spectra. Further, they may also contribute significantly to the local density of dark matter. A search for such flows has been performed by the Axion Dark Matter eXperiment Phase I at spectral resolutions of 84 mHz, 168 mHz, 546 mHz, and 1.09 Hz. Signal modulation due to terrestrial motion becomes significant at or below resolutions of order 1 Hz. Annual and daily modulation amplitudes of ~250 Hz and ~2 Hz were accounted for when identifying potential axion signals. This search produced limits on the local density of non-virialized axions over the 3.36--3.69 $\mu$eV mass range.