CONSTRUCTION AND OPERATION OF THE GENERATION-2 ADMX DETECTOR

The Snowmass 2013 Community Study Report, Cosmic Frontier CF3, Concluding Remarks stated: "Some candidates are perceived as more motivated than others. For example, the axion has the advantage of being a natural solution to a strong CP problem. Furthermore, for a dark-matter candidate, the QCD axion is special in that it has a fairly well bounded parameter space. The axion-photon couplings over the range of benchmark models extend over an order of magnitude. The upper end of the QCD axion mass range is set at a few meV by the limit from SN1987A (though there might be ways to evade this), the lower end, limited by the "misalignment" cosmological bound, is set at around a micro-eV (though, as discussed above, there are ways to evade this bound). The most promising approach to detecting the QCD axion is with the RF-cavity technique. Although the expected conversion into RF power within the cavity is extraordinarily weak, experiments will shortly start taking data for a definitive search. By "definitive", we mean one that will either find the axion with high confidence, if it exists, or if not, exclude it at high confidence. These experiments will sensitively explore the first two decades of allowed QCD axion mass where the dark-matter QCD axion is expected to be. Starkly, these searches have large discovery potential."

The Gen 2 Axion Dark Matter eXperiment (ADMX) is based on the idea that dark-matter axions in our Milky Way halo can be detected by their conversion into microwave photons within a high-Q cavity threaded by a large magnetic field. The signal power is of order of a yoctoWatt (10??24 Watts) of electromagnetic power, and signal detection therefore calls for deep cryogenic cooling and near quantum-limited Superconducting QUantum Interference Device (SQUID) microwave amplifiers.

We already received support for the first year of Gen 2 ADMX (2013-2014). These funds were applied to the procurement of a high flow-rate dilution refrigerator to cool the cavity and SQUID amplifiers to deep cryogenic temperatures. This proposal requests years 2, 3, and 4 Gen 2 ADMX continued project support. The key year 2 activity will be to install and commission the dilution refrigerator. Years 3 & 4 will include Gen 2 experiment operations, plus retrofits of new-frequency-range cavities and electronics into the experiment package. This Gen 2 timeline completes the construction and operation of the "definitive experiment" to search for QCD dark-matter axions.

Throughout the ADMX Gen 2 program, R&D will be undertaken to open up the higher-mass region of axion model space. This will be accomplished by instrumenting higher TM cavity modes. In addition, new cavity structures will be developed to access higher frequencies (and thereby higher masses) while exploiting the large volume within the ADMX magnet bore. Concurrently, new ultra-low noise microwave amplifiers at frequencies well above 2 GHz will be developed. Production SQUID amplifiers have been demonstrated to operate up to a few GHz, and other amplifier designs such as stripline resonator SQUIDs and Josephson Parametric Amplifiers (JPAs) have been show to reach well beyond that. This effort would develop and test amplifier designs that could be used to 10's of GHz and beyond. All these R&D tasks have prototypes already in operation, and the key R&D deliverable is to adapt these designs to a production environment.

This Gen 2 research program will allow ADMX to search for even the more pessimistically coupled QCD dark-matter axions at fractional axion halo densities for masses from 1 to over 40 microeV. This would expand the sensitivity to dark matter axions by orders of magnitude in both coupling and mass, thereby greatly increasing the likelihood of a discovery or ruling out the QCD dark-matter axion hypothesis at high confidence.