Collaborative Research: Novel Cavity Haloscopes for Axion Dark Matter at CM-Wavelengths

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Astronomical observations have established that the Universe is primarily “dark” with approximately 85% of the matter made of yet unknown particles. Uncovering the nature of this dark matter will have far reaching consequences in cosmology, astrophysics and particle physics. The quantum chromodynamic (QCD) axion arises in an extension of the Standard Model of particle physics that would simultaneously solve the mystery of dark matter and the long-standing strong charge-parity problem. This award supports researchers at Stanford University and the University of Washington in the development of a novel axion detector called a haloscope that will improve the scan rate of axion dark matter experiments by more than three orders of magnitude at frequencies between 4 – 7 GHz. The awarded activities include outreach and education via a Summer Research Program for Teachers, and a mentorship program for graduate students in the training of undergraduate researchers. Above frequencies of approximately 4 GHz , corresponding to an axion particle mass of a few tens of micro-electronvolts, there exists a large sensitivity gap between current measurements and the theoretical benchmarks. Improving the scan rate at these frequencies is crucial to enhancing the reach of axion search experiments. The large gain of the new haloscope design is achieved by implementing a space-filling thin-shell cavity that decouples the resonator’s volume from its resonant frequency. A full-sized science-grade cavity will be fabricated and installed in an existing 250mK testbed to demonstrate tunability and high quality factor under cryogenic conditions. The cavities will also be integrated with a Josephson Junction-based amplifier and a digitizer to obtain new exclusion limits on searches for dark photons. These developments establish key preparations for future axion searches involving large-volume solenoid magnet systems like those currently utilized for the ADMX project.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.