First results from the HAYSTAC axion search

The axion is a well-motivated cold dark matter (CDM) candidate first postulated to explain the absence of \(CP\) violation in the strong interactions. CDM axions may be detected via their resonant conversion into photons in a "haloscope" detector: a tunable high-\(Q\) microwave cavity maintained at cryogenic temperature, immersed a strong magnetic field, and coupled to a low-noise receiver. This dissertation reports on the design, commissioning, and first operation of the Haloscope at Yale Sensitive to Axion CDM (HAYSTAC), a new detector designed to search for CDM axions with masses above \(20\) \(\mu\mathrm{eV}\). I also describe the analysis procedure developed to derive limits on axion CDM from the first HAYSTAC data run, which excluded axion models with two-photon coupling \(g_{a\gamma\gamma} \gtrsim 2\times10^{-14}\) \(\mathrm{GeV}^{-1}\), a factor of 2.3 above the benchmark KSVZ model, over the mass range \(23.55 < m_a < 24.0\) \(\mu\mathrm{eV}\). This result represents two important achievements. First, it demonstrates cosmologically relevant sensitivity an order of magnitude higher in mass than any existing direct limits. Second, by incorporating a dilution refrigerator and Josephson parametric amplifier, HAYSTAC has demonstrated total noise approaching the standard quantum limit for the first time in a haloscope axion search.