First Results from a Microwave Cavity Axion Search at $24\mu \mathrm{eV}$

We report on the first results from a new microwave cavity search for dark matter axions with masses above $20\mu \mathrm{eV}$. We exclude axion models with two-photon coupling $g_{a\gamma \gamma }\gtrsim 2\times {10}^{-14}{\mathrm{GeV}}^{-1}$ over the range $23.55<m_a<24.0\mu \mathrm{eV}$. These results represent two important achievements. First, we have reached cosmologically relevant sensitivity an order of magnitude higher in mass than any existing limits. Second, by incorporating a dilution refrigerator and Josephson parametric amplifier, we have demonstrated total noise approaching the standard quantum limit for the first time in an axion search.

Figure 1

Simplified receiver diagram. Blue arrows indicate the path that a putative axion signal would take through the system, and black arrows indicate other paths. A vector network analyzer (VNA) is used to measure the cavity’s frequency response in both transmission and reflection.

Figure 2

(a) Histogram of the full grand spectrum constructed from real data after cutting synthetic axions. (b) Simulated axion signal in the presence of Gaussian white noise. We construct the simulated grand spectrum two ways: multiplying the noise by an empirical baseline and applying a Savitzky-Golay filter to mimic real data (black circle), and using an ideal flat baseline and no fitting (blue triangle). In both cases the single grand spectrum bin best aligned with the axion is histogrammed over many iterations of the simulation. The reduction in $\sigma$ is due to the fit-induced correlations also observed in real data. After correcting for the reduction in $\sigma$, the reduction in $\mu$ is the fit-induced axion power loss. The vertical line is an analytic calculation of the expected SNR.

Figure 3

Our exclusion limit at 90% confidence. The light green shaded region is a $1\sigma$ error band. The large notch around 5.704 GHz is the result of cutting spectra around a previously unidentified TE mode. The narrow notches correspond to frequencies where synthetic axion signals were injected in one of the scans. The inset shows this work (green) together with previous cavity limits from ADMX (magenta, Ref. [9]) and early experiments at Brookhaven (RBF, blue, Ref. [22]) and the University of Florida (UF, cyan, Ref. [23]). The axion model band [14] is shown in yellow.