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This Letter reports new results from the HAYSTAC experiment’s search for dark matter axions in our galactic halo. It represents the widest search to date that utilizes squeezing to realize subquantum limited noise. The new results cover $1.71\mathrm{\mu }\mathrm{eV}$of newly scanned parameter space in the mass ranges $17.28–18.44\mathrm{\mu }\mathrm{eV}$and $18.71–19.46\mathrm{\mu }\mathrm{eV}$. No statistically significant evidence of an axion signal was observed, excluding couplings $\left|g_{\gamma }\right|\ge 2.75\times \left|g_{\gamma }^{\mathrm{KSVZ}}\right|$and $\left|g_{\gamma }\right|\ge 2.96\times \left|g_{\gamma }^{\mathrm{KSVZ}}\right|$at the 90% confidence level over the respective region. By combining this data with previously published results using HAYSTAC’s squeezed state receiver, a total of $2.27\mathrm{\mu }\mathrm{eV}$of parameter space has now been scanned between $16.96–19.46\mathrm{\mu }\mathrm{eV}$ $\mu \mathrm{eV}$, excluding $\left|g_{\gamma }\right|\ge 2.86\times \left|g_{\gamma }^{\mathrm{KSVZ}}\right|$at the 90% confidence level. These results demonstrate the squeezed state receiver’s ability to probe axion models over a significant mass range while achieving a scan rate enhancement relative to a quantum-limited experiment. Published by the American Physical Society2025 

Abstract This study explores the primary effects of dielectric materials in a resonant cavity-based search for axion dark matter. While dielectrics prove beneficial in numerous cases, their incorporation may lead to less-than-optimal performance, especially for the lowest TM mode. Additionally, the stronger confinement of the electric field inside the dielectrics can exacerbate mode mixings, in particular for higher-order modes. Case studies have been carried out using a combination of analytical solutions and numerical simulations. The findings indicate dielectric cavities employing the TM 010 mode experience a significant reduction in sensitivity when compared to a similar search conducted in a cavity at equivalent frequency using no dielectrics. 

Microwave cavity haloscopes are among the most sensitive direct detection experiments searching for dark matter axions via their coupling to photons. When the power of the expected microwave signal due to axion–photon conversion is on the order of 10−24 W, having the ability to validate the detector response and analysis procedure by injecting realistic synthetic axion signals becomes helpful. Here, we present a method based on frequency hopping spread spectrum for synthesizing axion signals in a microwave cavity haloscope experiment. It allows us to generate a narrow and asymmetric shape in frequency space that mimics an axion’s spectral distribution, which is derived from a Maxwell–Boltzmann distribution. In addition, we show that the synthetic axion’s power can be calibrated with reference to the system noise. Compared to the synthetic axion injection in the Haloscope At Yale Sensitive to Axion Cold dark matter (HAYSTAC) Phase I, we demonstrated synthetic signal injection with a more realistic line shape and calibrated power.